Week Number Calculator
Find ISO week numbers from dates or convert ISO weeks back to dates
Date you want to convert to an ISO week.
The ISO year that contains the target week.
Week number inside the selected ISO year.
Weekday within the ISO week.
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How ISO Week Numbers Work
ISO weeks follow Monday starts and the first-Thursday rule.
Monday starts the week
ISO weekday 1 is Monday and ISO weekday 7 is Sunday. The result always reports the Monday-to-Sunday range for the selected week.
Week 1 has a Thursday
ISO week 1 is the week containing January 4, which is equivalent to the week containing the first Thursday of the ISO year.
Some years have 53 weeks
ISO years usually have 52 weeks. Years that start on Thursday, or leap years that start on Wednesday, have 53 ISO weeks.
Week Number Examples
Common ISO week conversions and New Year edge cases.
This Sunday belongs to ISO week 22, running May 25 through May 31.
Some early January dates are still in the previous ISO week year.
ISO week 22, weekday 7, maps back to Sunday, May 31, 2026.
Week Number Calculator FAQ
Answers about ISO week years, week starts, 53-week years, and week-to-date conversion.
What is an ISO week number?
ISO week numbers use Monday as the first day of the week. Week 1 is the week that contains the first Thursday of the ISO year.
Why can January dates belong to last year's week?
ISO week years are based on full weeks, not calendar-year boundaries. Early January can still be part of the previous ISO year, and late December can be part of the next ISO year.
Do all years have 52 weeks?
No. Most ISO years have 52 weeks, but some have 53 weeks. The calculator clamps week numbers to the selected ISO year.
Can I convert a week number back to a date?
Yes. Switch to ISO week to date, enter the ISO week year, week number, and weekday, and the calculator returns the matching calendar date.
Is this a payroll or fiscal week calculator?
No. This page follows ISO-8601 week rules. Payroll, retail, and fiscal calendars may use different week starts and year definitions.