Bend Allowance Calculator. Calculate bend allowance and bend deduction for sheet metal fabrication. Enter your material thickness, bend radius, bend angle, K-factor, and optional flange lengths to get the bend allowance (BA), bend deduction (BD), and total flat length — no bend allowance chart needed. Also try the Density Calculator.
Bend allowance (BA) is calculated using the formula: BA = (π / 180) × bend_angle × (inside_radius + K-factor × thickness). The K-factor represents the position of the neutral axis within the material. This formula gives you the arc length along the neutral axis through the bend zone. See also our Section Modulus Calculator.
The K-factor is a ratio that describes the position of the neutral axis relative to the sheet thickness. It ranges from 0.1 to 0.5. A K-factor of 0.5 means the neutral axis is at the center of the material. Typical values are 0.33 for mild steel, 0.38 for aluminum, and 0.44 for stainless steel.
The K-factor depends on the material type, the bend radius relative to material thickness, and the bending method used. Softer materials like aluminum tend to have higher K-factors. When the inside radius is smaller than the material thickness, the K-factor is typically around 0.33; for larger radii, it approaches 0.5.
Bend allowance is critical for determining the correct flat blank size before bending. If you cut the blank without accounting for bend allowance, your finished part will be the wrong size. Accurate bend allowance ensures that the final bent part meets the required dimensions without wasting material. You might also find our calculate Major Principal Stress σ₁, Minor Principal Stress σ₂ & Maximum Shear Stress τmax — Mohr's Circle useful.
Bend allowance (BA) is the arc length of the neutral axis through the bend — it tells you how much material length is consumed in the bend zone. Bend deduction (BD) is the amount subtracted from the total outside mold line lengths to get the flat blank length. BD = 2 × setback − BA.
The total flat length is calculated as: L = Flange A + Flange B + Bend Allowance. The bend allowance replaces the bent section in the flat blank. This is why knowing the correct bend allowance is essential before cutting your sheet metal blank.
Setback (SB) is the distance from the bend tangent line to the apex (mold line) of the bend. It is calculated as: SB = tan(bend_angle / 2) × (inside_radius + thickness). Setback is used to locate the bend lines and to calculate bend deduction.
Yes, bend angle has a direct proportional effect on bend allowance. A larger bend angle means a longer arc through the material, resulting in a larger bend allowance. For example, a 180° bend will have exactly twice the bend allowance of a 90° bend with all other parameters the same. Check out our use the Slenderness Ratio Calculator as well.