Week Number
Find the ISO week number for any date of the year.
CalculatorHow to use this tool
Find the ISO 8601 week number for any date, along with the Monday and Sunday that bound it and how many weeks the year contains. Payroll systems, production schedules, and European business calendars all run on these numbers.
- STEP 1Pick a date, or press Today.
- STEP 2Press Find Week Number.
- STEP 3Read the week number and the dates the week spans.
ISO weeks start on Monday, and week 1 is the week containing the year's first Thursday. That means early January can still belong to week 52 or 53 of the previous year.
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Examples
Frequently asked questions
Why does 1 January sometimes belong to last year's week 52?
Because an ISO week belongs to whichever year holds the majority of its days. If 1 January is a Friday, that week has more days in the old year, so it stays week 52 or 53.
Why do some years have 53 weeks?
A 365-day year is 52 weeks plus one day, so the surplus accumulates. Roughly every five to six years the calendar needs a 53rd week to absorb it.
Is this the same as a US week number?
No. US conventions often start the week on Sunday and treat the week containing 1 January as week 1, which can differ from the ISO answer by one.