Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 28, 2021	Mar. 29, 2020	Mar. 28, 2021	Mar. 29, 2020
Revenue	$ 754,764	$ 734,133	$ 1,519,868	$ 1,531,756
Gross profit	112,484	97,000	228,794	206,737
Selling, general and administrative expenses	(51,907)	(51,041)	(101,928)	(97,476)
Contingent consideration – fair value adjustments	230	1,571	193	1,571
Income from operations	60,807	47,530	127,059	110,832
Interest expense	(2,823)	(3,501)	(5,849)	(6,849)
Income before income tax expense	57,984	44,029	121,210	103,983
Income tax expense	(12,456)	(7,616)	(23,234)	(20,253)
Net income	45,528	36,413	97,976	83,730
Net income attributable to noncontrolling interests	(11)	(16)	(23)	(23)
Net income attributable to Tetra Tech	$ 45,517	$ 36,397	$ 97,953	$ 83,707
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.84	$ 0.67	$ 1.81	$ 1.53
Diluted (in dollars per share)	$ 0.83	$ 0.66	$ 1.79	$ 1.51
Weighted-average common shares outstanding:
Basic (in shares)	54,187	54,699	54,085	54,541
Diluted (in shares)	54,736	55,463	54,715	55,380
Subcontractor costs
Cost of revenue	$ (154,939)	$ (149,673)	$ (314,873)	$ (333,274)
Other costs of revenue
Cost of revenue	$ (487,341)	$ (487,460)	$ (976,201)	$ (991,745)