This bend allowance calculator gives you the length of sheet metal consumed by a single bend, the figure you add to your straight legs to lay out a correct flat pattern before folding. When metal is bent, the outside stretches and the inside compresses, and somewhere between them sits the neutral axis that neither stretches nor shrinks. The flat blank length depends on where that neutral axis lies, which the K-factor captures as a fraction of the material thickness. Enter the bend angle in degrees, the inside bend radius, the material thickness and your K-factor, all lengths in the same unit, and the tool returns the bend allowance from the standard formula (pi / 180) x angle x (radius + K x thickness). It also returns the bend deduction, which is what you subtract when you dimension parts to the outside mould lines rather than the tangent points, and the outside setback that links the two. Sheet metal workers, fabricators and press brake operators use these numbers to cut blanks that fold to the right finished size instead of guessing and scrapping material. Defaults show a 90 degree bend in 3 mm material over a 3 mm inside radius with a K-factor of 0.33, a realistic mild steel case. For production parts, confirm the K-factor against your own bend tests, because tooling, material and the bending method all shift where the neutral axis ends up.
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deg
mm
mm
6.27 mm
bend allowance (length added at the bend)
Bend deduction5.73 mm
Outside setback6.00 mm
Neutral axis offset (K x T)0.99 mm
All lengths use the same unit as your inputs, shown here as millimetres. The K-factor depends on material, tooling and method, so verify it against your own bend tests for production work. Estimate only.
How it works
Bend allowance is the arc length of the neutral axis through the bend. The neutral axis sits a distance of K times the thickness in from the inside face, so its radius is the inside radius plus K times thickness. Multiplying that radius by the bend angle in radians gives the arc length, which written with the angle in degrees is (pi / 180) x angle x (radius + K x thickness). The outside setback is the distance from the bend tangent point to the outside apex, equal to tan(angle / 2) x (radius + thickness). The bend deduction is then two outside setbacks minus one bend allowance: it is the amount to remove from the summed outside leg lengths to get the correct flat blank.
Worked example
Take a 90 degree bend in 3 mm sheet, an inside radius of 3 mm and a K-factor of 0.33. The neutral axis offset is 0.33 times 3, which is 0.99 mm, so the neutral radius is 3 plus 0.99, or 3.99 mm. Bend allowance is (pi / 180) x 90 x 3.99, which is 6.27 mm. The outside setback is tan(45 degrees) x (3 plus 3), which is 6.00 mm. Bend deduction is 2 times 6.00 minus 6.27, which is 5.73 mm. So to lay out this part you either add 6.27 mm between the tangent lines, or subtract 5.73 mm from the two outside leg lengths.
