Week number calculator

Find the ISO week number of any date

Runs in your browser. Nothing is uploaded.

How it works

ISO week numbers run from Monday to Sunday, and week 1 is the week with the year’s first Thursday. Most years have 52 weeks, and some have 53. Early January can belong to the last week of the year before, so 1 January 2021 is in week 53 of 2020.

Week 1 is the week with the year’s first Thursday.

ISO 8601 is the international standard for writing dates. It defines weeks so each one belongs to exactly one year.

Worked example. 1 January 2021 was a Friday, so it belongs to the last week of the year before: 2020-W53.

Examples

Date	ISO week
2026-01-01	2026-W01
2026-06-15	2026-W25
2026-12-31	2026-W53
2021-01-01	2020-W53

What this tool doesn’t cover

This uses ISO weeks. Some calendars count weeks from Sunday, or from 1 January.

Questions

Why is 1 January sometimes in week 52 or 53?

A week belongs to the year that has its Thursday, so early January can fall in the year before.

Which years have 53 weeks?

Years that start on a Thursday, and leap years that start on a Wednesday. 2026 is one.

What does 2026-W03 mean?

It’s the standard way to write week 3 of 2026.

What week of the year is it?

Enter a date, or use today, to see its ISO week number. Weeks start on Monday, and week 1 holds the year’s first Thursday.

Do all countries number weeks the same way?

No. This tool uses ISO 8601, common in Europe and Asia. The US often starts weeks on Sunday and counts from 1 January.

Was this useful?

Built by QuickToolz.

Sources: ISO 8601 date and time format (opens in a new tab)