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Earnings per Share (Tables) | 12 Months Ended
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Sep. 28, 2025
Earnings Per Share [Abstract] |
Summary of number of weighted-average shares used to compute basic and diluted EPS | The following table presents the number of weighted-average shares used to compute basic and diluted EPS (in thousands, except per share data):
| Fiscal Year Ended
| September 28, 2025 |  | September 29, 2024 |  | October 1, 2023
Net income attributable to Tetra Tech | $ | 247,724 |  |  | $ | 333,382 |  |  | $ | 273,420 |
Weighted-average common shares outstanding – basic | 264,713 |  |  | 267,364 |  |  | 266,015 |
Effect of diluted stock options and unvested restricted stock | 1,942 |  |  | 2,125 |  |  | 2,170 |
Shares issuable assuming conversion of convertible notes | 468 |  |  | 553 |  |  | — |
Weighted-average common stock outstanding – diluted | 267,123 |  |  | 270,042 |  |  | 268,185 |
Earnings per share attributable to Tetra Tech: |  |  |  |  |
Basic | $ | 0.94 |  |  | $ | 1.25 |  |  | $ | 1.03 |
Diluted | $ | 0.93 |  |  | $ | 1.23 |  |  | $ | 1.02 |

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- Definition Tabular disclosure of an entity's basic and diluted earnings per share calculations, including a reconciliation of numerators and denominators of the basic and diluted per-share computations for income from continuing operations. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 260 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482662/260-10-50-1 + Details
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