Is pipe bend radius measured to the inside or centerline?

In piping work, bend radius usually means centerline radius unless another radius is clearly stated. Inside radius is CLR minus half the outside diameter, and outside radius is CLR plus half the outside diameter.

A 1.5D bend has a centerline radius equal to about 1.5 times the pipe diameter. For example, a 4 inch OD pipe with a 1.5D bend has a 6 inch centerline radius.

How do I calculate bend arc length?

Convert the bend angle to radians and multiply by the centerline radius. A 90 degree bend has an angle of π/2 radians, so its centerline arc length is R × π/2.

Does this check wall thinning?

No. It only gives a simple strain indicator. Actual wall thinning and ovality depend on pipe material, wall thickness, tooling, mandrel support, heat, bend radius, and procedure.

Can this be used for conduit?

The geometry can be used for conduit layout as long as you know the outside diameter and centerline radius. Electrical code bend limits and pulling requirements must be checked separately.

Engineering

Bend Radius Calculator (Pipe)

Calculate pipe bend centerline radius, inside radius, outside radius, bend arc length, tangent length, and bend ratio from OD, radius factor, and bend angle.

What is pipe bend radius?

Pipe bend radius is usually the centerline radius, also called CLR. A 1.5D bend has a centerline radius about 1.5 times the pipe diameter, while a 3D bend has a larger, smoother radius. Larger radii generally reduce ovality, thinning, and bending strain.

Apex Piping’s close-radius pipe bending document describes bends by the centerline radius as a ratio of nominal pipe diameter, which is the same concept used by this calculator.

Worked example

OD = 4.5 in Bend radius factor = 1.5D Angle = 90° CLR = 4.5 × 1.5 = 6.75 in Arc length = 6.75 × π/2 = 10.60 in

The arc length is measured along the bend centerline. Cut length and fabrication allowance can differ depending on tangent lengths, bending method, and end-prep requirements.

Common mistakes in pipe bend calculations

Do not confuse inside radius, outside radius, and centerline radius. Also do not assume a bend is acceptable just because the geometry fits; piping codes, material ductility, wall thinning, ovality, pressure rating, and bend procedure qualification may control.

Common questions

In piping work, bend radius usually means centerline radius unless another radius is clearly stated. Inside radius is CLR minus half the outside diameter, and outside radius is CLR plus half the outside diameter.
A 1.5D bend has a centerline radius equal to about 1.5 times the pipe diameter. For example, a 4 inch OD pipe with a 1.5D bend has a 6 inch centerline radius.
Convert the bend angle to radians and multiply by the centerline radius. A 90 degree bend has an angle of π/2 radians, so its centerline arc length is R × π/2.
No. It only gives a simple strain indicator. Actual wall thinning and ovality depend on pipe material, wall thickness, tooling, mandrel support, heat, bend radius, and procedure.
The geometry can be used for conduit layout as long as you know the outside diameter and centerline radius. Electrical code bend limits and pulling requirements must be checked separately.