About the Sheet Metal Bend Allowance Calculator
This sheet metal bend allowance calculator works out how long to cut a flat blank so that it ends up the right size after bending. Enter the material thickness, the inside bend radius, the bend angle and the K-factor, plus the two flange lengths, and it returns the bend allowance (the length of the neutral axis through the bend), the bend deduction, the outside and inside setbacks and the developed flat length.
It is built for press-brake operators, sheet metal fabricators, CAD users setting up sheet metal rules, and engineering students. Flanges can be dimensioned to the outside of the part (the outside mold line, typical on drawings) or to the inside, and the table shows how the flat length changes for other K-factors so you can see how sensitive your part is.
The K-factor is the position of the neutral axis as a fraction of thickness from the inside surface. It depends on the material, the radius-to-thickness ratio and the bending method; values between about 0.3 and 0.5 are typical, with 0.44–0.45 a common starting point for air bending steel when the radius is close to the thickness. For critical parts, bend a test piece and back-calculate your shop’s K-factor.
With the default inputs, the flat blank length is 4.8957. Change any value above to recalculate instantly.
How to use the sheet metal bend allowance calculator
- 1Enter the material thickness and inside bend radius in the same unit.
- 2Enter the bend angle — the angle bent through, 90° for a right-angle bracket.
- 3Set the K-factor for your material and tooling (0.44 is a common default).
- 4Choose whether flange lengths are measured to the outside or inside, and enter both.
- 5Read the flat blank length, bend allowance and bend deduction, and check the K-factor table.
Formula and method
When sheet metal bends, the outside stretches and the inside compresses; somewhere between them is a neutral axis that keeps its original length. The K-factor locates that axis as a fraction of thickness from the inside surface, so the neutral radius is R + K·T and the bend allowance is that radius times the bend angle in radians.
The outside setback is the distance from the tangent point of the bend to the virtual sharp (where the outside surfaces would meet). Measured to the outside, the two flanges overlap by two setbacks, so the bend deduction 2·OSSB − BA is subtracted from their sum. When flanges are dimensioned to the inside, the inside setback R·tan(A/2) is removed from each and the bend allowance is added.
- BA
- Bend allowance — arc length of the neutral axis
- BD
- Bend deduction
- A
- Bend angle in degrees (angle bent through)
- R
- Inside bend radius
- T
- Material thickness
- K
- K-factor (neutral axis position ÷ thickness)
- OSSB
- Outside setback
Worked examples
90° bracket, 0.06" steel, 0.06" radius, 2" × 3" outside
BA = 90 × π/180 × (0.06 + 0.44 × 0.06) = 1.5708 × 0.0864 ≈ 0.1357 in. OSSB = tan 45° × 0.12 = 0.12 in, so BD = 0.24 − 0.1357 = 0.1043 in. The flat blank is 2 + 3 − 0.1043 ≈ 4.8957 in.
2 mm aluminum, 3 mm radius, 120° bend, K = 0.4
BA = 120 × π/180 × (3 + 0.4 × 2) = 2.0944 × 3.8 ≈ 7.959 mm. OSSB = tan 60° × 5 ≈ 8.660 mm, so BD = 17.321 − 7.959 ≈ 9.362 mm. With 40 and 60 mm outside flanges, the blank is 100 − 9.362 ≈ 90.638 mm.
Inside dimensions: 16 ga (0.0598") at 90°, R = 0.03", K = 0.33
BA = 1.5708 × (0.03 + 0.33 × 0.0598) ≈ 0.0781 in. Measured to the inside, each flange loses the inside setback of 0.03 in, so the flat length is 1 + 1.5 − 0.06 + 0.0781 ≈ 2.5181 in.
Frequently asked questions
What is the difference between bend allowance and bend deduction?+
Bend allowance is the length of material in the bend itself, measured along the neutral axis; you add it to the flat flange lengths measured to the tangent points. Bend deduction is the amount you subtract from the sum of the outside flange dimensions. Both give the same flat blank.
What K-factor should I use?+
Most sheet metal falls between 0.30 and 0.50. Tight bends (inside radius smaller than thickness) in soft material tend toward 0.33–0.42, a radius about equal to thickness gives roughly 0.44, and large radii approach 0.5. The best value comes from bending a test strip and measuring.
How do I find my K-factor from a test bend?+
Cut a strip of known length, bend it 90°, and measure both outside flange lengths. The bend deduction is their sum minus the strip length. Then BA = 2·OSSB − BD, and K = (BA ÷ angle in radians − R) ÷ T.
Is the bend angle the included angle?+
No. Use the angle the sheet is bent through, measured from flat. A right-angle bracket is a 90° bend; a part with a 135° included angle between flanges is bent through 45°.
Why is the flat length shorter than the sum of the outside dimensions?+
The outside dimensions both extend to the virtual sharp corner, so the corner region is counted twice. The material actually follows a rounded bend that is shorter than two setbacks, so the bend deduction is subtracted.