# ISOWEEKNUM function in Excel

> ISOWEEKNUM returns the ISO 8601 week number of a date: weeks start on Monday, and week 1 is the week that contains the year's first Thursday.

Source: https://spreadsheetpractice.com/functions/isoweeknum · Updated October 28, 2026 · Formulas and results on this page were run by the Spreadsheet Practice formula engine on October 30, 2026

## Quick answer

- **What it does:** ISOWEEKNUM returns the ISO 8601 week number of a date: weeks start on Monday, and week 1 is the week that contains the year's first Thursday.
- **Syntax:** `=ISOWEEKNUM(date)`
- **Most common use:** `=ISOWEEKNUM(A2)` returns 1 (January 1, 2026 is a Thursday, so it starts week 1)
- **Available in:** Excel 2013 and later
- **Category:** [Date functions](https://spreadsheetpractice.com/functions/category/date)
- **Returns:** A whole number from 1 to 53.
- **Google Sheets:** Works the same in Google Sheets.

## What it does

ISOWEEKNUM returns the ISO 8601 week number of a date: weeks start on Monday, and week 1 is the week that contains the year's first Thursday. It is the standard in much of Europe and in many company calendars, and it can put early January dates in week 52 or 53 of the year before.

## Syntax

`=ISOWEEKNUM(date)`

| Argument | Required? | What it is |
|---|---|---|
| `date` | Required | The date to number. |

## Examples

Each result was calculated by running the formula on the sample sheet.

| Cell | Formula | Result | What it shows |
|---|---|---|---|
| B2 | `=ISOWEEKNUM(A2)` | 1 | January 1, 2026 is a Thursday, so it starts week 1 |
| B3 | `=ISOWEEKNUM(A3)` | 53 | January 1, 2027 is a Friday: it belongs to week 53 of 2026 |
| B4 | `=WEEKNUM(A3)` | 1 | WEEKNUM's default system calls the same day week 1 |

### The sample sheet

Every formula above runs on this data:

|  | A | B |
|---|---|---|
| 1 | Date | Result |
| 2 | 1/1/2026 | 1 |
| 3 | 1/1/2027 | 53 |
| 4 | 12/31/2026 | 1 |

## Common mistakes

| What you see | Why, and how to fix it |
|---|---|
| Week 53 in January | Correct for ISO weeks: January 1–3 can belong to the last week of the previous year. |
| Doesn't match WEEKNUM | WEEKNUM uses a different system by default. WEEKNUM(date,21) gives ISO weeks too. |

## Tips

- The ISO year of a date is the year of its Thursday: =YEAR(A2-WEEKDAY(A2,2)+4).

## Related functions

- [WEEKNUM](https://spreadsheetpractice.com/functions/weeknum): WEEKNUM returns the week number of a date in its year, 1 to 54.
- [WEEKDAY](https://spreadsheetpractice.com/functions/weekday): WEEKDAY returns the day of the week for a date as a number.
- [YEAR](https://spreadsheetpractice.com/functions/year): YEAR returns the four-digit year from a date.

## Questions

### How do I get ISO week numbers in Excel?

=ISOWEEKNUM(A2), or =WEEKNUM(A2,21) in versions before Excel 2013.

### Why is January 1 in week 53?

In the ISO system a week belongs to the year that holds its Thursday. If January 1 falls on a Friday, Saturday or Sunday, it belongs to the last week of the previous year.

## Practice

A graded ISOWEEKNUM exercise ("ISO week numbers") is on [the ISOWEEKNUM page](https://spreadsheetpractice.com/functions/isoweeknum). It is checked instantly, then re-checked on changed data.

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