Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Oct. 01, 2023	Oct. 02, 2022	Oct. 03, 2021
Revenue	$ 4,522,550	$ 3,504,048	$ 3,213,513
Gross profit	725,029	575,559	498,400
Selling, general and administrative expenses	(305,107)	(234,784)	(222,972)
Acquisition and integration expenses	(33,169)	0	0
Right-of-use operating lease asset impairment	(16,385)	0	0
Contingent consideration – fair value adjustments	(12,255)	(329)	3,273
Income from operations	358,113	340,446	278,701
Interest income	5,898	1,780	917
Interest expense	(52,435)	(13,364)	(12,748)
Other non-operating income	89,402	19,904	0
Income before income tax expense	400,978	348,766	266,870
Income tax expense	(127,526)	(85,602)	(34,039)
Net income	273,452	263,164	232,831
Net income attributable to noncontrolling interests	(32)	(39)	(21)
Net income attributable to Tetra Tech	273,420	263,125	232,810
Net income attributable to Tetra Tech	$ 273,420	$ 263,125	$ 232,810
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 5.14	$ 4.91	$ 4.31
Diluted (in dollars per share)	$ 5.10	$ 4.86	$ 4.26
Weighted-average common shares outstanding:
Basic (in shares)	53,203	53,620	54,078
Diluted (in shares)	53,637	54,163	54,675
Subcontractor costs
Costs of revenue	$ (771,461)	$ (668,468)	$ (661,341)
Other costs of revenue
Costs of revenue	$ (3,026,060)	$ (2,260,021)	$ (2,053,772)