How to use this calculator
- Pick a date, or press "Today", to see its week number and the Monday and Sunday of that week.
- To go the other way, type a year and a week number to see which dates that week covers.
- Open the list below to see every week of the year with its dates.
How week numbers work
The international standard, ISO 8601, says:
- Weeks start on Monday and end on Sunday.
- Week 1 is the week with the year's first Thursday in it (which is also the week with 4 January in it).
- So a year has 52 or 53 weeks, and the first few days of January can belong to the last week of the year before, or the last days of December to week 1 of the next year.
US-style calendars usually start weeks on Sunday and count the week with 1 January in it as week 1, so their numbers can be one higher than ISO's for most of the year.
Worked examples
Every week of the year (ISO)
| Week | Monday | Sunday |
|---|
Frequently asked questions
Why does 1 January show week 52 or 53?
If 1 January falls on a Friday, Saturday or Sunday, that week's Thursday is still in December, so the week belongs to the year before. For example, Friday 1 January 2027 is in week 53 of 2026.
Which years have 53 weeks?
A year has 53 ISO weeks when it starts on a Thursday, or when it's a leap year starting on a Wednesday. 2026 starts on a Thursday, so it has 53 weeks; the next one is 2032.
Which week number should I use?
For work in the UK and Europe, and in most business software and spreadsheets set to ISO weeks, use the ISO week. If a US calendar or colleague counts from Sunday, use the US-style number. When it matters, write the dates as well as the week number.