Excel’s ISNUMBER function is one of a group of IS functions or “Information Functions” that can be used to find out information about a specific cell in a worksheet or workbook.
Instructions in this article apply to Excel for Microsoft 365, Excel 2019, 2016, 2013, and 2010.
The ISNUMBER function's job is to determine if the data in a certain cell is a number or not.
 If the data is a number or is a formula that returns a number as output, a value of TRUE is returned by the function — the example in row 1 in the image above.
 If the data is not a number, or the cell is empty, a FALSE value is returned — the example in rows 2 in the image above.
Additional examples show how this function is often used in conjunction with other Excel functions to test the outcome of calculations. This is usually done to gather information about a value in a particular cell before using it in other calculations.
The ISNUMBER Function's Syntax and Arguments
A function's syntax refers to the layout of the function and includes the function's name, brackets, and arguments.
The syntax for the ISNUMBER function is:
= ISNUMBER ( Value )
Value: (required) — Refers to the value or cell contents being tested.
This argument can be blank, or it can contain data such as:
 Text strings.
 Numbers.
 Error values.
 Boolean or logical values.
 Nonprinting characters.
It can also contain a cell reference or named range pointing to the location in the worksheet for any of the above types of data.
ISNUMBER and the IF Function
As mentioned, combining ISNUMBER with other functions, such as with the IF function, provides a way of finding errors in formulas that do not produce the right type of data as output.
ISNUMBER and SEARCH
Similarly, combining ISNUMBER with the SEARCH function creates a formula that searches the text strings for a match to the designated data.
If a matching number is found, the formula returns the value of TRUE, otherwise, it returns FALSE as a value.
ISNUMBER and SUMPRODUCT
Using the ISNUMBER and SUMPRODUCT functions in a formula checks a range of cells to see if they contain numbers or not.
The combination of the two functions gets around the limitation of ISNUMBER on its own of only checking one cell at a time for number data.
ISNUMBER checks each cell in the range to see if it holds a number and returns TRUE or FALSE depending upon the result.
Note, however, that even if one value in the selected range is a number, the formula returns an answer of TRUE, such as if the range contains:
 Empty cells.
 Text data.
 An error message (#DIV/0!).
 The copyright symbol (©).
 One number in cell A7 which is enough to return a value of TRUE in cell C9.
How to Enter the ISNUMBER Function
Options for entering the function and its arguments into a worksheet cell include:
 Type the complete function such as: =ISNUMBER(A2) or =ISNUMBER(456) into a worksheet cell.
 Select the function and its arguments using the ISNUMBER function dialog box.
Although it is possible to just type the complete function manually, many people find it easier to use the dialog box as it takes care of entering the function's syntax — such as brackets and comma separators between arguments.
ISNUMBER Function Dialog Box
The steps below outline the steps used to enter ISNUMBER into cell C2 in the image above.

Select cell C2, which is the location where the formula results will be displayed.

Select the Formulas tab.

Choose More Functions > Information from the ribbon menu to open the function dropdown list.

Select ISNUMBER in the list to bring up that function's dialog box.

Select cell A2 in the worksheet to enter the cell reference into the dialog box.

Select OK to close the dialog box and return to the worksheet.

The value TRUE appears in cell C2 since the data in cell A2 is the number 456.

If you select cell C2, the complete function = ISNUMBER ( A2 ) appears in the formula bar above the worksheet.