Accounting Policies (Policies)	9 Months Ended
Sep. 28, 2025
Accounting Policies [Abstract]
Basis Of Accounting	The interim consolidated financial statements include the accounts of ATI Inc. and its subsidiaries. Unless the context requires otherwise, “ATI” and “the Company” refer to ATI Inc. and its subsidiaries. The Company follows a 4-4-5 or 5-4-4 fiscal calendar, whereby each fiscal quarter consists of thirteen weeks grouped into two four-week months and one five-week month, and its fiscal year ends on the Sunday closest to December 31. Unless otherwise stated, references to years and quarters in this Quarterly Report on Form 10-Q relate to fiscal years and quarters, rather than calendar years and quarters. These unaudited consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles for interim financial information and with the instructions for Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and note disclosures required by U.S. generally accepted accounting principles for complete financial statements. In management’s opinion, all adjustments (which include only normal recurring adjustments) considered necessary for a fair presentation have been included. These unaudited consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto included in the Company’s 2024 Annual Report on Form 10-K. The results of operations for these interim periods are not necessarily indicative of the operating results for any future period. The December 29, 2024 financial information has been derived from the Company’s audited consolidated financial statements.
New Accounting Pronouncements Adopted	New Accounting Pronouncements Adopted In November 2023, the Financial Accounting Standards Board (FASB) issued new accounting guidance related to segment reporting disclosures. This guidance requires additional disclosures on an annual and interim basis of segment information, including significant segment expenses that are regularly provided to the chief operating decision maker (CODM) and the presentation and composition of other segment items, which is the difference between segment revenue less segment expenses and the measure of segment profit or loss. The guidance also requires that all current segment disclosures required on an annual basis be provided on an interim basis and requires disclosure of the title and position of the CODM and how the CODM uses the reported measure of segment profit or loss in assessing performance and allocating resources. This guidance does not change how an entity identifies its reportable segments. This new guidance, with the exception of interim disclosures, was effective for the Company in fiscal year 2024, and the Company adopted this new accounting guidance for annual disclosures effective January 1, 2024. The interim disclosures are effective for the Company in fiscal year 2025. The adoption of these changes did not have an impact on the Company’s consolidated financial statements other than disclosure requirements which are included in Note 11. Pending Accounting Pronouncements In December 2023, the FASB issued new accounting guidance related to income tax disclosures. This guidance requires an entity to disclose specific categories in its annual rate reconciliation and provide additional information for reconciling items that meet a quantitative threshold. This guidance also requires additional annual disclosures for income taxes paid and requires disaggregation of income before tax, between domestic and foreign, and income tax expense, between federal, state and foreign. This guidance also eliminates several current disclosure requirements related to: (1) the nature and estimate of the range of the reasonably possible change in the unrecognized tax benefits balance in the next 12 months, (2) making a statement that an estimate of the range cannot be made, and (3) disclosing the cumulative amount of each type of temporary difference when a deferred tax liability is not recognized because of the exceptions to comprehensive recognition of deferred taxes related to subsidiaries and corporate joint ventures. This new guidance includes annual disclosure requirements that will be effective for the Company for fiscal year 2025 and must be applied on a prospective basis with retrospective application permitted. The Company does not expect these changes to have an impact on the Company’s consolidated financial statements other than disclosure requirements. In November 2024, the FASB issued new accounting guidance related to expense disaggregation disclosures. This guidance requires entities to disclose specified information about certain costs and expenses including (1) the amounts of purchases of inventory, employee compensation, depreciation, and intangible asset amortization, (2) include certain amounts that are already required to be disclosed under current generally accepted accounting principles in the same disclosure as the other disaggregation requirements, (3) a qualitative description of the amounts remaining in relevant expense captions that are not separately disaggregated quantitatively, and (4) the total amount of selling expenses and, in annual reporting periods, an entity’s definition of selling expenses. This new guidance for annual disclosures will be effective for the Company for fiscal year 2027 and for interim disclosures will be effective for the Company for fiscal year 2028. The guidance can be applied prospectively or retrospectively and early adoption is permitted. The Company does not expect to early adopt this guidance and does not expect these changes to have an impact on the Company’s consolidated financial statements other than disclosure requirements.
Inventory	Inventories are stated at the lower of cost (first-in, first-out (FIFO) and average cost methods) or net realizable value.
Derivatives	As part of its risk management strategy, the Company, from time-to-time, utilizes derivative financial instruments to manage its exposure to changes in raw material prices, energy costs, foreign currencies, and interest rates. In accordance with applicable accounting standards, the Company accounts for most of these contracts as hedges. The Company sometimes uses futures and swap contracts to manage exposure to changes in prices for forecasted purchases of raw materials, such as nickel, and natural gas. Under these contracts, which are generally accounted for as cash flow hedges, the price of the item being hedged is fixed at the time that the contract is entered into, and the Company is obligated to make or entitled to receive a payment equal to the net change between this fixed price and the market price at the date the contract matures. The majority of ATI’s products are sold under contractual arrangements that include raw material surcharges and index mechanisms. However, as of September 28, 2025, the Company had entered into