ISO week number calculator
Companies and supply chains use ISO week numbers in plans and orders; this tool gives you the week number and its exact bounds.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter date.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
In ISO 8601 weeks start on Monday and week 1 holds the first Thursday of January. Week = floor((day of year - weekday number + 10) ÷ 7), with Monday = 1 and Sunday = 7.
Example
Inputs: Date = 2026-10-01. Result: Week number: 40; Monday (week start): Monday, September 28, 2026 (2026-09-28); Sunday (week end): Sunday, October 4, 2026 (2026-10-04).
Limits and caveats
- It follows ISO 8601 only; some countries and apps start weeks on Sunday or Saturday and number them differently.
Questions about this tool
Why can 1 January be in week 52 or 53?
If 1 January is a Friday, Saturday or Sunday, ISO puts it in the last week of the previous year.
When does a year have 53 weeks?
When it starts on a Thursday, or on a Wednesday in a leap year, like 2026 and 2020.
What does 2026-W40-4 mean?
ISO year 2026, week 40, day 4, which is Thursday.