How to use the Week Number Calculator
- Choose to find a week number or the dates of a week.
- Enter the date, or the year and week number.
- Read the result.
What does this tool do?
Under ISO 8601, used across Europe and in many businesses, weeks start on Monday and week 1 is the week containing the year's first Thursday. Years have 52 or 53 ISO weeks.
Because of that rule, the first days of January can belong to the last week of the previous year.
Why use it?
- Match week numbers used at work or school.
- Find the dates of a project week.
- Planning and payroll.
Example
January 1, 2027 is a Friday, so it belongs to ISO week 53 of 2026. Week 1 of 2027 starts on Monday, January 4.
Privacy
The calculation happens instantly in your browser. The numbers you enter are not sent to our servers or saved. There's no account to create and nothing to install.
Frequently asked questions
Why does January 1 sometimes show week 52 or 53?
ISO week 1 is the week containing the year's first Thursday. If January 1 falls on a Friday, Saturday or Sunday, those days belong to the last week of the previous year.
Do US calendars use the same week numbers?
Not always. Many US calendars start weeks on Sunday and count the week containing January 1 as week 1, which can differ from ISO week numbers by one. This calculator uses ISO 8601.
Last reviewed by the M2Toolkit team.