Excel WEEKNUM function
Returns the week number of the year for a date. In Spanish Excel it is called NUM.DE.SEMANA.
Syntax
English Excel
=WEEKNUM(serial_number, [return_type])
Spanish Excel (Mexico)
=NUM.DE.SEMANA(núm_de_serie, [tipo])
Spanish Excel (Spain, Latin America)
=NUM.DE.SEMANA(núm_de_serie; [tipo])
| Argument | What it is |
|---|---|
| serial_number | The date. |
| return_typeoptional | 1: weeks from Sunday; 2: from Monday; 21: ISO week. |
Example
WEEKNUM(date, 2) gives the week of the year with weeks starting on Monday. Handy for grouping sales or shifts by week. With 21 it uses the ISO week, common in Europe.
=WEEKNUM(A2,2)
Common mistake
Systems number weeks differently: 21 gives the ISO week many calendars in Europe use.
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Practice WEEKNUM
Type the formula in the sheet and check it instantly. Spanish or English function names both work. See all lessons
WEEKNUM
WEEKNUM(date, 2) gives the week of the year with weeks starting on Monday. Handy for grouping sales or shifts by week. With 21 it uses the ISO week, common in Europe.
In B2, calculate each date’s week number, with weeks starting on Monday.
| A | B | C | |
|---|---|---|---|
| 1 | Date | Week | |
| 2 | 01/01/2026 | ||
| 3 | 01/04/2026 | ||
| 4 | 01/05/2026 | ||
| 5 | 03/16/2026 | ||
| 6 | 11/30/2026 | ||
| 7 |
Tip: while typing, click a cell or drag across several to insert their reference.
Frequently asked questions
- What is WEEKNUM called in Spanish?
- In Spanish Excel it is NUM.DE.SEMANA. The formula =WEEKNUM(A2,2) is written =NUM.DE.SEMANA(A2;2) (or with commas in Mexico).
- Why does WEEKNUM use semicolons in some Excel versions?
- It depends on regional settings. Where the comma is the decimal mark (Spain, much of South America), Excel separates arguments with semicolons. English Excel and Mexican Spanish Excel use commas.
- What is the most common WEEKNUM mistake?
- Systems number weeks differently: 21 gives the ISO week many calendars in Europe use.