# Week number calculator

Find the ISO week number of any date

Use it at https://www.quicktoolz.com/tools/week-number-calculator. From QuickToolz.

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.

## How it works

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.

## Sources

- [ISO 8601 date and time format](https://www.iso.org/iso-8601-date-and-time-format.html)