Sheet Metal Bend Allowance Calculator

How to use it: enter the bend's inside radius (your tooling or press-brake die radius), material thickness, bend angle, and a K-factor for your material and process. All four results update immediately - no need to press calculate.

Formula. The neutral axis - the layer of material that neither stretches nor compresses through the bend - sits at radius R + K×T from the inside of the bend, where R is inside radius, T is thickness and K is the K-factor (0 = neutral axis at the inside face, 0.5 = at the mid-thickness). Bend allowance is the arc length of that neutral axis:

Bend allowance = (π/180) × angle × (R + K×T)

Bend deduction is the alternative way to dimension a flat pattern - instead of adding an arc length between flange tangent lines, you subtract a deduction from the sum of the flange lengths measured to where their outer faces would intersect (the "mold line"):

Outside setback = tan(angle ÷ 2) × (R + T) and Bend deduction = 2 × outside setback − bend allowance

Worked example. With the defaults above (90°, 2 mm inside radius, 1.5 mm thickness, K = 0.33): neutral radius = 2 + 0.33×1.5 = 2.495 mm; bend allowance = (π/2) × 2.495 ≈ 3.919 mm; outside setback = tan(45°) × (2 + 1.5) = 3.5 mm; bend deduction = 2×3.5 − 3.919 ≈ 3.081 mm. To flat-pattern a part with two 20 mm flanges bent 90°, either lay out 20 + 20 + 3.919 = 43.919 mm using flange lengths to the bend tangent lines, or 20 + 20 − 3.081 = 36.919 mm using flange lengths to the mold-line intersection - both describe the same physical part measured differently.

The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.

Bend angle

90 deg

Inside radius

2 mm

Material thickness

1.5 mm

K-factor

0.33

Primary result: Bend allowance: 3.919 mm.

• Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
• Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
• When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.

K-factor is an approximation of real material behavior, not a physical constant. It depends on alloy, temper, bend radius-to-thickness ratio, and whether the bend is air-bent, bottomed or coined. 0.33–0.5 is a common starting range for air bending mild steel and aluminium; confirm the value for your material and process with a test bend and a caliper before cutting production parts.

This calculator returns bend geometry only - it does not account for springback (the die angle needed to hit a target part angle), tooling minimum flange length, grain-direction cracking risk, or bends tighter than roughly one material thickness.

Units are millimetres for length and decimal degrees for angle; the K-factor itself is unitless.