How does ISO week numbering work?

Under ISO-8601, weeks start on Monday, and week 1 is the week that contains the year's first Thursday. That means the first few days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year. The calculator shows the correct ISO week-numbering year alongside the week.

Why is 1 January sometimes in week 52 or 53?

Because ISO week 1 is anchored to the first Thursday, not to 1 January. If 1 January is a Friday, Saturday or Sunday, it falls in the final week of the previous year. For example, 1 January 2021 is a Friday and belongs to week 53 of 2020.

Can a year have 53 weeks?

Yes. Most years have 52 ISO weeks, but a year has 53 when it starts on a Thursday, or on a Wednesday in a leap year. The calculator returns up to 53 and is correct for these long years.

Does this work offline and is anything sent to a server?

The calculation runs entirely in your browser, so nothing you type is sent anywhere, and once the page has loaded it keeps working with no connection. There is no sign-up and no limit on how many dates you check.

Week Number Calculator

Find the ISO-8601 week number for any date. Enter a date and the calculator shows its ISO week number, the ISO week-numbering year, and the Monday and Sunday that begin and end that week. Useful for planning, payroll and scheduling. Free and instant in your browser.

Leap-year accurate
100% free
No sign-up, no app
Instant as you type
Works offline after first load

Read the guide: What Is an ISO Week Number

Date

ISO week number

Week 25

ISO week-numbering year 2026. Weeks start on Monday.

Week number

ISO year

2026

Week of year

25 of 53

Week starts (Mon)

2026-06-15

Week ends (Sun)

2026-06-21

Your date

2026-06-15

Monday, 15 June 2026 falls in ISO week 25 of 2026, which runs from Monday, 15 June 2026 to Sunday, 21 June 2026.

How to use it

1
Pick a date

Choose any past or future date.
2
Read the week number

See the ISO week number from 1 to 53 and the year it belongs to under the ISO rules.
3
See the week range

Check the Monday that starts the week and the Sunday that ends it.

When it comes in handy

Planning and scheduling

Refer to a week by its number, the way many calendars, planners and project tools do.

Payroll and reporting

Group dates into ISO weeks for timesheets, rotas and weekly reports.

Coordinating across teams

Use a shared week number so "week 32" means the same thing to everyone.

Instant, accurate & 100% in your browser

The calculation runs right here in your browser, counting the real calendar so leap years and month lengths come out right. Nothing you type is sent to a server, there is no sign-up and no limit, and once the page has loaded it keeps working even with no connection.

Frequently asked questions

How does ISO week numbering work?: Under ISO-8601, weeks start on Monday, and week 1 is the week that contains the year's first Thursday. That means the first few days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year. The calculator shows the correct ISO week-numbering year alongside the week.
Why is 1 January sometimes in week 52 or 53?: Because ISO week 1 is anchored to the first Thursday, not to 1 January. If 1 January is a Friday, Saturday or Sunday, it falls in the final week of the previous year. For example, 1 January 2021 is a Friday and belongs to week 53 of 2020.
Can a year have 53 weeks?: Yes. Most years have 52 ISO weeks, but a year has 53 when it starts on a Thursday, or on a Wednesday in a leap year. The calculator returns up to 53 and is correct for these long years.
Does this work offline and is anything sent to a server?: The calculation runs entirely in your browser, so nothing you type is sent anywhere, and once the page has loaded it keeps working with no connection. There is no sign-up and no limit on how many dates you check.