Week Number Calculator
Find the ISO week number for any date. Enter a date and get its week number instantly, along with the year it belongs to.
How it works
- 1
Enter a date
Any date, past, present or future.
- 2
Get the week number
The ISO 8601 week number, from 1 to 52 or 53.
- 3
Check the week year
Early January dates can belong to the previous year’s last week.
Related tools
Other tools people use right after this one.
Built to get out of your way
487
Tools available
6
Languages
0
Watermarks added
2h
Until your files are erased
100%
Free to start
∞
Devices supported
Frequently asked questions
Because ISO weeks start on Monday and week 1 is defined by the first Thursday of the year, early January days can fall in the last week of the prior year.
How ISO week numbers work
ISO 8601 defines week 1 as the week containing the year’s first Thursday, and weeks run Monday to Sunday. This means the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year.
Week numbers are widely used in business planning, manufacturing schedules, payroll and project management to track progress consistently across the year.