What ISNUMBER does
ISNUMBER tells you whether Excel currently stores a value as numeric. Use it when you need a strict worksheet check instead of a conversion-friendly calculation.
That means ISNUMBER(19) returns TRUE, but ISNUMBER("19") returns FALSE because the quoted value is text, not a number.
Practical examples
Check whether a cell already contains a number
=ISNUMBER(A2)This returns TRUE only when A2 is numeric. It is useful in validation rules, cleanup formulas, and helper columns.
Detect numbers stored as text
=ISNUMBER("19")This returns FALSE. The result is important because many imports and pasted values look numeric but are still stored as text.
Gate a formula with IF and ISNUMBER
=IF(ISNUMBER(B2),B2*1.2,"Check input")This pattern lets you continue a calculation only when the source value is numeric. Otherwise, the formula shows a clear message instead of pushing bad data further into the sheet.
Common mistakes and notes
ISNUMBER does not coerce text numbers
If a value looks like a number but is stored as text, ISNUMBER still returns FALSE. That is by design and matches Microsoft's documentation for the IS functions.
Dates often return TRUE
Excel stores valid dates and times as serial numbers, so ISNUMBER usually returns TRUE for a real date cell even when the number formatting makes it look like a date.
ISNUMBER is different from VBA IsNumeric
Worksheet ISNUMBER checks the current stored type in Excel. VBA IsNumeric checks whether an expression can be evaluated as numeric, so it can return True for strings such as "19".