Online Tool Store
Week Number Lookup
Find the ISO 8601 week number and week year for any date, along with that week's Monday-to-Sunday date range. Runs entirely in your browser.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Example shown - replace with your own date.
ISO week number
ISO week year
Week range (Mon-Sun)
How the week number lookup works
- Pick any date.
- The tool finds the Thursday of that date's week, since ISO weeks are defined by which year contains that Thursday.
- It counts how many weeks from January 1's week that Thursday falls in, giving the ISO week number and week year.
FAQ
What is an ISO week number?
ISO 8601 defines week 1 of a year as the week containing that year's first Thursday, with weeks running Monday to Sunday - the standard used across most of Europe and in business/project planning.
Why can a date near January 1 belong to the previous or next ISO year?
Since ISO weeks are anchored to the first Thursday, the last few days of December can fall in week 1 of the following year, and the first few days of January can fall in the last week of the previous year.
Is this the same as the week number Excel shows?
Not always. Excel's default WEEKNUM function uses a Sunday-start week that doesn't follow ISO 8601 - use Excel's ISOWEEKNUM function for a match with this tool.
Is my date sent anywhere?
No. The week number is calculated entirely in your browser - nothing is uploaded.
How we compare
| Feature | Online Tool Store | Spreadsheet WEEKNUM formula | Calendar app week labels |
|---|---|---|---|
| True ISO 8601 week numbering | ✓ | Needs the right formula variant | Varies |
| Shows the full Monday-Sunday range | ✓ | ✗ | Varies |
| No spreadsheet needed | ✓ | ✗ | ✓ |
For a quick, correctly-computed ISO week number without a spreadsheet, Week Number Lookup avoids the formula pitfalls.