I don't see anything wrong with your formula except the missing = sign at the
beginning, and I'm sure you have that in your actual sheet.

Meanwhile, I would accomplish the same effect with this formula:

=MAX(AT1-AV1,0)

It will always subtract the second value from the first, but if the value is
less than zero, the MAX will cause the answer 0 to be returned.

Does that work better for you?
--
"Actually, I *am* a rocket scientist." -- JB

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"LCCHELP" wrote:

I have input the following if function

if(at1av1,at1-av1,0) however if the number in av1 is greater than at1 it
returns a negative number, rather than returning 0.

can someone enlighten me as to what I have missed.

thanks