How to find the week number of a date
- The Date starts on today, so the current week number shows straight away.
- Pick any other date to get its week number instead.
- Read the ISO week, the Monday to Sunday it covers, the ISO week date and the US week number. To go the other way, choose Dates of a week and enter a week and year to see its dates and every week of that year.
How ISO 8601 week numbers work
ISO 8601 is the international standard for week numbers and the usual system in Europe. Its rules are short:
- Weeks run from Monday to Sunday.
- Week 1 is the week that contains the year’s first Thursday. Put another way, it’s the first week with at least four days in the new year, and 4 January is always in it.
- A year has 52 or 53 weeks, and the last days of December or first days of January can belong to the neighbouring year.
The ISO week date writes a day as year, week and weekday: 2026-W41-7 is the seventh day (Sunday) of week 41 of 2026, which is 11 October 2026. Watch the year in it. Monday 29 December 2025 is 2026-W01-1, because that week’s Thursday is 1 January 2026.
ISO and US week numbers side by side
| Date | ISO week | US week (Sunday start) |
|---|---|---|
| Monday 29 December 2025 | Week 1 of 2026 | Week 53 of 2025 |
| Thursday 1 January 2026 | Week 1 | Week 1 |
| Sunday 11 October 2026 | Week 41 | Week 42 |
| Friday 1 January 2027 | Week 53 of 2026 | Week 1 |
| Monday 4 January 2027 | Week 1 | Week 2 |
When someone says “week 12”, ask which system they mean. Planners, payroll systems and spreadsheet functions don’t all agree. In Excel, ISOWEEKNUM gives the ISO number while WEEKNUM uses Sunday-start weeks by default.