financial hedging arrangements, primarily at the request of its customers related to firm orders, for an aggregate notional amount of approximately 1 million pounds of nickel with hedge dates through 2027. The aggregate notional amount hedged is approximately 2% of a single year’s estimated nickel raw material purchase requirements. These derivative instruments are used to hedge the variability of a selling price that is based on the London Metal Exchange (LME) index for nickel, as well as to hedge the variability of the purchase cost of nickel based on this LME index. Any gain or loss associated with these hedging arrangements is included in sales or cost of sales, depending on whether the underlying risk being hedged is the variable selling price or the variable raw material cost, respectively. At September 28, 2025, the outstanding financial derivatives used to hedge the Company’s exposure to energy cost volatility consisted of natural gas cost hedges. At September 28, 2025, the Company hedged approximately 60% of its forecasted domestic requirements for natural gas for the remainder of 2025 and approximately 35% for 2026. While most of the Company’s direct export sales are transacted in U.S. dollars, it uses foreign currency exchange contracts, from time-to-time, to limit transactional exposure to changes in currency exchange rates for those transactions denominated in a non-U.S. currency. The Company sometimes purchases foreign currency forward contracts that permit it to sell specified amounts of foreign currencies it expects to receive from its export sales for pre-established U.S. dollar amounts at specified dates. In addition, the Company may also hedge forecasted capital expenditures and designate cash balances held in foreign currencies as hedges of forecasted foreign currency transactions. At September 28, 2025, the Company had no material outstanding foreign currency forward contracts. The Company may enter into derivative interest rate contracts to maintain a reasonable balance between fixed- and floating-rate debt. There were no outstanding derivative interest rate contracts at September 28, 2025. There are no credit risk-related contingent features in the Company’s derivative contracts, and the contracts contain no provisions under which the Company has posted, or would be required to post, collateral. The counterparties to the Company’s derivative contracts are substantial and creditworthy commercial banks that are recognized market makers. The Company controls its credit exposure by diversifying across multiple counterparties and by monitoring credit ratings and credit default swap spreads of its counterparties. The Company also enters into master netting agreements with counterparties when possible. For derivative financial instruments that are designated as cash flow hedges, the gain or loss on the derivative is reported as a component of other comprehensive income (OCI) and reclassified into earnings in the same period or periods during which the hedged item affects earnings. For derivative financial instruments that are designated as fair value hedges, changes in the fair value of these derivatives are recognized in current period results. There were no outstanding fair value hedges as of September 28, 2025. The cash flow impact for all derivative financial instruments is reported in cash flows provided by operating activities on the consolidated statement of cash flows. The Company did not use net investment hedges for the periods presented. The effects of derivative instruments in the tables below are presented net of related income taxes, excluding any impacts of changes to income tax valuation allowances affecting results of operations or other comprehensive income, when applicable (see Note 15 for further explanation). The Company may also use derivative instruments that are not designated as hedges to protect the Company’s results from certain fluctuations in foreign exchange rates, as well as to offset a portion of the foreign currency gains and losses generated by the remeasurement of certain assets and liabilities denominated in non-functional currencies. Changes in the fair value of these foreign exchange contract derivatives not designated as hedging instruments are recorded in cost of sales or selling, general and administrative expenses on the consolidated statement of operations, and the Company recognized $0.6 million of expense and $2.3 million of income, net, for settled foreign currency forward contracts that were not designated as hedges during the third quarter and year-to-date period ended September 28, 2025, respectively, and $1.0 million and $0.5 million of income, net, during the third quarter and year-to-date period ended September 29, 2024, which offset foreign currency gains/losses in the relevant currency. We have no significant outstanding hedges that are not designated as of September 28, 2025.
Retirement Benefits	The Company has defined contribution retirement plans or defined benefit pension plans covering substantially all employees. Company contributions to defined contribution retirement plans are generally based on either a percentage of eligible pay or on hours worked. Benefits under the defined benefit pension plans are generally based on years of service and/or final average pay. The Company funds the U.S. pension plans in accordance with the Employee Retirement Income Security Act of 1974, as amended, and the Internal Revenue Code of 1986, as amended. The Company also sponsors several postretirement plans covering certain collectively bargained salaried and hourly employees. The plans provide health care and life insurance benefits for eligible retirees. In most retiree health care plans, Company contributions towards premiums are capped based on the cost as of a certain date, thereby creating a defined contribution. All defined benefit pension and retiree health care plans are closed to new entrants.
Commitments And Contingencies	Environmental liabilities are recorded when the Company’s liability is probable and the costs are reasonably estimable. In many cases, however, the Company is not able to determine whether it is liable or, if liability is probable, to reasonably estimate the loss or range of loss. Estimates of the Company’s liability remain subject to additional uncertainties, including the nature and extent of site contamination, available remediation alternatives, the extent of corrective actions that may be required, and the number, participation, and financial condition of other potentially responsible parties (PRPs). The Company adjusts its accruals to reflect new information as appropriate. Future adjustments could have a material adverse effect on the Company’s consolidated results of operations in a given period, but the Company cannot reliably predict the amounts of such future adjustments.

