Try tools|Options|calculation tab
and check Automatic

And you don't need =sum().

Just use:
=a1-a5



aaron wrote:

i do a simple sum(a1-a5). it will not return a value until i do an autosum
formula.

also- when i add a number to the the blank cell in the array, the sum
formula will not automatically calculate the sum until i do the autosum
function!

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Dave Peterson