Calculators
What Week Is It? Week Number Calculator
See today's week number at a glance, find the week number of any date, and list every week of the year with its dates. Switch between ISO 8601 weeks (Monday start, used in Europe and much of the world) and US weeks (Sunday start).
This week is…
ISO weeks start on Monday, and week 1 is the week with the year’s first Thursday. US weeks start on Sunday, and week 1 is the week with 1 January.
How to use the Week Number Calculator
- Read this week's number at the top.
- Pick any date to see its week number and the dates of that week.
- Switch between ISO and US numbering, or show all weeks of the year.
Frequently asked questions
- What week number is it today?
- It's shown at the top of the page, worked out from your device's date. ISO 8601 numbering is used unless you switch to US numbering.
- What is an ISO week number?
- In ISO 8601, weeks start on Monday and week 1 is the week that contains the year's first Thursday. Most of Europe and many businesses use it.
- How are US week numbers different?
- US weeks start on Sunday, and week 1 is the week that contains 1 January. Around New Year the two systems can give different numbers.
- Does a year have 52 or 53 weeks?
- Most have 52 ISO weeks. A year has 53 when it starts on a Thursday, like 2026, or is a leap year that starts on a Wednesday.
- Why does 1 January sometimes fall in week 52 or 53?
- With ISO numbering, the first days of January belong to the last week of the previous year when that week has its Thursday in December.
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