ISO Week Number Calculator
ISO 8601 week number for any date.
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Formula
ISO week = week of year where week 1 contains the year's first Thursday
ISO weeks start on Monday. Week 1 is the week with the first Thursday of January, which may place early January days in the previous year's week 52/53 and late December days in next year's week 1.
Worked example
30 August 2026 falls in ISO week 35 of 2026 — a Sunday near month-end, useful for sprint planning, payroll weeks, and European calendar systems that label weeks 1–52.
Where this can give the wrong answer
- Late December dates may belong to week 1 of the following ISO year — read both week number and ISO week-year rows.
- Early January can be week 52/53 of the prior ISO year even though the calendar year changed.
- US systems that start weeks on Sunday differ from ISO Monday-based weeks.
FAQ
- Around New Year's, the ISO week-year follows the Thursday rule, not January 1. 1 January 2025 might still be ISO week 52 of 2024 if it's before the first Thursday.