ISO Week Number Calculator
Find the ISO 8601 week number for any date, plus the Monday-to-Sunday range of that week and how many weeks are in its ISO year.
- ISO week number
- Week 31 of 2026
- Week runs
- 2026-07-27 to 2026-08-02
- Weeks in that ISO year
- 53
How it works
ISO 8601 defines week 1 of a year as the week (Monday to Sunday) containing that year's first Thursday — equivalently, the week containing January 4th. Every other week is numbered by counting whole weeks from there, which is why a date in late December or early January can fall in a week number belonging to the neighboring calendar year.
Most years have 52 ISO weeks, but a year has 53 when its January 1st falls on a Thursday, or on a Wednesday in a leap year — this happens roughly once every 5-6 years (2026, for example, is a 53-week ISO year).
FAQ
Why does January 1st sometimes belong to the previous year's week count?
Because ISO weeks always run Monday to Sunday, and week 1 is defined by containing the year's first Thursday rather than starting on January 1st itself. If January 1st is a Friday, Saturday, or Sunday, it falls in the last week of the previous ISO year instead of week 1 of the new one.
How is this different from just counting 7-day chunks from January 1st?
A naive count from January 1st would assign different week numbers to the same weekday across different years, since January 1st isn't always a Monday. The ISO 8601 standard anchors every week to a fixed Monday-Sunday grid instead, so, for example, week 10 always means the same relative point in the year's week grid regardless of what day January 1st happened to fall on.