Week Number Calculator
Find the ISO week number for any date of the year.
Last updated August 26, 2026
3 / 2026
ISO week number
15
Day of the year
About this tool
The ISO 8601 week numbering system, widely used in Europe, business planning, and project management, defines Week 1 as the week containing the year's first Thursday — which means the ISO week number can occasionally differ from a naive “week since January 1st” count. This tool calculates a date's ISO week number, ISO year, and day-of-year in one step.
ISO 8601 week rules
| Rule | Detail |
|---|---|
| Week starts on | Monday |
| Week 1 defined as | The week containing the year's first Thursday |
| Weeks per year | 52 or 53, depending on the year |
Frequently asked questions
Why might a date in early January show as week 52 or 53 of the previous year?
Because ISO weeks belong to whichever year contains that week's Thursday, the first few days of January can fall in the last ISO week of the prior year if January 1st isn't a Thursday or later in the week.
Is this the same as the week number Excel or Google Sheets shows by default?
Not necessarily — spreadsheet functions like WEEKNUM default to a US-style Sunday-start week unless you explicitly choose the ISO (Monday-start) mode, so results can differ by one for dates near a week boundary.
Need the exact number of days or weeks between two dates?
Use the Date Difference Calculator to count total days, complete weeks, and optional Monday–Friday business days.
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