Calculate bend allowance and setbacks for pipe and tube bending operations.
Bend allowance is the arc length of the neutral axis of a bent pipe or sheet, used to figure the flat-pattern length before forming. This calculator solves bend allowance from the bend radius, the included angle, and the material thickness (with a configurable K-factor for the neutral-axis offset). Use it for pipe bending on a press, sheet-metal layout, and tube fabrication.
Bend Allowance (BA) = (θ × π / 180) × (R + K × t) where θ is the bend angle in degrees, R is the inner bend radius, t is the material thickness, and K is the neutral-axis K-factor — typically 0.41 for thick-walled pipe, 0.33 for steel sheet up to 2 mm, and 0.5 for thin tubing. The K-factor describes where the neutral axis falls inside the wall: K = 0 puts it at the inside surface; K = 1 puts it at the outside. Flat-length L_flat = L_inside + BA + L_outside. Bend deduction = 2 × (R + t) × tan(θ/2) − BA. The calculator returns BA in millimeters or inches, depending on input.
A sheet-metal apprentice unfolding a 90° flange on 1.5 mm steel at 2 mm inside radius uses K = 0.33, computes BA = 3.05 mm, and lays out the flat blank from the part drawing accurately on the first cut.
A custom-exhaust fabricator pre-cuts a 90° bend on 50 mm OD × 2 mm wall steel tube at 75 mm bend radius. Calculator returns BA = 118 mm; fitter cuts the straight length to fit the chassis exactly.
A frame-builder bending an extruded aluminium profile uses K = 0.5 (closer to outer fiber on soft material) to get a generous bend allowance, avoiding the over-stretching that cracked the previous part.
K-factor is the position of the neutral axis inside the wall thickness as a fraction. Use 0.33 for steel sheet, 0.41 for steel pipe, 0.5 for soft aluminium or copper. Manufacturer's data is the best source for production work.
Larger radius produces longer bend allowance because the neutral arc is longer. Tight radii (R < 1 × t) deform the material plastically and require empirical adjustment of K.
No. Bend allowance is the arc length added at the bend. Bend deduction is the total amount subtracted from the sum of the leg lengths to get flat length. They are related: L_flat = L_outside_sum − bend_deduction.