Week Calculator โ€” ISO Week Number Lookup

Find the ISO week number of any date, plus the start/end of that week and the day of the year.

WEEK NUMBER

Week 17 of 2026

WEEK START

Apr 19, 2026

WEEK END

Apr 25, 2026

DAY OF YEAR

#110

๐Ÿ’ก Understanding ISO Week Numbers

ISO 8601 defines week numbering: Week 1 is the week containing January 4th (or equivalently, the week containing the first Thursday of the year). Weeks run Monday through Sunday. A year has either 52 or 53 weeks.

Week numbers are used extensively in business planning, manufacturing schedules, project management, and financial reporting. 'Ship by Week 42' is more precise than 'ship in mid-October' and is standard in European and international business.

Enter any date to see its ISO week number, the start and end dates of that week, and the day-of-year number. Week numbers provide a universal reference for scheduling across organizations.

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