Week number calculator
Find the ISO week number and week year.
Week number calculator
Your result appears here
Adjust the values to see your result automatically.
How to use Week number calculator
The Week number calculator takes a date and returns its ISO week number and ISO week year, written like 2026-W40, plus the ISO weekday. ISO weeks start on Monday and week 1 contains at least four days of the new year. A custom mode uses your interface locale first weekday and a minimum of 1 to 7 days in the first week.
- Adjust the values; an initial result is calculated from the defaults.
- Change the basic settings or open Advanced mode to fine-tune the result.
- Review and copy or download your result. Pause Live results to run manually. Sources or individual settings above 20,000 characters require an explicit run.
What this tool supports
Find the ISO week number and week year.
- Date
- Set this value for your task.
- Week convention
- ISO: Monday, minimum 4 days · Custom week numbering
- Minimum days in first week (custom)
- Range: 1 to 7
Limits and processing
Use finite numbers, valid dates and the ranges shown by each control. Invalid values produce an error without a downloadable result.
Frequently asked questions
What week number is it today?
Leave the date as today and keep the ISO convention. The result is shown in year-week form, such as 2026-W40, with the ISO week, ISO week year and weekday number where Monday is 1.
Why can early January belong to the previous year in ISO weeks?
ISO week 1 is the week with at least four days in the new year, starting on Monday. If January 1 falls on a Friday, Saturday or Sunday, those days belong to the last week of the previous ISO week year.
How do I count weeks starting on Sunday?
Choose Custom week numbering. It uses the first weekday of your interface locale, which may be Sunday, and the minimum days in the first week that you set from 1 to 7. The result shows both settings used.
Where is my input processed?
This operation processes its source in your browser. Copy and download are explicit actions; source input is not saved in an account or history.
What are the input limits?
Use finite numbers, valid dates and the ranges shown by each control. Invalid values produce an error without a downloadable result.