Week number calculator: find the ISO week number for any date, plus its week start and end dates. Free, no signup, runs locally in your web browser now.
Uses ISO 8601 week numbering: weeks start on Monday, and week 1 contains the first Thursday of the year.
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Calculation details
Every figure above is calculated locally in your browser from the assumptions shown. No inputs are sent anywhere. See the methodology section below for how this tool works.
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Worked example: March 4, 2026
March 4, 2026 falls on a Wednesday. The ISO rule assigns a date to the week of its Thursday, so the anchor day is Thursday, March 5. To number the week, the algorithm finds week 1 of 2026: January 1, 2026 is a Thursday, so week 1 runs Monday, December 29, 2025 through Sunday, January 4, 2026.
Count Mondays from the week-1 Monday: December 29, then January 5, 12, 19, 26, February 2, 9, 16, 23, and March 2. That is 9 full weeks after week 1, so the week starting March 2 is week 10. The calculator returns ISO week 10 of 2026, running March 2 through March 8, Monday to Sunday.
Internally the code normalizes every date to 12:00 UTC before doing arithmetic. That is why March 4 works even though the tool never asks for a timezone: UTC noon cannot be shifted into a different calendar day by any real-world offset, so the ISO week is stable no matter where the date is picked.
Where ISO weeks surprise people
The ISO year is not the calendar year. Enter December 29, 2025 and you get week 1 of 2026, because that Monday opens the year whose first Thursday falls on January 1, 2026. Reverse case: January 1, 2021 was a Friday, which belongs to week 53 of 2020, so a January date can report the previous year.
Most years have 52 ISO weeks, but long years have 53: years starting on a Thursday, or leap years starting on a Wednesday. 2026 starts on a Thursday, so it has 53 weeks, and December 28, 2026 is Monday of week 53. 2027 starts on a Friday, so it goes back to 52.
Two practical mismatches: US business calendars usually start weeks on Sunday and number weeks differently, so this tool can be one week off from an American payroll calendar. And a leap day shifts weekday alignments: 2024 was a leap year starting on a Monday, which is why it contained 52 weeks while 2026 contains 53 despite both spanning the same 366 or 365-day spans people expect.
