Week Number Calculator
Convert between dates and week numbers, and list every week of a year
| Week | From | To |
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Date to Week
ISO and US week numbers at once
Week to Dates
Start and end dates of any week
Full Year Table
Every ISO week of a year listed
Frequently asked questions
What is the difference between ISO and US week numbers?
ISO 8601 weeks run Monday to Sunday, and week 1 is the week containing the year's first Thursday — so ISO week 1 can begin in late December. US weeks run Sunday to Saturday, and week 1 is simply the week containing 1 January, however few days of it fall in the new year. The two numbers often differ by one.
Why does the ISO week-year sometimes differ from the calendar year?
An ISO week never splits across two week-years, so the days at either end of a calendar year get pulled into a neighbouring one. 1 January 2027 falls on a Friday, which belongs to the week that started on Monday 28 December 2026, so it is reported as week 53 of 2026 rather than week 1 of 2027. The calculator shows the week-year separately for exactly this reason.
When does a year have 53 ISO weeks instead of 52?
A year has 53 ISO weeks when 1 January falls on a Thursday, or when it is a leap year and 1 January falls on a Wednesday or Thursday. 2026 is a 53-week year because it starts on a Thursday; 2025 has 52. If you ask for week 53 of a 52-week year the calculator tells you the year is short rather than inventing a date.
Does my time zone affect the result?
No. All arithmetic runs on UTC calendar days, so a daylight-saving change can never shift a date into a neighbouring week. The date you type is treated as that calendar day exactly, wherever you are.