Bend deduction calculator
Bend deduction is what you subtract from the sum of the outside flange dimensions to get the flat blank length. It is the number most shop drawings are built around, because outside dimensions are the ones you can measure with a caliper.
The formula
The outside setback (OSSB) is how far the outside mould line — the point where the two flat outside faces would meet if the bend were a sharp corner — sits beyond the tangent point where the bend actually begins. There is one at each end of the bend, hence 2 × OSSB. Subtracting the bend allowance leaves the material the outside dimensions over-count.
Flat blank length is then A + B − BD, where A and B are the outside flange dimensions.
Worked example
An L-bracket in 2 mm mild steel, bent 90° to a 2 mm inside radius with a K-factor of 0.4, with outside legs of 34 mm and 54 mm:
Cut a strip 84.398 mm long, mark the bend line 30 mm from one end, and bending it to 90° gives outside legs of 34 mm and 54 mm.
Things that trip people up
- Mixing dimension references. Bend deduction only works on outside dimensions. If half your drawing is dimensioned to the bend line, convert it first or use bend allowance.
- Assuming the radius you asked for. In air bending the inside radius is set by the die opening, not by what the drawing says. Ask the shop what die they will use, or measure a sample.
- Using a textbook K-factor. 0.4 is a reasonable starting point for mild steel, but the real value depends on your material and tooling. Measure it once and reuse it.
- Deductions stacking across bends. Each bend removes its own deduction. A four-bend box needs four of them applied in the right places, which is where doing it by hand stops being practical.
Frequently asked questions
What is bend deduction?
Bend deduction is the amount you subtract from the sum of the two outside flange dimensions to get the flat blank length. It exists because the outside dimensions of a folded part add up to more than the material actually present.
What is the bend deduction formula?
BD = 2 × OSSB − BA, where OSSB = (R + T) × tan(angle / 2) is the outside setback and BA = (π / 180) × angle × (R + K × T) is the bend allowance.
Can bend deduction be negative?
Yes. At small bend angles, or with a large inside radius relative to thickness, the bend allowance exceeds twice the setback and the deduction goes negative — meaning the flat blank is longer than the sum of the outside dimensions.
Should I use bend deduction or bend allowance?
Use bend deduction when your flanges are dimensioned to the outside mould line, which is how most shop drawings and calipered measurements work. Use bend allowance when they are dimensioned to the bend line. Both give the same flat length.
Let the editor stack the deductions
Dimension the folded box the size you want it and Coobic back-solves the flat pattern across every bend, then exports it as DXF.
Open the editor