Week Number
Find the week number for any date — with the full Mon–Sun date range and a week-by-week year heatmap.
How week numbers work
Weeks run Monday to Sunday. Week 1 is the week containing the year's first Thursday — equivalently, the week that includes January 4. This means week 1 always has at least four days in the new year.
A quirk of this rule is that the week-year can differ from the calendar year for a few days around New Year's. December 29–31 may belong to week 1 of the next year, and January 1–3 may belong to week 52 or 53 of the previous year. The widget handles this automatically. Already have a specific week like 2026-W26 and need the date range? Use the ISO Week Converter.
Frequently asked questions
What is the current week number?
The widget above shows this week's number, calculated from your device's clock. The Mon–Sun date range of the current week is shown directly below it.
How are week numbers counted?
Weeks run Monday to Sunday. Week 1 is the week that contains the year's first Thursday — this always includes January 4. Most years have 52 weeks; some have 53.
Why does week 1 sometimes start in late December?
If January 1 falls on a Friday, Saturday, or Sunday, the year's first Thursday is in the following week — so those early January days belong to week 52 or 53 of the previous year. For example, January 1, 2021 was a Friday, so it fell in week 53 of 2020.
How many weeks are in a year?
Most years have 52 weeks. A year gets a 53rd week when January 1 is a Thursday, or when it is a leap year starting on a Wednesday.
How do I find the week number for any date?
Use the date picker at the top. The week number, Mon–Sun range, and the full-year heatmap all update instantly.