Week Number Calculator
Find the ISO week number of any date.
Good to know. ISO weeks start on Monday, and week 1 is the one with the year’s first Thursday. Really.
How it works
In plain words
ISO weeks start on Monday. Week 1 is the week that contains the year’s first Thursday. Find the Thursday of the date’s week, and count how many weeks it is into its year.
The formula
thursday = date − (weekday − 1) + 3 (Monday = 1) week = floor((thursday − 1 January) ÷ 7) + 1
Worked example
Sunday 15 March 2026 is in ISO week 11. That week runs from Monday 9 March to Sunday 15 March.
Sources
- ISO 8601, Date and time format Calendar dates in the Gregorian calendar, and weeks that start on Monday.
Sources checked September 29, 2026. 6 automated tests check its answers on every change.
About the week number calculator
Week numbers are everywhere in Europe and much of Asia: in offices, schools, factories, and holiday planning ("we’re off in week 29"). They are handy, but no one can work them out in their head.
This calculator uses the international standard, ISO 8601. Weeks start on Monday, and week 1 is the week that contains the first Thursday of the year. Another way to say it: week 1 is the first week with at least four days in the new year. Most years have 52 weeks; some have 53.
Because of that rule, the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next. For example, 1 January 2027 is a Friday, so it falls in week 53 of 2026. The calculator shows the week-numbering year so this is always clear.
The US often uses a different system, where weeks start on Sunday and week 1 is the week containing 1 January. Those week numbers can be one higher than ISO numbers for part of the year. If you’re coordinating with someone in the US, check which system they use.
Questions people ask
What week number is it today?
The calculator opens with today’s date, so the week number shown is this week’s ISO week.
When does week 1 start?
Week 1 is the week (Monday to Sunday) that contains the first Thursday of the year. It can start as early as 29 December.
Do all years have 52 weeks?
No. Years that start on a Thursday, and leap years that start on a Wednesday, have 53 ISO weeks.
Are US week numbers the same?
Not always. The US often starts weeks on Sunday and counts the week containing 1 January as week 1.