Week Number Calculator
Pick a date to see which ISO week it falls in, the year that week belongs to, and the Monday and Sunday that start and end the week.
Your numbers
How we calculate it
In the ISO 8601 system weeks run from Monday to Sunday, and week 1 is the week that contains the first Thursday of the year. The calculator finds the Thursday of the date’s week, takes the year that Thursday falls in as the week-year, and counts the weeks from the start of that year.
A year has 53 weeks when 28 December falls in week 53, and 52 weeks otherwise. For 15 March 2026 the answer is week 11 of 2026, which runs from Monday 9 March to Sunday 15 March, and 2026 has 53 weeks.
Assumptions and limits
- The first days of January can belong to week 52 or 53 of the year before, and the last days of December can belong to week 1 of the next year. That is why the week-year is shown separately.
- This is the ISO 8601 numbering. The United States and some other countries number weeks differently, with weeks starting on Sunday.
- Years from 1 to 9999 are supported.
Frequently asked questions
What week is it today?
Leave the date on today’s date, which is the default, and the calculator shows the current ISO week.
Why does 1 January sometimes show week 52 or 53?
Because week 1 must contain the first Thursday. If 1 January falls on a Friday, Saturday or Sunday, it still belongs to the last week of the previous year.
Which years have 53 weeks?
Years where 1 January is a Thursday, or where it is a Wednesday in a leap year. 2020 and 2026 are examples.
Related calculators
- Day of the Week CalculatorPick any date.
- Business Days CalculatorPick a start date and an end date, and decide whether to count the start day.
- Days Between Dates CalculatorPick two dates to see exactly how many days apart they are, along with the same gap in weeks, in years-months-days and in working weekdays.
- Leap Year CalculatorCheck one year, or pick a range.
Worked example (a fixed example using the starting numbers, it does not change as you type)
Inputs: Date = March 15, 2026.
- ISO week number 11
- Week-year 2026
- Weeks in that year 53
- Monday of that week Monday, March 9, 2026
- Sunday of that week Sunday, March 15, 2026
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