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.
Monday, May 18, 2026
W21
2026-W21-1
ISO Week Details
ISO Year2026
ISO Week21
ISO Day of Week1 (Mon)
Week RangeMay 18 - May 24
Weeks in 202653
Day Names (ISO)
Mon
1
Tue
2
Wed
3
Thu
4
Fri
5
Sat
6
Sun
7
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