Global week rules
Week Number
Translate dates into week systems, reverse a week number into its date range, or inspect a complete year.
Build-date example
ISO week 34 of 2026
2026-08-19, using Monday as the first weekday and four minimum days in week one.
Primary result
ISO week 34 of 2026
August 17–23, 2026 · Wednesday
United States comparison
Week 34 of 2026August 16–22, 2026Why week numbers differ
A week number is not a property of a date alone. It also depends on which weekday starts a week and how many new-year days a partial week needs before it becomes week one.
ISO 8601 uses Monday and four minimum days. The United States regional default uses Sunday and one minimum day. Other regions use their own CLDR defaults.
Calendar year and week year
Dates near January 1 and December 31 can belong to a neighboring week year. Every result therefore shows both the week number and week year, plus the exact seven-day range.
Coverage and limitations
The calculator uses the proleptic Gregorian calendar for years 0001–9999. It does not calculate fiscal, retail 4-4-5, payroll, school, sprint, or holiday weeks.
Sources
Regional defaults use the bundled Unicode CLDR snapshot 46-bundled-v1, reviewed 2026-08-19. No runtime request is made to Unicode.
Frequently asked questions
What is the current ISO week number?
The calculator opens with your browser-local date and shows its ISO week year and number as the primary result.
Why can a week year differ from a calendar year?
The first or last dates of a calendar year can belong to an adjacent week year, depending on the first-day and minimum-days rule.
What is the difference between ISO and U.S. week numbers?
ISO weeks start Monday and require four days in week one. The U.S. regional default starts Sunday and requires one day.
Are ISO and U.S. the only week systems?
No. Regional defaults vary. The calculator includes a reviewed CLDR snapshot and custom rule controls.
What do first day and minimum days mean?
They determine where each week begins and how many new-year days a partial week needs before it becomes week one.
Do holidays affect week numbers?
No. Week numbering uses calendar dates and the selected week rule, not holiday calendars.