Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 29, 2024	Oct. 01, 2023	Oct. 02, 2022
Revenue	$ 5,198,679	$ 4,522,550	$ 3,504,048
Gross profit	866,440	725,029	575,559
Selling, general and administrative expenses	(356,024)	(305,107)	(234,784)
Acquisition and integration expenses	(7,138)	(33,169)	0
Right-of-use operating lease asset impairment	0	(16,385)	0
Contingent consideration – fair value adjustments	(2,541)	(12,255)	(329)
Income from operations	500,737	358,113	340,446
Interest income	7,288	5,898	1,780
Interest expense	(44,559)	(52,435)	(13,364)
Other non-operating income	0	89,402	19,904
Income before income tax expense	463,466	400,978	348,766
Income tax expense	(130,023)	(127,526)	(85,602)
Net income	333,443	273,452	263,164
Net income attributable to noncontrolling interests	(61)	(32)	(39)
Net income attributable to Tetra Tech	333,382	273,420	263,125
Net income attributable to Tetra Tech	$ 333,382	$ 273,420	$ 263,125
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 1.25	$ 1.03	$ 0.98
Diluted (in dollars per share)	$ 1.23	$ 1.02	$ 0.97
Weighted-average common shares outstanding:
Basic (in shares)	267,364	266,015	268,100
Diluted (in shares)	270,042	268,185	270,815
Subcontractor costs
Costs of revenue	$ (876,817)	$ (771,461)	$ (668,468)
Other costs of revenue
Costs of revenue	$ (3,455,422)	$ (3,026,060)	$ (2,260,021)