What does pie in math equal?

'pie' does not equal anything in mathematics. 'Pie' can be eaten as an Apple pie or a meat pie.

If you mean 'pi' , note the spelling. then it approximates to.

pi =~=3.14

pi =~=3.1416

pi =~=3.141592

pi = 3.141592....

It is an irrational number, which casually means that the decimals go infinity, and the decimal digits are NOT in any regular order.

To date 'pi' has been calculated , by super dupa computers, to at least 50,000,000,000 ( 50 billion) places and still going.

NB 'pi' is the small/lower case Classical Greek letter 'p' , and refers to proportion as in C = pi d ( Of circles). 'pi' is the constant of proportion.