Week Number Calculator
This calculator finds the ISO-8601 week number for any date — the numbering used on wall calendars, in project planning and in many countries’ official calendars. ISO weeks always run Monday to Sunday, and week 1 of each year is defined as the week containing 4 January.
How to use the week number calculator
- Enter a date, or leave it blank to use today.
- Read the week number and the year that week belongs to (its “week-year”, which can differ from the calendar year at the very start or end of the year).
- Check the Monday–Sunday date range shown for that week.
Formula
ISO-8601 defines week 1 of a year as whichever week contains 4 January, with weeks running Monday to Sunday. One consequence: the last few days of December can fall in week 1 of the next year, and the first few days of January can fall in the last week of the previous year — because weeks belong to whichever calendar year contains their Thursday.
Worked examples
A mid-year date
1 October 2026 falls in week 40 of 2026, running from Monday 28 September to Sunday 4 October 2026.
A year-boundary date
1 January 2027 falls in week 53 of 2026 (not week 1 of 2027) — that week runs from Monday 28 December 2026 to Sunday 3 January 2027, and its Thursday, 31 December, is still in 2026.
Why ISO weeks sometimes disagree with the calendar year
The rule in one sentence
A week belongs to whichever ISO year contains its Thursday. Since ISO weeks run Monday–Sunday, this means a week that starts in late December but has its Thursday in January belongs to the new year’s week 1 — and symmetrically, a week that starts in early January but has its Thursday still in December belongs to the previous year’s final week.
52 or 53 weeks?
Most years have exactly 52 ISO weeks. A year has 53 instead whenever 1 January falls on a Thursday, or — in a leap year — on a Wednesday or Thursday. This calculator reports the exact count for whichever year the date you enter belongs to, rather than assuming 52.
Where ISO week numbers are used
- European business and government calendars, invoicing and payroll cycles.
- Project management and sprint planning (“week 40”).
- Manufacturing and logistics date codes.
Note that the United States commonly uses a different, non-ISO week-numbering convention (weeks starting Sunday, week 1 containing 1 January); this tool reports the ISO-8601 standard specifically, which is the internationally defined one.
Practice problems
Try these yourself first, then check your answer. The answers come from the calculator above.
- What ISO week number does 15 March 2026 fall in?
Show answer
ISO week number: 11
Common mistakes to avoid
- Assuming 1 January is always in week 1 — it depends on which weekday 1 January falls on; sometimes it is still in the final week of the previous year.
- Mixing up ISO week numbering with the “week of the month” numbering some calendars use, which is a different and less standardized system.
- Expecting every year to have exactly 52 weeks — most have 52, but years whose 1 January or 31 December lands appropriately can have 53.
Frequently asked questions
What week number is it today?
Leave the date field blank and the calculator uses today’s date automatically.
Why does the week sometimes belong to a different year than the date?
ISO weeks belong to whichever year contains that week’s Thursday. A date right at the start of January or the end of December can therefore fall in a week numbered for the other year.
Do all years have 53 weeks?
No — most years have 52 ISO weeks; a year has 53 only when 1 January falls on a Thursday (or, in a leap year, a Wednesday or Thursday).
What day does an ISO week start on?
Monday. ISO weeks run Monday through Sunday, unlike calendars that start the week on Sunday.
Reviewed September 26, 2026 by the SolveCalcPro editorial team. Found a mistake? Tell us; see our editorial policy.