Week Number Calculator
Enter a date to get its ISO 8601 week number — the standard used in European calendars, logistics and payroll.
How ISO week numbers work
ISO 8601 weeks start on Monday, and week 1 is the week containing the year's first Thursday (equivalently: the week containing January 4). That means January 1–3 can belong to week 52 or 53 of the previous ISO year, and December 29–31 can already be week 1 of the next.
Frequently asked questions
Why does my spreadsheet show a different week number?
Excel's default WEEKNUM starts week 1 on January 1 and weeks on Sunday — a US convention. Use ISOWEEKNUM for the ISO number this tool shows.
Can a year have 53 weeks?
Yes — years starting on a Thursday (and leap years starting on a Wednesday) have 53 ISO weeks. It happens about every 5–6 years; 2026 has 53.
Who uses week numbers?
Manufacturing and logistics ("delivery in week 34"), European business calendars, payroll systems and broadcast scheduling.
Last updated: 2026-07-13