Find the ISO week number for a calendar date or look up the Monday-to-Sunday dates in a specific ISO week.
ISO 8601 divides the calendar into Monday-to-Sunday weeks. Week 1 is the week containing January 4, which is equivalently the week containing the year's first Thursday.
Because weeks cannot be split at New Year, an early-January or late-December date can belong to a different ISO week-year than its calendar year.
Weeks begin Monday and end Sunday.
Week 1 = the week containing January 4ISO weekday numbers run from Monday = 1 through Sunday = 7.
Monday = selected date − (ISO weekday − 1); Sunday = Monday + 6 daysDecember 28 always belongs to the final ISO week, so its week number is either 52 or 53.
ISO week number of December 28August 13, 2026 is in ISO week 33 of 2026. That week starts Monday, August 10 and ends Sunday, August 16.
January 1, 2021 is in ISO week 53 of 2020, even though its calendar year is 2021.
January 4, 2021 is Monday in ISO week 1 of 2021.
Use Find Week Dates to list all seven dates for a valid ISO week-year and week number.
Compare the calendar year and ISO week-year fields around January 1 and December 31.
This calculator uses only ISO 8601. Some locales use Sunday-start or other week-number conventions.
ISO 8601 defines Monday as day 1 and Sunday as day 7, giving each week a stable seven-day range for international planning.
Most ISO week-years contain 52 weeks. Some contain a 53rd week because 365- and 366-day calendar years do not divide evenly into seven-day weeks.
Like the Day of the Week Calculator, this tool uses the proleptic Gregorian calendar consistently, including dates before local Gregorian adoption. Historical regional calendars may differ.
Choose Date to week, enter a date, and calculate its ISO week number and week-year.
It identifies a Monday-to-Sunday week under ISO 8601. Week 1 contains January 4.
Every ISO week starts Monday and ends Sunday.
A week is assigned as a whole. If January 1 falls before the first ISO week's Thursday-aligned boundary, it remains in the preceding ISO week-year.
It is the year assigned to an ISO week and can differ from the date's calendar year near New Year.
An ISO week-year contains either 52 or 53 complete weeks.
Yes. The calculator determines this from the ISO week containing December 28.
Select Find Week Dates, enter the ISO week-year and week 20, then calculate.
It is the Monday-to-Sunday week containing January 4, or equivalently the year's first Thursday.
No. Local calendars may use different starting days or numbering rules; this calculator is explicitly ISO 8601.
No. The calculator uses date-only values without time or timezone conversion.
It consistently applies proleptic Gregorian calendar rules.