How it works
A bend consumes material along its neutral axis — the layer that neither stretches nor compresses, sitting a fraction K of the thickness in from the inside surface. The arc length of that neutral axis is the bend allowance: BA = (π/180) · angle · (R + K·t), where R is the inside radius, t the thickness and K the K-factor. The outside setback is OSSB = (R + t) · tan(angle/2), and the bend deduction — what you subtract from the summed outside flange lengths to get the flat blank — is BD = 2 · OSSB − BA. Add the bend allowance along the neutral axis, or subtract the bend deduction from outside dimensions; both give the same flat length.
Worked example
A 90° bend in 2 mm sheet with a 3 mm inside radius and K = 0.33: the bend
allowance is
(π/180) × 90 × (3 + 0.33×2) ≈ 5.75 mm, the outside setback is
(3 + 2) × tan 45° = 5.0 mm, and the bend deduction is
2 × 5.0 − 5.75 ≈ 4.25 mm. So a flat with two 50 mm outside flanges
lays out at 50 + 50 − 4.25 = 95.75 mm. The calculator returns
exactly this.
Frequently asked questions
- What is bend allowance in sheet metal?
- Bend allowance is the arc length of the neutral axis through a bend — the amount of material consumed by the bend. It is BA = (π/180) × angle × (R + K·t), where R is the inside radius, t the thickness and K the K-factor. You add it to the flat lengths along the neutral axis to lay out a blank.
- What is the K-factor?
- The K-factor is where the neutral axis sits inside the bend, given as a fraction of the material thickness: the neutral axis is K·t in from the inside surface. It is typically about 0.33 to 0.45 — lower for air bending, higher for bottoming and coining.
- What is the difference between bend allowance and bend deduction?
- They measure the same bend two ways. Bend allowance (BA) adds to the flat lengths measured along the neutral axis. Bend deduction (BD = 2·OSSB − BA) subtracts from the sum of the outside flange lengths. Most shops lay out flats from outside dimensions, so they use the bend deduction.
- How do I find the flat (blank) length?
- Sum the outside flange lengths and subtract one bend deduction for each bend: flat length = Σ(outside flanges) − Σ(bend deduction). For a single bend that is the two outside legs minus one bend deduction. Use this calculator to get the bend deduction per bend.
- Does the material and tooling change the K-factor?
- Yes. K is empirical, not fixed: air bending sits around 0.33, while bottoming and coining push the neutral axis outward toward 0.40–0.45. It also shifts with material, grain direction and the radius-to-thickness ratio, so verify it with a test bend on production material and tooling.
- Does this work in metric and imperial?
- Yes — enter the inside radius and thickness in mm or inches and the bend allowance, bend deduction and setback convert to your unit system. The bend angle and K-factor are unitless.
Method & assumptions
- The K-factor is empirical and varies with material, tooling and bending method — air bending ≈ 0.33, bottoming and coining higher (up to ~0.45). Verify it with a test bend on production material.
- The bend angle is the degrees the flange turns from flat (90° = a right-angle bend), not the included angle between the legs.
- Bend allowance and bend deduction are for one bend; sum one deduction per bend across a multi-bend part for the flat length.
Related calculators
- Bend Deduction Calculator — Flat (blank) length and bend deduction for a press-brake bend.