Bend Allowance Calculator
Bend allowance, bend deduction and outside setback for a sheet-metal bend, from thickness, bend angle, inside radius and K-factor.
- Rev
- 2026-08-01
- Formulas
- 4
- Assumptions
- 4
Bending stretches the outside of the material and compresses the inside. Somewhere between the two is a surface whose length does not change — the neutral axis. Bend allowance is the arc length of that neutral axis through the bend, and it is what turns a folded part into a flat blank.
Work in one unit system throughout: enter thickness and radius in the same unit you want the answer in. The angle is always in degrees.
Swept from flat — a right angle is 90°.
0.44 is the air-bending default.
Bend allowance (BA)
3.393mm
Bend deduction (BD)
2.607mm
Outside setback
3.000mm
Neutral radius
2.160mm
R / t
1.00
Flat length (L)
—
Enter both legs
Substitution
BA = 90.0 × (π/180) × (1.500 + 0.440 × 1.500) = 3.393 mm
OSSB = tan(90.0/2) × (1.500 + 1.500) = 3.000 mm
BD = 2 × 3.000 − 3.393 = 2.607 mm
Assumptions
A number without its assumptions is not citable. These are the conditions the result is valid under.
- K = 0.44 is the default here because it is the long-standing shop value for air-bent mild steel at a moderate R/t. It is a starting point, not a constant: K rises with R/t, from roughly 0.33 at R/t below 1 to about 0.5 at high R/t.
- The formulas are geometric. They do not model springback, so the punch angle needed to reach a given finished angle is always larger than A.
- Valid for A below 180°. Bends approaching 180° (hems) are a separate calculation.
- Bending across the grain is assumed. Bending with the grain needs a larger inside radius in most alloys and can crack at the tabulated minimum.
Formulas
What the tool computes, written out. Every symbol is defined.
Bend allowance
BA = A × (π / 180) × (R + K × T)
- BA
- bend allowance — arc length of the neutral axis
- A
- bend angle in degrees, swept from flat (a 90° bend uses A = 90)
- R
- inside bend radius
- T
- material thickness
- K
- K-factor — neutral-axis position as a fraction of T, measured from the inside surface
Outside setback
OSSB = tan(A / 2) × (R + T)
The distance from the bend tangent line to the outside mold line, per leg.
Bend deduction
BD = 2 × OSSB − BA
Flat blank length
L = leg₁ + leg₂ − BD
Legs measured to the outside mold line. Using tangent-to-tangent legs instead, L = leg₁ + leg₂ + BA — the two are equivalent.
Questions
6 questionsWhat is bend allowance?
Bend allowance is the arc length of the neutral axis through a bend. Add it to the two flat legs measured tangent-to-tangent — that is, up to where the bend starts — and you get the flat blank length before forming.
What is the difference between bend allowance and bend deduction?
They describe the same bend from two directions. Bend allowance is added to legs measured to the bend tangent lines. Bend deduction is subtracted from legs measured to the outside mold lines, which is how parts are usually dimensioned on a drawing. Bend deduction = 2 × outside setback − bend allowance.
What K-factor should I use?
0.44 is a safe default for air-bent mild steel and aluminum at R/t of roughly 1–3. Below R/t = 1 the neutral axis shifts inward and K drops toward 0.3–0.35; at large radii it approaches 0.5. Bottoming raises K slightly and coining raises it further. If the tolerance is tight, bend a test coupon and back-calculate K from the measured flat.
Is the bend angle the included angle?
No. A is the angle swept from flat, so a right-angle bracket uses A = 90°. The included angle you would measure between the two legs is 180° − A. Entering the included angle by mistake is the single most common error with this formula.
Does the bending method change the result?
Yes, through K. Air bending leaves the material largely unrestrained and gives the lowest K; bottoming forces the material into the die; coining thins the material at the bend and pushes the neutral axis closest to the middle. Same geometry, different flat length.
Why does my calculated blank not match the shop floor?
Because the geometry is only part of the story. Springback, tool wear, punch radius drift, material lot variation and grain direction all move the result. Treat the calculated value as the starting point for a first article, then correct K from the measurement.
Standards and references
- Neutral-axis / K-factor method as used in press-brake practice; see the site's bend radius and K-factor chart for the underlying tables.