ISO week numbers
Many businesses, schools and governments number weeks of the year using ISO 8601, the international standard. Its rules:
- Weeks start on Monday.
- Week 1 is the week that contains the first Thursday of the year (the same as the week containing January 4).
- A year has 52 or 53 weeks.
Because of this, the first days of January can belong to the last week of the previous year, and the last days of December to week 1 of the next.
Examples
- October 17, 2026 is in week 42 (Monday October 12 to Sunday October 18). 2026 has 53 weeks because it starts on a Thursday.
- January 1, 2027 is still in 2026-W53.
- December 30, 2024 was already in 2025-W01.
US week numbers
Some US calendars count weeks from Sunday, with week 1 being the week containing January 1. Those numbers can be one higher than ISO for part of the year. If your company uses a fiscal calendar, its weeks may differ again.
How this works
The page finds the Thursday of the week containing your date (weeks run Monday to Sunday). The year of that Thursday is the ISO week-numbering year, and the week number is how many weeks that Thursday is into its year. The week's Monday and Sunday are shown, and the number of weeks in the year comes from the week containing December 28.
Frequently asked
What week number is it today?
Open the page: today's date is selected and its ISO week is shown at the top.
When does week 1 start?
On the Monday of the week that contains the year's first Thursday.
Which years have 53 weeks?
Years starting on a Thursday, and leap years starting on a Wednesday, such as 2026 and 2032.
Why does my US calendar show a different week?
It probably counts weeks from Sunday with January 1 in week 1, not the ISO rule.
Sources
Reviewed Oct 17, 2026 · test vectors fixtures/calculators/date/week-number-calculator/