#NUM!
A number is invalid or the result is impossible to represent.
What it means
The formula is well-formed and the arguments are numbers, but the maths cannot produce a valid result: an out-of-domain input, a value too large to represent, or an iterative calculation that never converges.
How to fix it
- Validate the input range before calling, e.g. =IF(A1<0, "", SQRT(A1)).
- Supply a closer starting guess to an iterative function.
- Rescale the calculation so intermediate values stay representable.
Common causes
- A domain error such as =SQRT(-1).
- A result too large or too small for the number range.
- IRR/RATE-style iterative functions that fail to converge.
- A negative or zero argument where the function requires a positive one.
A formula that produces it
=SQRT(-1)