Sheet Metal Bend Allowance Calculator

Find the bend allowance, bend deduction and flat blank length for a sheet metal bend.

16 ga steel is 0.0598″.
How far the flange bends from flat. 90 for an L.
Flat Blank (Optional)
Flange lengths measured to the outside of the bend (outside mold line).
Fractions and decimals both work.
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In. Bend Allowance
Bend deduction
Outside setback
Flat blank length
Neutral axis radius
Metric
Radius to Thickness

Bend Chart

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PartT × RAngleKBABDRemove

How to Calculate Bend Allowance

When sheet metal bends, the inside compresses and the outside stretches. Somewhere in between, a neutral axis keeps its length. The K-factor says where it sits as a fraction of the thickness from the inside face. Bend allowance is the length of that neutral axis through the bend.

Bend allowance BA = π ÷ 180 × angle × (R + K × T)
Outside setback OSSB = tan(angle ÷ 2) × (R + T)   Bend deduction BD = 2 × OSSB − BA
Flat length = flange 1 + flange 2 − BD   (flanges to the outside mold line)

Equivalently, flat length = the two straight flat sections + BA.

Worked Example

Typical K-Factors

SituationK
Tight air bends (R less than T), soft materials0.33
Air bending, R about 1 to 3 × T0.40–0.45
Large radius (R over 3 × T)0.45–0.50

Rules of thumb only. The best K-factor comes from bending a test strip on your own press brake and tooling and back-calculating.

Tips

In air bending, the inside radius is set mostly by the die opening (roughly 15% of the V-die width for mild steel), not the punch radius. Enter the bend angle as the amount the metal moves from flat: a 90° L bracket is 90, and a bend to a 135° included angle is 45.