Week Number Calculator
Find the ISO 8601 week number for any date, plus the day of the year and weekday.
Find the week number for any date
The Week Number Calculator tells you the ISO 8601 week number for any date, along with the day of the year and the weekday. Pick a date — it defaults to today — and the answers appear instantly. It is the quick way to answer "what week is it?" for planning, reporting, and scheduling.
What is an ISO week number?
The ISO 8601 standard numbers the weeks of the year from 1 to 52 or 53. Under this system, weeks start on Monday, and week 1 is defined as the week containing the year's first Thursday (equivalently, the week containing January 4). This avoids ambiguity at year boundaries and is the standard used in business, manufacturing, and much of Europe.
Why week numbers matter
- Project planning — teams schedule by "week 32" rather than specific dates.
- Manufacturing and logistics — production and delivery are often planned by week.
- Reporting — weekly metrics are grouped by ISO week for consistency.
- Payroll and rotas — shift patterns frequently reference week numbers.
How to use it
- Select a date (today is pre-filled).
- Read the ISO week number.
- Check the day of the year and weekday for extra context.
Week-number facts
| Detail | ISO 8601 rule |
|---|---|
| Week starts on | Monday |
| Week 1 is | The week with the year's first Thursday |
| Weeks per year | 52 or 53 |
Year boundaries
Because of the first-Thursday rule, the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year. The calculator reports the correct ISO week-year in these cases, so you never miscount around New Year.
Private and free
All calculation happens in your browser using your device's date. Nothing is uploaded, and the tool is completely free with no limits.
Frequently asked questions
How is the ISO week number defined?
Weeks start on Monday and week 1 is the week containing the year's first Thursday (the week with January 4).
Why might early January be week 52 or 53?
Under ISO rules, days before the first Thursday belong to the final week of the previous year.
How many weeks are in a year?
Most years have 52 ISO weeks; some have 53, depending on which day the year starts and whether it is a leap year.
Is it free and private?
Yes. It runs entirely in your browser with nothing uploaded, and it is completely free.