Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Dec. 27, 2020	Dec. 29, 2019
Revenue	$ 765,104	$ 797,623
Gross profit	116,310	109,737
Selling, general and administrative expenses	(50,058)	(46,435)
Income from operations	66,252	63,302
Interest expense	(3,026)	(3,349)
Income before income tax expense	63,226	59,953
Income tax expense	(10,778)	(12,636)
Net income	52,448	47,317
Net income attributable to noncontrolling interests	(12)	(7)
Net income attributable to Tetra Tech	$ 52,436	$ 47,310
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.97	$ 0.87
Diluted (in dollars per share)	$ 0.96	$ 0.85
Weighted-average common shares outstanding:
Basic (in shares)	53,927	54,560
Diluted (in shares)	54,637	55,438
Subcontractor costs
Cost of revenue	$ (159,933)	$ (183,600)
Other costs of revenue
Cost of revenue	$ (488,861)	$ (504,286)