Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jul. 03, 2022	Jun. 27, 2021	Jul. 03, 2022	Jun. 27, 2021
Revenue	$ 890,231	$ 801,633	$ 2,601,485	$ 2,321,500
Gross profit	144,584	125,696	419,524	354,490
Selling, general and administrative expenses	(60,679)	(55,889)	(173,879)	(157,625)
Income from operations	83,905	69,807	245,645	196,865
Interest expense, net	(2,919)	(2,737)	(8,967)	(8,585)
Income before income tax expense	80,986	67,070	236,678	188,280
Income tax expense	(22,329)	(15,146)	(56,473)	(38,380)
Net income	58,657	51,924	180,205	149,900
Net income attributable to noncontrolling interests	(7)	(21)	(26)	(44)
Net income attributable to Tetra Tech, basic	58,650	51,903	180,179	149,856
Net income attributable to Tetra Tech, diluted	$ 58,650	$ 51,903	$ 180,179	$ 149,856
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 1.10	$ 0.96	$ 3.35	$ 2.77
Diluted (in dollars per share)	$ 1.09	$ 0.95	$ 3.32	$ 2.74
Weighted-average common shares outstanding:
Basic (in shares)	53,507	54,117	53,777	54,095
Diluted (in shares)	54,006	54,666	54,328	54,698
Subcontractor costs
Cost of revenue	$ (169,745)	$ (163,590)	$ (502,024)	$ (478,461)
Other costs of revenue
Cost of revenue	$ (575,902)	$ (512,347)	$ (1,679,937)	$ (1,488,549)