How to use the sheet metal bend allowance calculator
- Enter the material thickness, the inside bend radius and the bend angle (the angle the metal is turned from flat; 90 for a right angle corner). Use one length unit throughout.
- Enter your K-factor: where the neutral line sits as a share of the thickness from the inside face. It depends on the material and the tooling, so use your shop's figure.
- Enter the two outside flange lengths, measured on the outside of the part to the outside mold line of the bend.
- Read the flat pattern length, the bend allowance and bend deduction, and the table of where the flat length goes.
Formula
bend allowance BA = angle (radians) x (inside radius + K x thickness)
outside setback OSSB = tan(angle / 2) x (inside radius + thickness)
bend deduction BD = 2 x OSSB - BA flat length = first flange + second flange - BD
The neutral line is the line inside the metal that neither stretches nor shrinks in the bend, so its length is the same flat or bent. The bend allowance is the arc length of that line. A flat part is each flange minus the setback, plus the arc.
A 90 degree bend in 0.06 inch sheet
For a thickness of 0.06, an inside radius of 0.06, K = 0.33, a 90 degree bend and outside flanges of 2 and 3, the bend allowance is 1.5708 x (0.06 + 0.33 x 0.06) = 0.12535. The outside setback is tan 45 degrees x 0.12 = 0.12, so the bend deduction is 2 x 0.12 - 0.12535 = 0.11465 and the flat length is 5 - 0.11465 = 4.88535.
The flat pieces are 2 - 0.12 = 1.88 and 3 - 0.12 = 2.88, which with the arc of 0.12535 add to the same 4.88535. For a 135 degree bend the bend allowance is 0.18802, the setback 0.28971 and the flat length 4.60861.
Where the K-factor comes from
The K-factor is not a fixed constant of the metal. It changes with the material, the thickness, the bend radius, the angle and how the bend is made, such as air bending or bottoming. Shops usually measure it by bending test pieces. A common range is somewhere between 0.3 and 0.5, and the default here is only an example, so enter the number that matches your process.
Bend deduction is the amount to subtract from the sum of the outside lengths and bend allowance is the length of material actually used in the bend. They are two ways of writing the same flat pattern.
Check before cutting
Bend one scrap piece and measure before you cut a batch. Material grain, springback and tool wear all affect the result. If a flange is shorter than the setback, the bend would use up the whole flange, and the page tells you so. For other bending jobs, see the Conduit Offset Bend Calculator, and for the weight of the blank, the Metal Weight Calculator.
More engineering tools
For wire runs, cross sections and orbits, try the Orbital Mechanics Calculator, Wire Voltage Drop Calculator and Section Properties Calculator.
Assumptions and limits
- A single bend with a constant inside radius and an angle between 0 and 179 degrees.
- The bend angle is the angle turned from flat, so a right angle corner is 90.
- The flange lengths are outside dimensions measured to the outside mold line.
- The K-factor and radius are inputs you must know for your material and tooling.
- The example numbers are an illustration. Replace them with your own.
Frequently asked questions
What is the difference between inside and outside setback?
Outside setback is measured from the outside mold line to the tangent point of the bend, using radius plus thickness. Inside setback uses just the radius. This page uses the outside value for the flat length.
Why is the flat length shorter than the sum of the outside lengths?
The outside lengths run to the outside mold line, which cuts across the corner past the metal. The metal along the neutral line is shorter than that path, and the bend deduction takes the difference away. The deduction is never negative when the K-factor is between 0 and 1.
Does it work for several bends?
Not in one step. Calculate each bend, then subtract each bend deduction from the total of the outside lengths of the part.
How do I find my K-factor?
Bend a test piece, measure its flat blank and finished flanges, and solve for K. Material data sheets and your press brake supplier may give starting values for your tooling.