Generally, you'd use the function ABS()
If A1 is itself a formula, you simply have to put the ABS() around the value, ie.
A1: =ABS(A5*A6) [as an example]
If A1 is an entry cell, you have four choices as I see it:
1) Put this formula in B1: =ABS(A1) and then use B1 in your calculations.
2) Put ABS(A1) in every formula where you use A1.
3) Put Data -> Validation on the cell, and restrict input to positive values.
4) Learn how to use VBA/Macros with Worksheet Events, so that if a negative value is entered, you automatically change it to a positive one. (Probably the most difficult of these suggestions.)
Scott
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