How do you calculate 45 degree steel elbow weight-- need a formula?

The easiest way is to find the centerline length and multiply that by the linear weight of the pipe size being used.

Alternatively, this formula should give the weight of an elbow in pounds:

w=0.071*pi^2*R*(ro^2-ri^2)

Where:

w= weight of elbow (lb)

R=curve radius of elbow (in)

ro=outside radius of pipe (in)

ri=inside radius of pipe (in)