Consolidated Statements of Income - USD ($) shares in Thousands	12 Months Ended
	Sep. 27, 2020	Sep. 29, 2019	Sep. 30, 2018
Revenue	$ 2,994,891,000	$ 3,107,348,000	$ 2,964,148,000
Gross profit	446,535,000	408,183,000	384,458,000
Selling, general and administrative expenses	(204,615,000)	(200,230,000)	(190,120,000)
Acquisition and integration expenses	0	(10,351,000)	0
Contingent consideration – fair value adjustments	14,971,000	(1,085,000)	(4,252,000)
Impairment of goodwill	(15,800,000)	(7,755,000)	0
Income from operations	241,091,000	188,762,000	190,086,000
Interest income	1,375,000	1,732,000	1,824,000
Interest expense	(14,475,000)	(15,358,000)	(17,348,000)
Income before income tax expense	227,991,000	175,136,000	174,562,000
Income tax expense	(54,101,000)	(16,375,000)	(37,605,000)
Net income	173,890,000	158,761,000	136,957,000
Net income attributable to noncontrolling interests	(31,000)	(93,000)	(74,000)
Net income attributable to Tetra Tech	$ 173,859,000	$ 158,668,000	$ 136,883,000
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 3.21	$ 2.89	$ 2.46
Diluted (in dollars per share)	$ 3.16	$ 2.84	$ 2.42
Weighted-average common shares outstanding:
Basic (in shares)	54,235	54,986	55,670
Diluted (in shares)	55,022	55,936	56,598
Subcontractor costs
Costs of revenue	$ (646,319,000)	$ (717,711,000)	$ (763,414,000)
Other costs of revenue
Costs of revenue	$ (1,902,037,000)	$ (1,981,454,000)	$ (1,816,276,000)