ISO Week Number Calculator
What week number is it? This calculator gives the ISO 8601 week for any date you pick, and shows the current week up top. ISO weeks start on Monday and belong to the year that contains their Thursday — a rule that occasionally puts early-January dates in week 52 or 53 of the previous year. The tool handles that correctly.
| ISO week number | |
| ISO week-year | |
| Week starts (Mon) | |
| Week ends (Sun) | |
| Day of year | |
| Weeks in this year |
How to use the ISO Week Number Calculator
The card at the top always shows the current ISO week so you can answer "what week is it?" at a glance. To check a specific date, use the date picker — every field below updates instantly. The ISO week number is the headline value; the ISO week-year matters at year boundaries, because a date in early January can legitimately belong to week 52 or 53 of the previous year.
The week starts and week ends rows give you the Monday and Sunday that bracket the date's week, which is handy for filtering reports or naming sprints. Weeks in this year tells you whether the ISO year has 52 or 53 weeks — a detail that trips up anyone hard-coding 52.
All calculation is done locally using the date you select, with no timezone surprises: the picker gives a calendar date and the math is performed in UTC to keep week boundaries stable.
How ISO 8601 week numbers work
The ISO 8601 standard defines weeks precisely so that systems in different countries agree on what "week 23" means. Three rules capture it: weeks start on Monday; week 1 is the week containing the year's first Thursday; and equivalently, week 1 is the week containing 4 January. Because the year is assigned by the week's Thursday, the first few days of January sometimes fall in the last week of the previous year, and the last few days of December sometimes fall in week 1 of the next year.
This is why the "ISO week-year" can differ from the calendar year. For example, 1 January 2023 was a Sunday, so it belonged to week 52 of ISO year 2022; the ISO week-year only rolled over to 2023 on Monday 2 January. Conversely, 31 December 2025 falls in week 1 of ISO 2026. Treating the calendar year and the ISO week-year as interchangeable is a classic off-by-one bug in reporting dashboards.
Most ISO years have 52 weeks, but roughly every five to six years a "long" year has 53 — these are the years that either start on a Thursday or are leap years starting on a Wednesday. If you generate weekly buckets or sprint numbers, assuming 52 will eventually drop or duplicate a week, so it is worth reading the "weeks in this year" value rather than hard-coding it.
Common use cases
- Sprint and cycle naming. Teams often label sprints by ISO week (for example "2026-W14"), so you need the canonical number, not a homemade count.
- Weekly reporting. Bucket events into ISO weeks to compare like-for-like periods across years.
- Cross-team scheduling. European calendars show week numbers by default; this converts a date your colleagues mention into one you can find.
- Validating code. Check a date library's week output against a known-correct reference, especially near year boundaries.