Week Number Calculator
Find the ISO week number for a date, or the dates in a given week.
What the Week Number Calculator does
ISO 8601 numbers weeks from Monday to Sunday, with week 1 being the week that contains the first Thursday of the year. This means early January can fall in the last week of the previous ISO year.
Formula
Week 1 contains the first Thursday of the year, equivalently 4 JanuaryWeeks run Monday to SundayA year has 52 or 53 ISO weeks
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| What do you want to find? | one of 2 options | Yes | — |
| Date | date | In some modes | Shown What do you want to find? is Week number from a date. |
| Week number | number | In some modes | Accepts 1 or more, up to 53. Shown What do you want to find? is Dates from a week number. |
| Year | number | In some modes | Accepts 1900 or more, up to 2200. Shown What do you want to find? is Dates from a week number. |
How to use it
- Choose What do you want to find?.
- Fill in the remaining inputs the form shows for your choice.
- Select Calculate.
Worked example
Which ISO week contains 14 August 2026?
- Date
- 2026-08-14
Week 33 of 2026, running Monday 10 August to Sunday 16 August.
Frequently asked questions
Why can 1 January be in week 52 or 53?
Because ISO weeks never split across the Monday boundary. If 1 January falls on a Friday, it belongs to the final week of the previous ISO year.
Do all systems use ISO weeks?
No. Some US conventions start weeks on Sunday and define week 1 as the one containing 1 January. This calculator uses the ISO standard.