**WEEKNUM Function in Microsoft Excel**

**Part 1: Introduction**

**Definition**

The WEEKNUM function in Excel returns the week number of a specific date. For example, the week containing January 1 is the first week of the year and is numbered week 1.

**Purpose**

The purpose of the WEEKNUM function is to calculate the week number of a specific date in the year. This can be useful for scheduling, project planning, and other date-related calculations.

**Syntax & Arguments**

The syntax for the WEEKNUM function is as follows:

```
WEEKNUM(serial_number,[return_type])
```

The WEEKNUM function has the following arguments:

**serial_number:**This is a required argument. It represents a date within the week. Dates should be entered using the DATE function or as results of other formulas or functions. For example, use DATE(2008,5,23) for the 23rd day of May 2008. Problems can occur if dates are entered as text.**return_type:**This is an optional argument. It is a number that determines on which day the week begins. The default is 1.

**Return Value**

The WEEKNUM function returns the week number of a specific date.

**Remarks**

- Excel stores date as sequential serial numbers to be used in calculations. By default, January 1, 1900, is serial number 1. January 1, 2008, is serial number 39448 because it is 39,448 days after January 1, 1900.
- Suppose Serial_number is out of range for the current date base value, a #NUM! Error is returned.
- Suppose Return_type is out of the range specified in the table above, a #NUM! Error is returned.

**Part 2: Examples**

**Example 1: Basic WEEKNUM Function**

**Purpose:** To find the week number of a specific date.

**Data tables and formulas:**

A | B | |
---|---|---|

1 | Date | Week Number |

2 | 2023-07-29 | =WEEKNUM(A2) |

**Explanation:** In this example, we want to find the week number of the date in cell A2. The formula `=WEEKNUM(A2)`

returns the week number of the date.

**Example 2: WEEKNUM Function with Different Return Types**

**Purpose:** To find the week number of a specific date with a week beginning on Monday.

**Data tables and formulas:**

A | B | |
---|---|---|

1 | Date | Week Number |

2 | 2023-07-29 | =WEEKNUM(A2, 2) |

**Explanation:** In this example, we want to find the week number of the date in cell A2 with a week beginning on Monday. The formula `=WEEKNUM(A2, 2)`

returns the week number of the date.

**Example 3: WEEKNUM Function with Dates as Results of Other Formulas**

**Purpose:** To find the week number of a date that results from another formula.

**Data tables and formulas:**

A | B | C | |
---|---|---|---|

1 | Year | Month | Week Number |

2 | 2023 | 7 | =WEEKNUM(DATE(A2, B2, 1)) |

**Explanation:** In this example, we want to find the week number of the first day of the month specified in cells A2 and B2. The formula `=WEEKNUM(DATE(A2, B2, 1))`

returns the week number of the first day of the month.

**Example 4: WEEKNUM Function with Dates Entered as Text**

**Purpose:** To find the week number of a date entered as text.

**Data tables and formulas:**

A | B | |
---|---|---|

1 | Date | Week Number |

2 | “2023-07-29” | =WEEKNUM(DATEVALUE(A2)) |

**Explanation:** In this example, we want to find the week number of the date in cell A2, which is entered as text. The formula `=WEEKNUM(DATEVALUE(A2))`

returns the week number of the date.

**Example 5: WEEKNUM Function with Dates in Different Formats**

**Purpose:** To find the week number of a date in a different format.

**Data tables and formulas:**

A | B | |
---|---|---|

1 | Date | Week Number |

2 | 29/07/2023 | =WEEKNUM(DATEVALUE(A2)) |

**Explanation:** In this example, we want to find the week number of the date in cell A2, which is in a different format. The formula `=WEEKNUM(DATEVALUE(A2))`

returns the week number of the date.

**Example 6: WEEKNUM Function with IF Function**

**Purpose:** To find the week number of a date and return a custom message if the week number is greater than 26.

**Data tables and formulas:**

A | B | |
---|---|---|

1 | Date | Message |

2 | 2023-07-29 | =IF(WEEKNUM(A2)>26, “Second half of the year,” “First half of the year”) |

3 | 2023-01-15 | =IF(WEEKNUM(A3)>26, “Second half of the year,” “First half of the year”) |

4 | 2023-12-20 | =IF(WEEKNUM(A4)>26, “Second half of the year,” “First half of the year”) |

**Explanation:** In this example, we want to determine if column A’s date falls in the first or second half of the year. The formula `=IF(WEEKNUM(A2)>26, "Second half of the year", "First half of the year")`

returns a custom message based on the week number of the date.

