About this calculator
The calculator also shows the simpler count from 1 January and the week of the month.
How to use it
- Enter a date, or leave it as today.
- Read the week number and its date range.
The formula
ISO week = week containing the Thursday of the date’s Monday-to-Sunday week
- week 1
- the week with the first Thursday of the year
Worked example
3 October 2026
- ISO week 40 of 2026.
- The week runs from Monday 28 September to Sunday 4 October 2026.
What the result means
Some years have 53 ISO weeks.
Assumptions
- ISO 8601 week rules.
Limitations
- Some companies use their own week numbering.
Frequently asked questions
What week number is it today?
Leave the date as today to see it.
Can 1 January be in week 52 or 53?
Yes, if the year starts on Friday, Saturday or Sunday.
Do weeks start on Monday?
In the ISO system, yes.
Last reviewed on 3 October 2026. Found a mistake? Tell us.