Week Number
Today's ISO week number, the week of any date, and the dates of any week — by the ISO 8601 rules, 53-week years included.
How to use this tool
- Today's week number is shown immediately.
- Pick any date to get its week, or enter a week number and year to get that week's Monday–Sunday range.
How it works
This uses the ISO 8601 standard — the one European planning, manufacturing, and logistics run on: weeks start Monday, and week 1 is the week containing the year's first Thursday. That definition means January 1 can belong to the previous year's week 52/53, and some years have 53 weeks. US calendars sometimes use a different convention (week 1 = week containing January 1), so if a colleague's number differs by one, that's why.
Frequently asked questions
What week number is it today?
The big number at the top — computed by the ISO 8601 rules from your device's date.
Why does my calendar app show a different week number?
It's probably using the US convention (week 1 contains January 1, weeks start Sunday) instead of ISO 8601. Around New Year the two can disagree by one.
Which years have 53 weeks?
ISO years starting on a Thursday — or leap years starting on a Wednesday. It happens roughly every 5–6 years; the tool handles it automatically.