Excel ISOWEEKNUM function
Returns the ISO week number (weeks start on Monday and week 1 holds the year’s first Thursday). In Spanish Excel it is called ISO.NUM.DE.SEMANA.
Syntax
English Excel
=ISOWEEKNUM(date)
Spanish Excel (Mexico)
=ISO.NUM.DE.SEMANA(fecha)
Spanish Excel (Spain, Latin America)
=ISO.NUM.DE.SEMANA(fecha)
| Argument | What it is |
|---|---|
| date | The date. |
Example
YEARFRAC(start, end) gives the years between two dates with decimals: 2.5 is two and a half years. DATEVALUE turns a date stored as text (like "2024-03-15", typical of a CSV) into a real date. ISOWEEKNUM gives the ISO week, used in logistics and payroll.
=ISOWEEKNUM(E2)
Common mistake
January 1 can fall in week 52 or 53 of the previous year. It is the same as WEEKNUM(date,21).
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Practice ISOWEEKNUM
Type the formula in the sheet and check it instantly. Spanish or English function names both work. See all lessons
Seniority, text dates and ISO weeks
YEARFRAC(start, end) gives the years between two dates with decimals: 2.5 is two and a half years. DATEVALUE turns a date stored as text (like "2024-03-15", typical of a CSV) into a real date. ISOWEEKNUM gives the ISO week, used in logistics and payroll.
In C2, calculate each employee’s years of service as of the cutoff date in I1.
| A | B | C | D | E | F | G | H | I | J | |
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | Employee | Hired | Years | Last payment (text) | Last payment | ISO week | Cutoff | 12/31/2025 | ||
| 2 | Ana | 03/15/2019 | 2025-11-28 | |||||||
| 3 | Beto | 07/01/2022 | 2025-12-05 | |||||||
| 4 | Carla | 01/20/2015 | 2025-12-12 | |||||||
| 5 | Dani | 10/07/2024 | 2025-12-19 | |||||||
| 6 | Elena | 12/31/2020 | 2025-12-26 | |||||||
| 7 |
Tip: while typing, click a cell or drag across several to insert their reference. F4 anchors the reference with $.
Frequently asked questions
- What is ISOWEEKNUM called in Spanish?
- In Spanish Excel it is ISO.NUM.DE.SEMANA. The formula =ISOWEEKNUM(E2) is written =ISO.NUM.DE.SEMANA(E2) (or with commas in Mexico).
- Why does ISOWEEKNUM 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 ISOWEEKNUM mistake?
- January 1 can fall in week 52 or 53 of the previous year. It is the same as WEEKNUM(date,21).