Date
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How this calculator works
Under ISO 8601, weeks start on Monday and week 1 is the week containing the year's first Thursday. That means the first days of January can belong to week 52 or 53 of the previous year. North American calendars often count differently: weeks start on Sunday and week 1 is the week containing January 1. Both are shown.
Formula
- ISO week 1 = the week (Monday to Sunday) containing the first Thursday of the year
- ISO week = floor((day of year of that week's Thursday − 1) / 7) + 1
- US week = Sunday-start week counted from the week containing January 1
Worked example
January 1, 2027
- Date
- 2027-01-01
ResultWeek 53
Friday, January 1, 2027 is in ISO week 53 of 2026.
Frequently asked questions
What week number is it today?
Leave the date as today and the headline shows the current ISO week number.
Why is January 1 sometimes in week 52 or 53?
ISO week 1 must contain the first Thursday of the year. If January 1 falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous year.
How many weeks are in a year?
ISO years have 52 weeks, or 53 when the year starts on a Thursday (or on a Wednesday in a leap year). 2026 has 53 ISO weeks.
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Results are estimates based on the values you enter and the published method shown above. They do not replace professional medical, financial or legal advice. Method reviewed October 7, 2026. About this site