ISO week numbers and US week numbers are not the same
Ask two people what week it is and you can get two different answers, because two conventions are in common use and they disagree for part of most years.
| ISO 8601 | US / broadcast | |
|---|---|---|
| Week starts | Monday | Sunday |
| Week 1 is | the week holding the first Thursday | the week holding 1 January |
| Weeks in a year | 52 or 53, never a part week | up to 54, with part weeks at both ends |
| Used by | Europe, most of Asia, business and logistics software | US retail and broadcast scheduling |
The calculator shows both side by side for whatever date you enter, and says plainly when they disagree. If someone in a project plan says "week 32" without saying which convention, that is worth a message rather than a guess.
The first-Thursday rule
ISO 8601 never splits a week across two years. Every week belongs entirely to the year that holds its Thursday, which is the same as saying week 1 is the week containing the first Thursday of January, and also the same as saying it is the week containing 4 January.
That is why 1 January is not always in week 1. When it falls on a Friday, Saturday or Sunday, its week has its Thursday in the old year, so those days carry the previous year's last week number. 1 January 2027 is a Friday, so it lands in ISO week 2026-W53. Going the other way, 30 December 2024 was a Monday whose Thursday fell in January, so it belongs to 2025-W01.
This is the single most common source of off-by-one week bugs. A date can carry an ISO week-year different from its calendar year, and the two must be kept together: 2026-W15 is unambiguous, while "week 15" on its own is not.
Why some years have 53 weeks
A year is 365 days, which is 52 weeks and one day over, or two days over in a leap year. Those spare days accumulate, and every five or six years they add up to a whole extra week. A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday.
Recent and upcoming long years: 2015, 2020, 2026, 2032, 2037. If your reporting compares week 53 against the same week last year, there usually is no same week last year, and that is a reconciliation problem worth knowing about in advance. The calculator reports the week count for whichever ISO year your date falls in.
What the date field accepts
The field takes ISO 8601 (2026-04-08), US order (04/08/2026), European dotted order (08.04.2026), long form (8 April 2026) and a Unix timestamp in seconds or milliseconds. Everything is computed in UTC, so a timestamp near midnight resolves to its UTC date rather than to your local one.
The whole calculation runs in your browser. No date you enter is sent anywhere, and the page keeps working offline once loaded.
Common questions
- What week number is it this week?
- The calculator at the top of this page opens on today and shows the current week under both conventions: the ISO 8601 week number used across Europe and most business software, and the US week number that counts from the week containing 1 January. They agree for most of the year and differ around the turn of it.
- How many weeks are in a year?
- 52 in most years and 53 in a few. An ISO year has 53 weeks when it begins on a Thursday, or when it is a leap year beginning on a Wednesday. 2015, 2020 and 2026 are 53-week years; the next after that is 2032.
- Is 1 January always in week 1?
- Not under ISO 8601. If 1 January falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous year, because its week has its Thursday in that year. Under the US convention 1 January is in week 1 by definition, which is exactly where the two systems part company.
- How do I calculate the ISO week number myself?
- Take the Thursday of the week your date falls in, by moving to Monday and then adding three days. The year of that Thursday is the ISO week-year. Count the days from 1 January of that year to the Thursday, divide by 7 and add 1, and you have the week number.
- What does 2026-W15 mean?
- It is the ISO 8601 notation for a whole week: week 15 of the ISO week-year 2026, running Monday 6 April to Sunday 12 April 2026. Adding a day digit, as in 2026-W15-3, names Wednesday of that week, since ISO numbers days 1 for Monday through 7 for Sunday.
- Which countries use week numbers?
- They are part of everyday planning across Northern and Central Europe, particularly in Germany (Kalenderwoche), the Nordic countries and the Netherlands, where a meeting in "week 32" needs no explanation. In the US they turn up mostly in retail, broadcast and payroll calendars rather than in general conversation.