Week Number Calculator
Find the ISO week number for any date.
Quick answer: enter your details below and click Find Week Number — your result appears instantly, right here in your browser. Nothing you enter is uploaded or stored.
W
500 New Tools
Enter your details below.
How to use
- Enter a date as YYYY-MM-DD.
- Click Find Week Number.
- Read the ISO week number and its ISO year.
About this tool
Enter a date and the tool returns its ISO week number, where weeks start on Monday and week 1 is the week containing the first Thursday of the year. That rule is why 1 January can belong to the previous year's last week, and 31 December can belong to the next year's first week.
Frequently asked questions
Do ISO weeks start on Monday or Sunday?
Monday, always. A Sunday belongs to the week that started the previous Monday.
How many ISO weeks can a year have?
52 or 53, depending on where the Thursdays fall.