Week Number Calculator
Find the ISO 8601 week number for any date, and the date range of a week.
"We'll ship in week 32" is unambiguous only if everyone agrees on how weeks are numbered — and they often don't. The ISO 8601 standard, used across Europe, business planning and manufacturing, defines week 1 as the week containing the year's first Thursday, with weeks running Monday to Sunday. That definition occasionally puts early-January dates in the previous year's final week, or late-December dates in week 1 of the next year.
Enter any date and this tool gives its ISO week number and ISO week-year, plus the Monday–Sunday date range that week covers, so "week 32" always resolves to specific dates.
How to use Week Number Calculator
- 1
Pick a date
Any calendar date.
- 2
Read the ISO week number
Plus the ISO week-year, which can differ near year boundaries.
- 3
See the week's dates
The Monday–Sunday range that week spans.
Features
- ISO 8601 week numbering
- Handles year-boundary edge cases correctly
- Shows the week's Monday–Sunday date range
Frequently asked questions
Common mistakes to avoid
- Assuming week 1 always contains January 1 — under ISO rules it may not.
- Mixing ISO week numbers with US week numbering, which can differ by one.
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