Accounting Policies (Policies)

9 Months Ended

Sep. 28, 2025

The interim consolidated financial statements include the accounts of ATI Inc. and its subsidiaries. Unless the context requires otherwise, “ATI” and “the Company” refer to ATI Inc. and its subsidiaries.

The Company follows a 4-4-5 or 5-4-4 fiscal calendar, whereby each fiscal quarter consists of thirteen weeks grouped into two four-week months and one five-week month, and its fiscal year ends on the Sunday closest to December 31. Unless otherwise stated, references to years and quarters in this Quarterly Report on Form 10-Q relate to fiscal years and quarters, rather than calendar years and quarters.

These unaudited consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles for interim financial information and with the instructions for Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and note disclosures required by U.S. generally accepted accounting principles for complete financial statements. In management’s opinion, all adjustments (which include only normal recurring adjustments) considered necessary for a fair presentation have been included. These unaudited consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto included in the Company’s 2024 Annual Report on Form 10-K. The results of operations for these interim periods are not necessarily indicative of the operating results for any future period. The December 29, 2024 financial information has been derived from the Company’s audited consolidated financial statements.

New Accounting Pronouncements Adopted

In November 2023, the Financial Accounting Standards Board (FASB) issued new accounting guidance related to segment reporting disclosures. This guidance requires additional disclosures on an annual and interim basis of segment information, including significant segment expenses that are regularly provided to the chief operating decision maker (CODM) and the presentation and composition of other segment items, which is the difference between segment revenue less segment expenses and the measure of segment profit or loss. The guidance also requires that all current segment disclosures required on an annual basis be provided on an interim basis and requires disclosure of the title and position of the CODM and how the CODM uses the reported measure of segment profit or loss in assessing performance and allocating resources. This guidance does not change how an entity identifies its reportable segments. This new guidance, with the exception of interim disclosures, was effective for the Company in fiscal year 2024, and the Company adopted this new accounting guidance for annual disclosures effective January 1, 2024. The interim disclosures are effective for the Company in fiscal year 2025. The adoption of these changes did not have an impact on the Company’s consolidated financial statements other than disclosure requirements which are included in Note 11.

