๐ ISO Week Date Converter
Convert between the ISO 8601 week date format (ISO year-week number-weekday, e.g. 2024-W01-1) and standard Gregorian calendar dates. Correctly handles week-spanning years, including years with a 53rd week.
How to use
- Choose "Date โ ISO week" or "ISO week โ Date".
- Enter a date, or an ISO year, week number, and weekday.
- The converted result is shown automatically.
How the calculation works
An ISO 8601 week date expresses a date as a year, a week number and a weekday. For example, 2026-W38-6 is Saturday of week 38 of 2026 (September 19, 2026). Rules โข Weeks start on Monday (1) and end on Sunday (7) โข Week 1 is the week containing the yearโs first Thursday โข A year has 52 or 53 weeks A year has 53 weeks when January 1 is a Thursday, or a Wednesday in a leap year. This tool converts dates to week dates and back. Entering week 53 for a year that has only 52 weeks is treated as invalid instead of rolling over into the next year.
Worked example
Saturday, September 19, 2026 โ 2026-W38-6 Friday, January 1, 2027 โ 2026-W53-5 (week 53 of the previous year) Monday, December 30, 2024 โ 2025-W01-1 (week 1 of the next year) 2026-W53-5 โ January 1, 2027 2027-W53 โ invalid (2027 has 52 weeks) Years with 53 weeks include 2020, 2026 and 2032.
Things to be aware of
- The ISO year (the week-numbering year) can differ from the calendar year for a few days around New Year.
- The compact form drops the hyphens, as in 2026W386.
- Results can differ from week numbers that start on Sunday or count the week containing January 1 as week 1.
FAQ
What is an ISO week date?
A date format defined by ISO 8601, where weeks start on Monday and week 1 is the week containing the year's first Thursday. A date is expressed as year-week-weekday (1=Monday to 7=Sunday), e.g. "2024-W01-1".
Why do some years have a 53rd week and others don't?
An ISO week-numbering year is either 364 days (52 weeks) or 371 days (53 weeks) long, depending on which weekday January 1st and December 31st fall on. Since this doesn't align exactly with the 365/366-day Gregorian year, some years end up with 53 ISO weeks.
Is the ISO week number the same as other week numbering systems?
Often close, but not necessarily identical. ISO 8601 has a precisely defined rule (Monday start, week 1 contains the first Thursday), while other regions or conventions may use different rules (e.g. Sunday start), so always check which system you need.