Sheet-metal flat pattern estimate
Bend allowance is measured along the neutral axis. Add flange dimensions to calculate the developed length of a single bend.
Engineering calculator
Bend Allowance Calculator
Calculate bend allowance, bend deduction and the flat length of a single sheet-metal bend from thickness, inside radius and K-factor.
Bend allowance & deduction
BA = (π / 180) × θ × (R + K × T)
OSSB = (R + T) × tan(θ / 2)
BD = 2 × OSSB - BA
T is material thickness; R is the finished inside bend radius; K is the distance from the inside face to the neutral axis divided by T. The bend angle θ is the rotation from flat, in degrees. The inside included angle is 180° - θ.
Reference: SOLIDWORKS K-factor formulaFlat pattern dimensions
Outside flange dimensions: L = A + B - BD
Straight tangent lengths: L = A + B + BA
Outside dimensions end at the virtual sharp where the outer faces would intersect. Each must be at least the outside setback. Straight tangent lengths exclude the curved bend. The two dimension references are not interchangeable.
This model covers one constant-radius bend, with bend angles greater than 0° and less than 180°. It does not model hems, coining or springback compensation. Confirm the finished radius and K-factor with a test bend before releasing a flat pattern.