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Sheet Metal Bend Allowance on a Box and Pan Brake

Guide · Updated October 28, 2026

Sheet stretches on the outside of a bend and compresses on the inside. Somewhere between is the neutral axis, which keeps its length. Bend allowance is the length of that neutral axis through the bend, and it tells you how long to cut the flat blank.

Check your brake angle

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Step by step

  1. Know the thickness and radius: Measure T with a mic; the inside radius R depends on the brake nose or finger.
  2. Choose a K-factor: K is where the neutral axis sits as a fraction of T, typically 0.3–0.5; 0.44 is a common default for air bending mild steel.
  3. Calculate BA: BA = A × (π ÷ 180) × (R + K × T), A in degrees.
  4. Calculate setback: OSSB = tan(A ÷ 2) × (R + T).
  5. Bend deduction: BD = 2 × OSSB − BA.
  6. Flat length: Flat = sum of outside leg lengths − BD for each bend.
  7. Test bend: Bend a scrap strip and adjust K to match your brake.

Worked example: 90° L-bracket

Mild steel, T = 0.060 in, R = 0.060 in, K = 0.44, outside legs 1.000 and 1.500 in. BA = 90 × 0.017453 × (0.060 + 0.44 × 0.060) = 1.5708 × 0.0864 = 0.1357 in. OSSB = tan 45° × (0.060 + 0.060) = 0.120 in. BD = 2 × 0.120 − 0.1357 = 0.1043 in. Flat = 1.000 + 1.500 − 0.1043 = 2.3957 in, so cut 2.396 in. Bend line: 1.000 − 0.120 = 0.880 in from one end is the start of the bend; on a box and pan brake you usually line the scribed line up with the edge of the finger.

Finding your own K-factor

Cut a strip exactly 3.000 in, bend 90° in the middle, measure both outside legs. If they total 3.110 in, BD = 3.110 − 3.000 = 0.110 in. Then BA = 2 × OSSB − BD, and K = (BA ÷ (A × π ÷ 180) − R) ÷ T. With OSSB 0.120: BA = 0.130, K = (0.130 ÷ 1.5708 − 0.060) ÷ 0.060 = 0.38.

Using the box and pan fingers

Remove fingers to make room for already-bent flanges. Put the bend line at the nose; clamp; lift the leaf to the angle plus springback.

Springback

Steel springs back a few degrees; overbend and check with a protractor.

Common problems

Part too long: BD underestimated; measure your K. Cracks at the bend: radius too tight for the material or bending along the grain. Bend not at the line: sheet slipped when clamping.

Tips

FAQ

What is bend allowance?

The neutral-axis length through the bend.

Formula?

BA = A × π/180 × (R + K × T).

Typical K-factor?

0.3–0.5; 0.44 is a common default.

What is bend deduction?

2 × outside setback − BA.

How do I find my K?

Bend a measured test strip.

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