ISO Week Calculator
Calculate ISO 8601 week numbers and convert between dates and ISO week format.
Date to ISO Week
ISO Week to Date
Week 1, 2026
Dec 29, 2025 - Jan 4, 2026
About ISO Weeks
ISO 8601 defines weeks as starting on Monday. Week 1 is the week containing the first Thursday of the year. This means a year can have 52 or 53 weeks.
Tuesday, April 7, 2026
W15
2026-W15-2
ISO Week Details
ISO Year2026
ISO Week15
ISO Day of Week2 (Tue)
Week RangeApr 6 - Apr 12
Weeks in 202653
Day Names (ISO)
Mon
1
Tue
2
Wed
3
Thu
4
Fri
5
Sat
6
Sun
7