Week Number Calculator
Find the ISO week number for any date plus day of week, day of year and days remaining with optional year calendar view
Select a date to see its week number and other details
Find the ISO week number for any date, plus other calendar information.
What's a Week Number?
Week number identifies which week of the year a date falls in. ISO 8601 standard: weeks run Monday to Sunday, and Week 1 is the week containing the first Thursday of the year. So 1 January can be in Week 1, 52, or 53 of the previous or current year depending on which day it falls on. Most European calendars use ISO numbering; US calendars often use a different system (Week 1 starts 1 January regardless).
Used in: industrial scheduling (manufacturing weeks), academic calendars, project planning, financial reporting (52-week years for businesses), agricultural and supply chain planning. A typical year has 52 weeks; some years have 53 (when 1 Jan and 31 Dec fall in the same week). 2024 was a 53-week year; 2025 has 52.
Week Numbers in 2025
| Date | ISO Week |
|---|---|
| 1 Jan 2025 (Wed) | Week 1 |
| 6 Jan 2025 (Mon) | Week 2 |
| 31 Mar 2025 (Mon) | Week 14 |
| 30 Jun 2025 (Mon) | Week 27 |
| 29 Sep 2025 (Mon) | Week 40 |
| 29 Dec 2025 (Mon) | Week 1 (2026) |
| 31 Dec 2025 | Week 1 (2026) |
Frequently Asked Questions
Why doesn't 31 December always end in Week 52?
ISO 8601 ties week numbering to Mondays and Thursdays, not calendar boundaries. If 31 December falls on Monday-Wednesday, it's in Week 1 of the next year (because that week's Thursday is in next year). It's logical but can confuse people.
Do US calendars use ISO numbering?
Less commonly. US business calendars often use 'Week 1 starts 1 January' (so 1 Jan = Week 1 always). ISO is the global standard but adoption varies. Always confirm which system a date is using.
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