Surface Area Formula

Area of a Circle

This will work for straight pipe as well as curved pipe. It does not work well on square pipe.

I made the assumption that you wanted the inside, circular area. Your question is too ambiguous to answer with intelligence.

D= outside cross-sectional diameter of elbow (or bend)

R = radius of center line of elbow

θ = degrees of bend of elbow

A = (πD)(2πR)(θ/360) = π²DRθ/180 ≈ 0.055 DRθ.

[You may think of this as (pipe circumference) x (swept circumference of a full circle)(fraction of circle thet the elbow is).]