Week Number Calculator: ISO and US Week of the Year
Find the week number of any date in the ISO 8601 system and the US Sunday-start system, see the dates of that week, and list every week of a year with its start and end.
Today is in ISO week
Week 45
of 2026
- ISO week date
- 2026-W45-7
- ISO week 45 (Monday to Sunday)
- Mon 2 Nov 2026 to Sun 8 Nov 2026
- US week (Sunday start)
- Week 46 of 2026
- US week 46 (Sunday to Saturday)
- Sun 8 Nov 2026 to Sat 14 Nov 2026
- ISO weeks in 2026
- 53
- Weeks left after this one
- 8
Find the dates of a week number
Monday, 2 November 2026 to Sunday, 8 November 2026
All ISO weeks of 2026
About the Week Number Calculator
The Week Number Calculator tells you which week of the year any date falls in. It opens on today's date, so the big number at the top is the current week, and you can pick any other date to see its week straight away. It shows two numbering systems side by side, because they do not always agree: the international ISO 8601 system used across Europe, in most business software and in many calendars, and the US system used by many American calendars and spreadsheets.
For each date you get the ISO week number and its week-year, the ISO week date (for example 2026-W45-5, meaning the fifth day of week 45), the Monday and Sunday of that ISO week, the US week number with its Sunday and Saturday, how many ISO weeks the year has, and how many weeks are left after the current one. A second panel works the other way round: enter a year and a week number to get that week's Monday and Sunday. A third panel lists every ISO week of the year with its dates.
How to use it
Find the week of a date
- Leave the Date field on today's date, or pick any date from 1900 to 2199.
- Read the headline week number, then the details below it for both systems.
Find the dates of a week
Type an ISO year and a week number in the lookup panel. The tool shows the Monday that starts the week and the Sunday that ends it. If a year has only 52 weeks and you ask for week 53, it tells you the valid range instead.
See the whole year
Press Show all weeks for a table of every ISO week of the year in the lookup panel, with its Monday and Sunday. The table scrolls on its own, so the page stays easy to read on a phone.
How ISO week numbers are worked out
ISO weeks start on Monday. Week 1 is the week that contains the first Thursday of the year, which is the same as the week that contains 4 January. With the day of the year as the ordinal (1 January is 1) and the ISO weekday from Monday = 1 to Sunday = 7, the week number is
A result of 0 means the date belongs to the last week of the previous year, and a result of 53 in a 52-week year means it belongs to week 1 of the next year. A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. 2026 is such a year: it starts on a Thursday, so it has 53 weeks and 1 January 2027 still belongs to week 53 of 2026.
ISO weeks and US weeks
The US system starts weeks on Sunday and always counts the week that contains 1 January as week 1, even if that week has only one day in January. Because of this, the two systems can differ by one week, and the US system can reach week 53 or even 54 in a year with only 52 ISO weeks. When you share a week number with colleagues abroad, say which system you mean, or use the ISO week date, which leaves no doubt.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device. The date you pick is never stored.
Related tools
To count working days in a week or a project, use the Business Days Calculator. The Days Until Calculator counts down to a date, and the Date Calculator adds days, weeks and months to any date.
Frequently asked questions
What is an ISO week number?
It is the week numbering defined by the ISO 8601 standard. Weeks run from Monday to Sunday, and week 1 is the week that contains the first Thursday of the year, which is also the week that contains 4 January. Most of Europe and most business software use it.
Why does 1 January sometimes belong to week 52 or 53?
Because an ISO week belongs to the year that holds its Thursday. If 1 January falls on a Friday, Saturday or Sunday, that week's Thursday is in December, so the day counts as the last week of the previous year. For example, 1 January 2027 is in week 53 of 2026.
How many weeks are in a year?
An ISO year has 52 or 53 weeks. It has 53 when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. 2026 has 53 ISO weeks and 2027 has 52. The calculator shows the count for the year of the date you pick.
How is the US week number different?
US weeks start on Sunday, and week 1 is always the week that contains 1 January, however few January days it has. So the US number can differ from the ISO number by one, and can reach 53 or 54. The calculator shows both so you can match whichever calendar you use.
How do I find the dates of a week number?
Use the lookup panel. Enter the ISO year and the week number, and the tool shows the Monday that starts the week and the Sunday that ends it. If you ask for week 53 in a 52-week year, it tells you the valid range.
What does an ISO week date like 2026-W45-5 mean?
It is the year, the week and the day of the week in one code. 2026-W45-5 is the fifth day, Friday, of ISO week 45 of 2026. Day 1 is Monday and day 7 is Sunday. It avoids any doubt about which numbering system is meant.