ISO week number calculator
Week numbers run production plans, shipping schedules and reports; this gives the standard week number of any date.
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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
Under ISO 8601 weeks start on Monday and week 1 is the week with the year's first Thursday (the one containing 4 January). Week = ⌊(Thursday of the week − 1 January) ÷ 7⌋ + 1.
Example
Inputs: Date = 2026-10-01. Result: Week number: 40; Week-year: 2,026; Week starts (Monday): Monday, September 28, 2026 (2026-09-28).
Limits and caveats
- Follows ISO 8601; week systems that start on Sunday or on 1 January can give a different number.
Questions about this tool
Why is 1 January sometimes in week 52 or 53?
If 1 January falls Friday to Sunday it belongs to the last week of the previous year, since week 1 must hold the first Thursday.
When does a year have 53 weeks?
When it starts on a Thursday, or on a Wednesday in a leap year.
Does this match Excel ISOWEEKNUM?
Yes, both follow the same standard.