Why does ISO week 1 start in December?

ISO week 1 is the week containing 4 January, so its Monday may be in the previous calendar year. Week 1 of 2025 is 30 December 2024 to 5 January 2025. Enter the week year, not necessarily the calendar year of the starting date.

What happens if a year has no week 53?

The result shows an error rather than rolling into another year. ISO 2025 has 52 weeks; ISO 2026 has 53. Choose US to apply Sunday and 1 January rules instead. US 2028 has a one-day week 54 on 31 December.

Can I copy the start date and end date separately?

Yes. Week starts and Week ends each have a copy button with YYYY-MM-DD output. Copy beside the controls copies the range with its convention and week year. If the browser refuses clipboard access, select the displayed text and copy it by hand.

Does this create a printable calendar or spreadsheet?

It provides a browser table of week ranges and copyable dates. It does not generate monthly calendar grids, print templates, calendar files, Excel workbooks or software-specific formulas.

Which years and schedules are supported?

Gregorian week years 1 through 9999 under ISO or the US Sunday and 1 January convention. The final ISO range may show early January of year 10000 as its endpoint. Fiscal, GPS, month-relative and employer-specific weeks are not calculated.

Week Number to Date

Enter a week-numbering year and week to find the dates it covers. ISO weeks run Monday to Sunday and may cross the calendar year. US weeks run Sunday to Saturday and stop at the year boundary. This opens on ISO week 1 of 2025, which starts in December 2024.

ISO week date rangeReadyEnter a year and week number.

Lookup

Week year

Week number

Convention

ISO 8601: weeks run Monday to Sunday, and week 1 is the week holding 4 January. Standard across Europe and in most software. A week belongs to the year that holds its Thursday, so the first days of January can fall in the last week of the year before.

Enter the week-numbering year, which can differ from the calendar year under ISO. US ranges stop at the year boundary.

ISO and US conventions. ISO 8601 is the international standard and the one meant by "week 36" in most of Europe and in most software. The US convention is what a spreadsheet returns from WEEKNUM unless you ask it for the ISO rule. Away from year boundaries, how far apart they run depends on the weekday 1 January falls on: in a year opening Sunday through Thursday they agree every day except Sundays, when the US number is one higher, and in a year opening on a Friday or a Saturday the US number is one higher all week and two higher on Sundays. At a year boundary, compare the week year too. This page uses Gregorian calendar dates from 0001 through 9999; it does not model fiscal, GPS, month-relative or employer schedules. Date lookup shows both week numbers; reverse lookup uses the selected rule.

Common questions

Why does ISO week 1 start in December?: ISO week 1 is the week containing 4 January, so its Monday may be in the previous calendar year. Week 1 of 2025 is 30 December 2024 to 5 January 2025. Enter the week year, not necessarily the calendar year of the starting date.
What happens if a year has no week 53?: The result shows an error rather than rolling into another year. ISO 2025 has 52 weeks; ISO 2026 has 53. Choose US to apply Sunday and 1 January rules instead. US 2028 has a one-day week 54 on 31 December.
Can I copy the start date and end date separately?: Yes. Week starts and Week ends each have a copy button with YYYY-MM-DD output. Copy beside the controls copies the range with its convention and week year. If the browser refuses clipboard access, select the displayed text and copy it by hand.
Does this create a printable calendar or spreadsheet?: It provides a browser table of week ranges and copyable dates. It does not generate monthly calendar grids, print templates, calendar files, Excel workbooks or software-specific formulas.
Which years and schedules are supported?: Gregorian week years 1 through 9999 under ISO or the US Sunday and 1 January convention. The final ISO range may show early January of year 10000 as its endpoint. Fiscal, GPS, month-relative and employer-specific weeks are not calculated.

Exact Gregorian week ranges under the selected ISO or US rule. ISO weeks are seven days; US ranges are clipped at 1 January and 31 December.