ISO Week Calculator

Calculate ISO week numbers

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ISO Week

W5

2026-W05-5

ISO Year

2026

ISO Weekday

5 (Friday)

Week 5 Range

Start (Monday):Jan 26, 2026End (Sunday):Feb 1, 2026

Days in This ISO Week

Mon

26

Tue

27

Wed

28

Thu

29

Fri

30

Sat

31

Sun

1

Find Date from ISO Week

Monday, December 29, 2025

2026 ISO Weeks Overview

Total weeks:53First week starts:Dec 29, 2025Last week ends:Jan 3, 2027Week type:Long year
ISO Week Rules

ISO 8601 week date standard:

  • Weeks start on Monday (day 1) and end on Sunday (day 7)
  • Week 1 is the week containing the first Thursday of the year
  • Equivalently: Week 1 contains January 4th
  • A year can have 52 or 53 weeks
  • Dec 31 might be in week 1 of next year
  • Jan 1 might be in week 52/53 of previous year

ISO Week format: YYYY-Www-D

Example: 2024-W15-3 = Wednesday of week 15, 2024

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