What this calculator does
Day of the week & week number works out which weekday a date falls on, plus ISO week and day of year. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaISO week 1 is the week containing the first Thursday of the year
The inputs explained
| Field | What to enter |
|---|---|
| Date | A calendar date. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Examples across different dates
The same calculation at a range of dates, with every other input held at its starting value. These are generated relative to the day the page was built.
Examples across different dates
| Date | Day of the week | ISO week number | Day of the year |
|---|---|---|---|
| 2026-08-11 | Tuesday | 33 | 223 of 365 |
| 2026-09-03 | Thursday | 36 | 246 of 365 |
| 2026-11-02 | Monday | 45 | 306 of 365 |
| 2027-01-31 | Sunday | 4 | 31 of 365 |
| 2027-08-04 | Wednesday | 31 | 216 of 365 |