Consolidated Statements of Income - USD ($) shares in Thousands	12 Months Ended
	Oct. 03, 2021	Sep. 27, 2020	Sep. 29, 2019
Revenue	$ 3,213,513,000	$ 2,994,891,000	$ 3,107,348,000
Gross profit	498,400,000	446,535,000	408,183,000
Selling, general and administrative expenses	(222,972,000)	(204,615,000)	(200,230,000)
Acquisition and integration expenses	0	0	(10,351,000)
Contingent consideration – fair value adjustments	3,273,000	14,971,000	(1,085,000)
Impairment of goodwill	0	(15,800,000)	(7,755,000)
Income from operations	278,701,000	241,091,000	188,762,000
Interest income	917,000	1,375,000	1,732,000
Interest expense	(12,748,000)	(14,475,000)	(15,358,000)
Income before income tax expense	266,870,000	227,991,000	175,136,000
Income tax expense	(34,039,000)	(54,101,000)	(16,375,000)
Net income	232,831,000	173,890,000	158,761,000
Net income attributable to noncontrolling interests	(21,000)	(31,000)	(93,000)
Net income attributable to Tetra Tech	$ 232,810,000	$ 173,859,000	$ 158,668,000
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 4.31	$ 3.21	$ 2.89
Diluted (in dollars per share)	$ 4.26	$ 3.16	$ 2.84
Weighted-average common shares outstanding:
Basic (in shares)	54,078	54,235	54,986
Diluted (in shares)	54,675	55,022	55,936
Subcontractor costs
Costs of revenue	$ (661,341,000)	$ (646,319,000)	$ (717,711,000)
Other costs of revenue
Costs of revenue	$ (2,053,772,000)	$ (1,902,037,000)	$ (1,981,454,000)