How to use this calculator
- Enter the bend angle. Enter the degrees the flange turns from flat (90° for a right-angle bend).
- Enter the inside radius. Enter the inside bend radius formed by the punch.
- Enter the thickness. Enter the sheet material thickness.
- Set the K-factor. Set the K-factor (~0.33 for air bending, up to ~0.45 for bottoming).
- Read the results. Read the bend allowance, bend deduction and outside setback; subtract one bend deduction per bend from the summed outside flanges for the flat length.
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. For a chart-oriented starting point, use the bend allowance chart and then run the calculator with the exact bend setup. For formula-first searches, see the bend allowance formula page.
Worked example
Verified against the live calculator
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). Use the K-factor calculator to back-solve it from 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.