Toolstia / Week number calculator
Week number calculator
Enter a date to find its ISO 8601 week number, or enter a year and a week number to see the dates that week runs from and to. Useful for planning, payroll, production schedules and anywhere weeks are referred to by number.
Week number of a date
Dates in a given week
Weeks follow the ISO 8601 standard used across Europe and much of the world: weeks start on Monday and week 1 is the week that contains the first Thursday of the year. A few days at the start or end of a year can belong to the neighbouring year’s week numbering. The United States and some other countries number weeks differently.
How it works
- Choose a date to see its week number.
- Or enter a year and a week to get the Monday and Sunday of that week.
- Check how many weeks the year has.
The ISO week rule
ISO 8601 weeks start on Monday and week 1 is the week that contains the first Thursday of the year, which is the same as the week containing 4 January.
52 or 53 weeks
Most years have 52 ISO weeks and some have 53, for example 2020 and 2026. The tool tells you which.
Year boundaries
A date such as 30 December 2024 belongs to week 1 of 2025, and 1 January 2021 belongs to week 53 of 2020. The ISO week-year can therefore differ from the calendar year at the start and end of the year.
Other numbering systems
The United States and some countries start weeks on Sunday and count the week containing 1 January as week 1, which can give a different number. This tool uses ISO 8601 only.
Questions
What week is it now?
Open the page: it starts with today’s date.
Why does week 53 sometimes exist?
Because a year has 52 weeks and one or two extra days, which add up to an extra week every five or six years.
Is anything uploaded?
No. The conversion happens in your browser and nothing is sent anywhere.