How to use it
- Choose ISO 8601 (used in Europe and most of the world, and in ISO-style dates such as 2026-W45) or US (Sunday start).
- Pick a date. You see its week number, the first and last day of that week and how many weeks the year has. The line under the result shows the current week.
- To go the other way, enter a year and a week number under “Dates of a week”.
How week numbers are counted
ISO 8601. Weeks run from Monday to Sunday. Week 1 is the week with the year’s first Thursday, which is the same as the week that contains 4 January. A week belongs to the year of its Thursday, so the first days of January can be in the last week of the previous year, and the last days of December in week 1 of the next.
To find the week of a date: move to the Thursday of the same week, count the days from 1 January of that Thursday’s year, divide by 7, round down and add 1.
US. Weeks run from Sunday to Saturday, week 1 is the one that contains 1 January (even if it has only one day of the new year), and the count restarts on 1 January. So the week around New Year has two numbers: the last one of the old year and week 1 of the new.
Worked example
Tuesday, 3 November 2026, ISO 8601:
- the Thursday of that week is 5 November, the 309th day of 2026;
- (309 − 1) ÷ 7 = 44, rounded down, + 1 = week 45;
- the week runs from Monday 2 to Sunday 8 November.
In the US system, 1 January 2026 is a Thursday, so 3 November is also in week 45, from Sunday 1 to Saturday 7 November. Week 1 of 2026 in ISO 8601 is 29 December 2025 to 4 January 2026.
Tips
- Say which system you mean when you plan with week numbers across countries: “week 1” can be a different week in a US and a European calendar.
- Spreadsheets. In Excel, ISOWEEKNUM gives ISO weeks and WEEKNUM without a second argument gives US weeks.
- To count the days between two dates, use the days between dates calculator.