How to use it
- Type the Year you want, from 1900 to 2100.
- Choose the Week system: ISO 8601 is the international standard; the US system starts each week on Sunday.
- Add a Date to see the week it falls in and the year that week is numbered in.
- Read All weeks of the year for the first and last day of every week.
ISO 8601 week numbers
In ISO 8601 a week runs from Monday to Sunday. Week 1 is the week that holds the year's first Thursday, which is also the week that holds 4 January. The weeks of a year can begin in December of the year before and end in January of the year after.
A year has 53 weeks when 1 January is a Thursday, or a Wednesday in a leap year. Otherwise it has 52 weeks.
Weeks in an ISO year = 53 if 1 January is a Thursday, or a Wednesday in a leap year
Weeks in an ISO year = 52 otherwise
Week 1 starts on the Monday of the week that holds 4 January.US week numbers
In the US system a week runs from Sunday to Saturday. Week 1 is the week that contains 1 January, and the last week is the one that contains 31 December. A year therefore has 53 or 54 weeks.
A week that spans the turn of the year is counted in both years: it is the last week of one year and week 1 of the next, so the same days can carry two numbers.
What this calculator does not cover
- Fiscal, school or company week numbers, which often start on another day or use their own rules.
- Years before 1900 or after 2100.
Worked example: 2026 and the turn of the year
1 January 2026 was a Thursday, so 2026 has 53 ISO weeks. Week 1 runs from Monday 29 December 2025 to Sunday 4 January 2026, and week 53 runs from Monday 28 December 2026 to Sunday 3 January 2027.
1 January 2027 is a Friday. Its week is Monday 28 December 2026 to Sunday 3 January 2027, which holds Thursday 31 December 2026. So with 1 January 2027 in the Date box, the output reads Week 53 of 2026, but the date is in 2027.
| Week | First day | Last day |
|---|---|---|
| 1 | December 29, 2025 | January 4, 2026 |
| 53 | December 28, 2026 | January 3, 2027 |
Frequently asked questions
How do I find the week number of a date?
Enter the date and the year. The Week of the date output gives the week number in the chosen system. In ISO numbering that week can belong to the year before or after the calendar year of the date.
Why does a year sometimes have 53 weeks?
An ISO year has 53 weeks when 1 January is a Thursday, or a Wednesday in a leap year. Otherwise it has 52. For example, 2020 and 2026 have 53 weeks, while 2025 and 2027 have 52.
What is the difference between ISO and US week numbers?
ISO weeks start on Monday, and week 1 holds the first Thursday of the year. US weeks start on Sunday, and week 1 holds 1 January. The same date can have a different week number in each system.
Why is 1 January 2027 in week 53 of 2026?
1 January 2027 is a Friday. Its Monday-to-Sunday week, from 28 December 2026 to 3 January 2027, contains Thursday 31 December 2026, so the week is counted in 2026 and week 1 of 2027 starts on 4 January.
Does the calculator send my dates anywhere?
No. The weeks are calculated in your browser, and nothing you type is stored or sent to us.
Why does the "This week" result sometimes say it is not in this year?
It shows the current week only when that week is numbered in the year you chose. In early January or late December, ISO numbering can place the current week in the neighbouring year, and the message says so.
Sources
- Wikipedia: ISO week date (the ISO 8601 week rules)
- Wikipedia: Week (the US system that starts on Sunday)
Figures and text last checked on Tuesday, November 10, 2026.