Bend deduction, and why it sizes your blank
Bend deduction (BD) is the number you subtract from the sum of a parts outside flange dimensions to get the flat blank length. Measure the outside legs, add them up, subtract the bend deduction for each bend, and you have the strip of material to cut before forming. It is the press-brake operators shortcut to a correct flat pattern.
The bend deduction formula
BD = 2 x OSSB – BA. The outside setback OSSB = (R + T) x tan(A/2), and the bend allowance BA = (A x pi/180) x (R + K x T), where A is the bend angle, R the inside radius, T the thickness, and K the K-factor. This tool computes all three and then applies them to your flange lengths.
Flat blank length
For a single bend joining two flanges, flat length = A + B – BD, using outside flange dimensions. Enter your two legs above and the calculator returns the blank size directly — the actual decision: how long to shear the strip.
Multiple bends
Each bend removes its own deduction. For a part with several bends, total flat length = sum of outside dimensions – sum of all bend deductions. For complex multi-bend parts, the Flat Pattern calculator chains the bends for you.
Bend deduction vs bend allowance
| Method | How you use it |
|---|---|
| Bend deduction | Subtract from summed OUTSIDE dimensions |
| Bend allowance | Add to flange lengths measured to the bend lines |
Both land on the same blank length; pick the one that matches how your drawing is dimensioned.
Worked example
A 90-degree bend in 2 mm steel, R = 3 mm, K = 0.33: OSSB = (3 + 2) x tan(45) = 5 mm, BA = 5.75 mm, so BD = 2 x 5 – 5.75 = 4.25 mm. Two 50 mm outside flanges then need a blank of 50 + 50 – 4.25 = 95.75 mm.
FAQ
Why is bend deduction larger for sharper angles?
Sharper bends pull the outside corner further past the bend, so more material must be removed. As the angle approaches a tight fold, both setback and deduction grow.
Are flange dimensions inside or outside?
Outside. Bend deduction is defined against the outside mold-line dimensions of the formed part.