If the diameter = d = 8".

Radius = half of diameter, or 4".

Circumference = 2 * pi * r.

pi = 3.14159265.

So circumference =

0.5 * d * pi * 2, or just

Circumf = d * pi.

Which comes out as 8 * 3.14159265 = 25.1327412. Inches, of course, don't lose the units in there.

That's a pretty big pipe, actually.

Radius = half of diameter, or 4".

Circumference = 2 * pi * r.

pi = 3.14159265.

So circumference =

0.5 * d * pi * 2, or just

Circumf = d * pi.

Which comes out as 8 * 3.14159265 = 25.1327412. Inches, of course, don't lose the units in there.

That's a pretty big pipe, actually.