Week Number Calculator
Which week of the year a date falls in, by the ISO 8601 rule that business and software use — and which dates any week number covers. Nothing is uploaded.
| Day | Date |
|---|
| Year | 1 January is in | 31 December is in | ISO weeks |
|---|
A week is never split between two years, so the first and last few days of a calendar year can belong to the week-year on either side of it.
How to use it
- Pick a date on the left to get its week number.
- Or type a year and a week number on the right to get the seven dates that week covers.
- The bottom table shows how the first and last days of nearby years fall, which is where the surprises are.
The ISO 8601 rule
Two sentences cover the whole thing. Weeks run Monday to Sunday. Week 1 is the week containing the first Thursday of January.
The Thursday is not arbitrary. Thursday is the middle day of a Monday-to-Sunday week, so the week containing the first Thursday is the first week with at least four of its seven days in the new year. The same rule is often stated as “the week containing 4 January”, which is easier to check and means exactly the same thing.
The consequence is that a week is never cut in half. Every week belongs wholly to one year, which is what makes week numbers usable for payroll, shift rotas, production planning and anything that has to add up over a year without double counting a day.
Why 1 January is sometimes in week 52 or 53
If 1 January falls on a Friday, Saturday or Sunday, its week has four or more days still in the old year, so the whole week counts as the old year's last week. 1 January 2027 is a Friday, and it sits in 2026-W53.
It happens at the other end too. 31 December 2024 was a Tuesday, and that week had most of its days in January, so it was 2025-W01. Neither case is a bug: it is the no-split-weeks rule doing what it is for. This is why a week number is written with its own year attached — 2026-W01 — and why that year is called the week-year rather than just the year.
52 weeks, or 53
Seven times fifty-two is 364, one day short of a normal year, so the extra day has to go somewhere. Most years have 52 ISO weeks; a year has 53 when 1 January is a Thursday, or when it is a leap year starting on a Wednesday. That works out at roughly 71 years in every 400.
A 53-week year is the one that breaks reports and spreadsheets built on the assumption that a year is 52 weeks. If a formula anywhere divides an annual figure by 52, it will be wrong in those years.
The other numbering
North American calendars usually start the week on Sunday and give week 1 to whichever week contains 1 January, even if only one day of it is in the new year. That is a different rule, and it gives a different answer for a great many dates — usually one higher than the ISO number. Both are shown above so you can see when they part company.
If you are sending a week number to somebody else, say which one you mean. “Week 14” on its own has caused more than one meeting to be booked seven days out.
Frequently asked questions
How is the ISO week number worked out?
Weeks run Monday to Sunday, and week 1 is the week that contains the first Thursday of January. The same rule can be stated as the week containing 4 January, or the first week with most of its days in the new year. Every week belongs to exactly one year and no week is ever split between two.
Why does 1 January sometimes fall in week 52 or 53?
Because a week is never split. If 1 January is a Friday, Saturday or Sunday, then that week has most of its days in the old year, so the whole week counts as the last week of the old year. 1 January 2027 is a Friday and sits in week 53 of 2026. The reverse happens too: 31 December 2024 was a Tuesday, in week 1 of 2025.
Why do two calendars give different week numbers for the same day?
There are two common conventions. ISO 8601 starts weeks on Monday and gives week 1 to the week holding the first Thursday. The North American convention starts weeks on Sunday and gives week 1 to whatever week holds 1 January, however few days of it there are. The two disagree by one for much of the year, so always say which one you mean.
Is anything I type sent anywhere?
No. The arithmetic happens in JavaScript inside your own browser. Nothing is uploaded and nothing is saved - close the tab and it is gone.