Pending Accounting Pronouncements

In December 2023, the FASB issued new accounting guidance related to income tax disclosures. This guidance requires an entity to disclose specific categories in its annual rate reconciliation and provide additional information for reconciling items that meet a quantitative threshold. This guidance also requires additional annual disclosures for income taxes paid and requires disaggregation of income before tax, between domestic and foreign, and income tax expense, between federal, state and foreign. This guidance also eliminates several current disclosure requirements related to: (1) the nature and estimate of the range of the reasonably possible change in the unrecognized tax benefits balance in the next 12 months, (2) making a statement that an estimate of the range cannot be made, and (3) disclosing the cumulative amount of each type of temporary difference when a deferred tax liability is not recognized because of the exceptions to comprehensive recognition of deferred taxes related to subsidiaries and corporate joint ventures. This new guidance includes annual disclosure requirements that will be effective for the Company for fiscal year 2025 and must be applied on a prospective basis with retrospective application permitted. The Company does not expect these changes to have an impact on the Company’s consolidated financial statements other than disclosure requirements.

In November 2024, the FASB issued new accounting guidance related to expense disaggregation disclosures. This guidance requires entities to disclose specified information about certain costs and expenses including (1) the amounts of purchases of inventory, employee compensation, depreciation, and intangible asset amortization, (2) include certain amounts that are already required to be disclosed under current generally accepted accounting principles in the same disclosure as the other disaggregation requirements, (3) a qualitative description of the amounts remaining in relevant expense captions that are not separately disaggregated quantitatively, and (4) the total amount of selling expenses and, in annual reporting periods, an entity’s definition of selling expenses. This new guidance for annual disclosures will be effective for the Company for fiscal year 2027

and for interim disclosures will be effective for the Company for fiscal year 2028. The guidance can be applied prospectively or retrospectively and early adoption is permitted. The Company does not expect to early adopt this guidance and does not expect these changes to have an impact on the Company’s consolidated financial statements other than disclosure requirements.

Inventory

Inventories are stated at the lower of cost (first-in, first-out (FIFO) and average cost methods) or net realizable value.

Derivatives

As part of its risk management strategy, the Company, from time-to-time, utilizes derivative financial instruments to manage its exposure to changes in raw material prices, energy costs, foreign currencies, and interest rates. In accordance with applicable accounting standards, the Company accounts for most of these contracts as hedges.

The Company sometimes uses futures and swap contracts to manage exposure to changes in prices for forecasted purchases of raw materials, such as nickel, and natural gas. Under these contracts, which are generally accounted for as cash flow hedges, the price of the item being hedged is fixed at the time that the contract is entered into, and the Company is obligated to make or entitled to receive a payment equal to the net change between this fixed price and the market price at the date the contract matures.

The majority of ATI’s products are sold under contractual arrangements that include raw material surcharges and index mechanisms. However, as of September 28, 2025, the Company had entered into financial hedging arrangements, primarily at the request of its customers related to firm orders, for an aggregate notional amount of approximately 1 million pounds of nickel with hedge dates through 2027. The aggregate notional amount hedged is approximately 2% of a single year’s estimated nickel raw material purchase requirements. These derivative instruments are used to hedge the variability of a selling price that is based on the London Metal Exchange (LME) index for nickel, as well as to hedge the variability of the purchase cost of nickel based on this LME index. Any gain or loss associated with these hedging arrangements is included in sales or cost of sales, depending on whether the underlying risk being hedged is the variable selling price or the variable raw material cost, respectively.

At September 28, 2025, the outstanding financial derivatives used to hedge the Company’s exposure to energy cost volatility consisted of natural gas cost hedges. At September 28, 2025, the Company hedged approximately 60% of its forecasted domestic requirements for natural gas for the remainder of 2025 and approximately 35% for 2026.

