Earnings Per Share (''EPS'') - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	9 Months Ended
Jul. 02, 2017	Jun. 26, 2016	Jul. 02, 2017	Jun. 26, 2016
Number of weighted-average shares used to compute basic and diluted EPS:
Net income attributable to Tetra Tech	$ 29,983	$ 25,694	$ 83,407	$ 52,678
Weighted-average common shares outstanding - basic	57,184	57,796	57,108	58,483
Effect of dilutive stock options and unvested restricted stock	977	820	1,008	745
Weighted-average common stock outstanding - diluted	58,161	58,616	58,116	59,228
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.52	$ 0.44	$ 1.46	$ 0.90
Diluted (in dollars per share)	$ 0.52	$ 0.44	$ 1.44	$ 0.89

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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