ISO Week Number to Date
Turn an ISO 8601 year and week number into the precise calendar date and the full Monday-to-Sunday span of that week.
Day of Week
How does this calculator work?
Enter an ISO year, week number (1-53), and weekday (Monday=1 to Sunday=7). The tool finds the Monday of ISO week 1 (the week containing January 4th), then adds (week-1)×7 plus (day-1) days to return the exact calendar date and the full Monday-to-Sunday range of that week.
Formula
How this is calculated
The ISO 8601 standard fixes the first week of a year as the week containing January 4th, and every ISO week runs Monday (day 1) through Sunday (day 7). You supply the ISO year, the week number W (1-53), and the weekday d (1=Monday .. 7=Sunday).
The calculation first builds January 4th of the chosen year, then steps back to the Monday of that week by subtracting (isoWeekday(Jan 4) − 1) days, giving the Monday of ISO week 1. Adding (W − 1)×7 days lands on the Monday of the requested week, and adding a further (d − 1) days selects the chosen weekday. The same week-1 Monday plus (W − 1)×7 days and its +6-day Sunday define the full week range.
All arithmetic is done in UTC day-counts to avoid daylight-saving shifts. Note the ISO year can differ from the calendar year for dates in early January or late December: a date may belong to week 1 of the next year or week 52/53 of the previous year. Week 53 only exists in long ISO years (those starting on a Thursday, or a Wednesday in a leap year); requesting week 53 of a short year rolls into the following January.
Frequently asked questions
Because ISO week 1 is the week containing January 4th, its Monday can fall on December 29, 30, or 31 of the prior calendar year. Those late-December days then belong to week 1 of the new ISO year.
An ISO year has 53 weeks when January 1st is a Thursday, or when it is a Wednesday in a leap year. Otherwise the year has 52 weeks, and requesting week 53 spills over into the next January.
In ISO 8601, day 1 is always Monday and day 7 is Sunday, unlike some locale conventions that start the week on Sunday.
Also known as
TG we-Calculate Editorial Team. (2026). ISO Week Number to Date [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/week-number-to-date
TG we-Calculate Editorial Team. "ISO Week Number to Date." TG we-Calculate. 2026. https://we-calculate.com/calculator/week-number-to-date.
TG we-Calculate Editorial Team, "ISO Week Number to Date," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/week-number-to-date
@misc{wecalculate_week_number_to_date, title = {ISO Week Number to Date}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/week-number-to-date}}, year = {2026}, note = {TG we-Calculate} }
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