While most of the Company’s direct export sales are transacted in U.S. dollars, it uses foreign currency exchange contracts, from time-to-time, to limit transactional exposure to changes in currency exchange rates for those transactions denominated in a non-U.S. currency. The Company sometimes purchases foreign currency forward contracts that permit it to sell specified amounts of foreign currencies it expects to receive from its export sales for pre-established U.S. dollar amounts at specified dates. In addition, the Company may also hedge forecasted capital expenditures and designate cash balances held in foreign currencies as hedges of forecasted foreign currency transactions. At September 28, 2025, the Company had no material outstanding foreign currency forward contracts.

The Company may enter into derivative interest rate contracts to maintain a reasonable balance between fixed- and floating-rate debt. There were no outstanding derivative interest rate contracts at September 28, 2025.

There are no credit risk-related contingent features in the Company’s derivative contracts, and the contracts contain no provisions under which the Company has posted, or would be required to post, collateral. The counterparties to the Company’s derivative contracts are substantial and creditworthy commercial banks that are recognized market makers. The Company controls its credit exposure by diversifying across multiple counterparties and by monitoring credit ratings and credit default swap spreads of its counterparties. The Company also enters into master netting agreements with counterparties when possible.

For derivative financial instruments that are designated as cash flow hedges, the gain or loss on the derivative is reported as a component of other comprehensive income (OCI) and reclassified into earnings in the same period or periods during which the hedged item affects earnings. For derivative financial instruments that are designated as fair value hedges, changes in the fair value of these derivatives are recognized in current period results. There were no outstanding fair value hedges as of September 28, 2025. The cash flow impact for all derivative financial instruments is reported in cash flows provided by operating activities on the consolidated statement of cash flows. The Company did not use net investment hedges for the periods presented. The effects of derivative instruments in the tables below are presented net of related income taxes, excluding any impacts of changes to income tax valuation allowances affecting results of operations or other comprehensive income, when applicable (see Note 15 for further explanation).

The Company may also use derivative instruments that are not designated as hedges to protect the Company’s results from certain fluctuations in foreign exchange rates, as well as to offset a portion of the foreign currency gains and losses generated by the remeasurement of certain assets and liabilities denominated in non-functional currencies. Changes in the fair value of these foreign exchange contract derivatives not designated as hedging instruments are recorded in cost of sales or selling, general and administrative expenses on the consolidated statement of operations, and the Company recognized $0.6 million of expense and $2.3 million of income, net, for settled foreign currency forward contracts that were not designated as hedges during the third quarter and year-to-date period ended September 28, 2025, respectively, and $1.0 million and $0.5 million of income, net, during the third quarter and year-to-date period ended September 29, 2024, which offset foreign currency gains/losses in the relevant currency. We have no significant outstanding hedges that are not designated as of September 28, 2025.

Retirement Benefits

The Company has defined contribution retirement plans or defined benefit pension plans covering substantially all employees. Company contributions to defined contribution retirement plans are generally based on either a percentage of eligible pay or on hours worked. Benefits under the defined benefit pension plans are generally based on years of service and/or final average pay. The Company funds the U.S. pension plans in accordance with the Employee Retirement Income Security Act of 1974, as amended, and the Internal Revenue Code of 1986, as amended. The Company also sponsors several postretirement plans covering certain collectively bargained salaried and hourly employees. The plans provide health care and life insurance benefits for eligible retirees. In most retiree health care plans, Company contributions towards premiums are capped based on the cost as of a certain date, thereby creating a defined contribution. All defined benefit pension and retiree health care plans are closed to new entrants.

Commitments And Contingencies

Environmental liabilities are recorded when the Company’s liability is probable and the costs are reasonably estimable. In many cases, however, the Company is not able to determine whether it is liable or, if liability is probable, to reasonably estimate the loss or range of loss. Estimates of the Company’s liability remain subject to additional uncertainties, including the nature and extent of site contamination, available remediation alternatives, the extent of corrective actions that may be required, and the number, participation, and financial condition of other potentially responsible parties (PRPs). The Company adjusts its accruals to reflect new information as appropriate. Future adjustments could have a material adverse effect on the Company’s

consolidated results of operations in a given period, but the Company cannot reliably predict the amounts of such future adjustments.