Sheet Metal Bend Allowance Calculator
Calculate how much material a bend uses so your flat blank comes out the right size. Enter the sheet thickness, inside bend radius, bend angle and K-factor (or estimate it from the radius-to-thickness ratio) to get the bend allowance, bend deduction, outside setback and the flat length of a two-flange part.
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Typical: 0.33 (sharp bends) to 0.5 (large radius)
From the edge to the outside mold line (virtual sharp)
How to use the Sheet Metal Bend Allowance Calculator
- Choose units and enter the material thickness and inside bend radius.
- Enter the angle and say whether it is the bend angle (90° for an L) or the included angle between flanges.
- Enter a K-factor (0.44 is a common default for air bending) or let the calculator estimate one from R/T.
- Add the two outside flange dimensions to get the flat blank length and bend lines.
- Check the angle table for other bends of the same material and radius.
Bend allowance formulas
| Quantity | Formula |
|---|---|
| Bend allowance | BA = A × (π ÷ 180) × (R + K × T) |
| Outside setback | OSSB = tan(A ÷ 2) × (R + T) |
| Bend deduction | BD = 2 × OSSB − BA |
| Flat length (outside dims) | L = flange A + flange B − BD |
| Flat length (flat legs) | L = (A − OSSB) + BA + (B − OSSB) |
Example: T = 2 mm, R = 2 mm, 90° bend, K = 0.44. BA = 1.5708 × (2 + 0.88) = 4.524 mm, OSSB = 4 mm, BD = 8 − 4.524 = 3.476 mm. Flanges of 50 and 30 mm need a 76.524 mm blank.
Typical K-factor values
| Inside radius vs thickness | Soft (aluminium, copper) | Medium (mild steel) | Hard (stainless) |
|---|---|---|---|
| R < T (air bending) | 0.33 | 0.38 | 0.40 |
| R = T to 3T | 0.40 | 0.43 | 0.45 |
| R > 3T | 0.50 | 0.50 | 0.50 |
These are common rule-of-thumb starting values for air bending. Bottoming and coining shift the neutral axis inward (lower K). For tight tolerances, derive K from a test bend on your own press brake and tooling.
Frequently asked questions
What is bend allowance?
Bend allowance is the length of the neutral axis through the bend, the material that ends up in the curved part: BA = A × (π ÷ 180) × (R + K × T), where A is the bend angle in degrees, R the inside radius, T the thickness and K the K-factor.
What is the difference between bend allowance and bend deduction?
Bend allowance is added to the flat flange lengths (measured to the bend tangent lines). Bend deduction is subtracted from the sum of the outside dimensions (measured to the virtual sharp): BD = 2 × OSSB − BA, with OSSB = tan(A/2) × (R + T). Both give the same flat length.
What is the K-factor?
The K-factor is the position of the neutral axis as a fraction of the thickness, measured from the inside of the bend. It is usually between 0.3 and 0.5: lower for sharp bends in harder material, approaching 0.5 for large radii. Calibrate it from a test bend for exact results.
How do I find the K-factor from a test piece?
Bend a strip of known flat length, measure the outside flange lengths, and compute BD = flange A + flange B − flat length. Then BA = 2 × OSSB − BD and K = (BA ÷ (A × π/180) − R) ÷ T.
What is the DIN 6935 K-factor estimate?
DIN 6935 gives a correction factor k = 0.65 + 0.5 × log₁₀(R/T) for R/T below 5 (k = 1 above that). The K-factor used here is k ÷ 2, e.g. R = T gives K = 0.325 and R = 5T gives K = 0.5.
Is the bend angle the same as the included angle?
No. The bend angle is how far the sheet is bent from flat; the included angle is the angle left between the flanges. They add up to 180°, so a 90° bend has a 90° included angle, but a 45° bend has a 135° included angle.
Why is bend deduction undefined for 180° hems?
For a 180° fold the outside mold lines are parallel and never meet, so the outside setback is infinite. Use the bend allowance with flange lengths measured to the bend tangent lines instead.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.