Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Apr. 03, 2022	Mar. 28, 2021	Apr. 03, 2022	Mar. 28, 2021
Revenue	$ 852,744	$ 754,764	$ 1,711,255	$ 1,519,868
Gross profit	135,173	112,484	274,940	228,794
Selling, general and administrative expenses	(60,653)	(51,677)	(113,200)	(101,735)
Income from operations	74,520	60,807	161,740	127,059
Interest expense, net	(3,144)	(2,823)	(6,048)	(5,849)
Income before income tax expense	71,376	57,984	155,692	121,210
Income tax expense	(18,327)	(12,456)	(34,144)	(23,234)
Net income	53,049	45,528	121,548	97,976
Net income attributable to noncontrolling interests	(9)	(11)	(19)	(23)
Net income attributable to Tetra Tech	$ 53,040	$ 45,517	$ 121,529	$ 97,953
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.99	$ 0.84	$ 2.25	$ 1.81
Diluted (in dollars per share)	$ 0.98	$ 0.83	$ 2.23	$ 1.79
Weighted-average common shares outstanding:
Basic (in shares)	53,834	54,187	53,916	54,085
Diluted (in shares)	54,346	54,736	54,492	54,715
Subcontractor costs
Cost of revenue	$ (153,103)	$ (154,939)	$ (332,280)	$ (314,873)
Other costs of revenue
Cost of revenue	$ (564,468)	$ (487,341)	$ (1,104,035)	$ (976,201)