**Example 7: WEEKNUM Function with SUM Function**

**Purpose:** To calculate the total week numbers of a range of dates.

**Data tables and formulas:**

A | B | |
---|---|---|

1 | Date | Week Number |

2 | 2023-07-29 | =WEEKNUM(A2) |

3 | 2023-08-05 | =WEEKNUM(A3) |

4 | 2023-08-12 | =WEEKNUM(A4) |

5 | Total | =SUM(B2:B4) |

**Explanation:** In this example, we want to calculate the total week numbers of the dates in cells A2 to A4. The formula `=SUM(B2:B4)`

returns the real week numbers of the dates.

**Example 8: WEEKNUM Function with VLOOKUP Function**

**Purpose:** To find the week number of a date and return a corresponding value from another table.

**Data tables and formulas:**

A | B | |
---|---|---|

1 | Date | Week Number |

2 | 2023-07-29 | =WEEKNUM(A2) |

3 | 2023-08-05 | =WEEKNUM(A3) |

4 | 2023-08-12 | =WEEKNUM(A4) |

D | E | |
---|---|---|

1 | Week Number | Value |

2 | 30 | “Value for week 30” |

3 | 31 | “Value for week 31” |

4 | 32 | “Value for week 32” |

5 | Value for A2 | =VLOOKUP(B2, D2:E4, 2, FALSE) |

**Explanation:** In this example, we want to find the week number of the date in cell A2 and return a corresponding value from another table. The formula `=VLOOKUP(B2, D2:E4, 2, FALSE)`

returns the corresponding value for the week number from the other table.

**Example 9: WEEKNUM Function with AVERAGE Function**

**Purpose:** To calculate the average week number of a range of dates.

**Data tables and formulas:**

A | B | |
---|---|---|

1 | Date | Week Number |

2 | 2023-07-29 | =WEEKNUM(A2) |

3 | 2023-08-05 | =WEEKNUM(A3) |

4 | 2023-08-12 | =WEEKNUM(A4) |

5 | Average | =AVERAGE(B2:B4) |

**Explanation:** In this example, we want to calculate the average week number of the dates in cells A2 to A4. The formula `=AVERAGE(B2:B4)`

returns the average week number of the dates.

**Example 10: WEEKNUM Function with MAX Function**

**Purpose:** To find the maximum week number of a range of dates.

**Data tables and formulas:**

A | B | |
---|---|---|

1 | Date | Week Number |

2 | 2023-07-29 | =WEEKNUM(A2) |

3 | 2023-08-05 | =WEEKNUM(A3) |

4 | 2023-08-12 | =WEEKNUM(A4) |

5 | Max | =MAX(B2:B4) |

**Explanation:** In this example, we want to find the maximum week of dates in cells A2 to A4. The formula `=MAX(B2:B4)`

returns the maximum week number of the dates.

**Example 11: WEEKNUM Function with MIN Function**

**Purpose:** To find the minimum week number of a range of dates.

**Data tables and formulas:**

A | B | |
---|---|---|

1 | Date | Week Number |

2 | 2023-07-29 | =WEEKNUM(A2) |

3 | 2023-08-05 | =WEEKNUM(A3) |

4 | 2023-08-12 | =WEEKNUM(A4) |

5 | Min | =MIN(B2:B4) |

**Explanation:** In this example, we want to find the minimum week number of the dates in cells A2 to A4. The formula `=MIN(B2:B4)`

returns the minimum week number of the dates.

**Example 12: WEEKNUM Function with COUNTIF Function**

**Purpose:** To count the dates that fall in a specific week.

**Data tables and formulas:**

A | B | |
---|---|---|

1 | Date | Week Number |

2 | 2023-07-29 | =WEEKNUM(A2) |

3 | 2023-08-05 | =WEEKNUM(A3) |

4 | 2023-08-12 | =WEEKNUM(A4) |

5 | Count of Week 31 | =COUNTIF(B2:B4, 31) |

**Explanation:** In this example, we want to count the dates that fall in week 31. The formula `=COUNTIF(B2:B4, 31)`

returns the count of dates that fall in week 31.

**Part 3: Tips and Tricks**

- Use the WEEKNUM function in combination with other date functions for more complex calculations.
- Be careful when entering dates as text. Using the DATE function or the result of another formula or process is better to avoid problems.
- Remember that the return_type argument is optional and defaults to 1 if omitted. This means that the week begins on Sunday. If you want the week to start on a different day, specify the return_type argument.
- Remember that Excel stores date as sequential serial numbers for calculations. By default, January 1, 1900, is serial number 1.
- Suppose you get a #NUM! Error, check if the serial_number is out of range for the current date base value or if the return_type is out of the range specified in the table above.