ISO week number

Many businesses, especially in Europe and in manufacturing and logistics, plan by week number: delivery in week 44, sprint for W45. ISO 8601 defines exactly how weeks are numbered so that everyone gets the same answer. This tool finds the ISO week number for any date and shows which days belong to that week.

How to use it

1. Pick a date.
2. Read the ISO week number, the ISO week-year and the weekday number.

The ISO 8601 rules

• Weeks start on Monday.
• Week 1 is the week that contains the year's first Thursday, which is the same as the week containing 4 January.
• A year has 52 or 53 weeks.
• Dates are written as YYYY-Www-D, where D is 1 for Monday to 7 for Sunday.

Examples

• 27 October 2026 (Tuesday) is 2026-W44-2. Week 44 runs from Monday 26 October to Sunday 1 November.
• 1 January 2027 (Friday) is 2026-W53-5. It belongs to week 53 of 2026, because the week's Thursday falls in 2026.
• 29 December 2025 (Monday) is 2026-W01-1, because that week contains Thursday 1 January 2026.

Why 53 weeks?

A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday. 2026 starts on a Thursday, so it has 53 weeks.

Common bugs in code

• Using the calendar year instead of the ISO week-year around New Year. In date format strings, the ISO week-year is often a different token, for example GGGG in Moment.js or RRRR in date-fns, and %G with %V in strftime.
• US week numbering starts weeks on Sunday and counts the week containing 1 January as week 1, so it often differs by one from ISO.