Week Number Calculator
Find the ISO week number for any date. See week start (Monday) and end (Sunday), day of year, and weeks left in the year.
Select a date
ISO Week Number
W20
2026 · 53 weeks total
Monday, 11 May 2026 – 17 May
Year progress — Week 20 of 53
33 weeks left
Week Number Calculator
The Week Number Calculator finds the ISO 8601 week number for any date. It also shows the week's Monday–Sunday span, day of year, quarter, and year progress.
ISO 8601 week number rules
- Weeks run Monday to Sunday
- Week 1 is the week containing the first Thursday of the year
- Some years have 53 weeks (years that start or end on Thursday)
- December 29–31 can belong to week 1 of the following year