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Calculate bend deduction, bend allowance, outside setback and flat blank length from sheet thickness, inside radius, bend angle and K-factor, in metric or imperial units.
Use this page when the drawing gives flange lengths to the bend intersection and you need the deduction that converts those dimensions into a flat blank. For reverse K-factor work from a measured part, use the dedicated K-factor calculator.
Bend deduction, also called bend compensation or flat-pattern deduction, is the amount subtracted from the sum of two mold-line flange lengths to obtain the flat blank length. For a single bend, BD = 2 × OSSB − BA, where OSSB = (R + t) × tan(α/2), BA = (π/180) × α × (R + Kt), R is inside radius, t is thickness, α is bend angle and K describes the neutral axis. If 50 mm and 30 mm flanges produce an 80 mm mold-line total and BD is 4.24 mm, the flat blank is 75.76 mm. Typical air-bending K-factors begin around 0.33-0.35, depending on material, radius-to-thickness ratio, tooling and bending method. Do not use the result blindly when the drawing dimensions stop at tangent points, because that convention requires a different calculation. Zhejiang Balford Mechatronics Co., Ltd. checks bending routes at its 12,000 m² facility and confirms critical patterns with first-article bends.
The result is a geometric flat-pattern estimate. Material direction, tooling, springback, bottoming and the press-brake controller convention can move the actual blank. Confirm the K-factor with a first-article bend.
For a single bend, the flat length is the sum of mold-line flange lengths minus bend deduction.
| Material / K | t | Inside R | BA | OSSB | BD |
|---|---|---|---|---|---|
| Mild steel / 0.33 | 1.0 mm | 1.0 mm | 2.09 mm | 2.00 mm | 1.91 mm |
| Mild steel / 0.33 | 2.0 mm | 2.0 mm | 4.18 mm | 4.00 mm | 3.82 mm |
| Mild steel / 0.33 | 3.0 mm | 3.0 mm | 6.27 mm | 6.00 mm | 5.73 mm |
| Stainless / 0.35 | 1.0 mm | 1.5 mm | 2.91 mm | 2.50 mm | 2.09 mm |
| Stainless / 0.35 | 2.0 mm | 3.0 mm | 5.81 mm | 5.00 mm | 4.19 mm |
| Aluminium / 0.33 | 2.0 mm | 2.0 mm | 4.18 mm | 4.00 mm | 3.82 mm |
Calculated for a 90-degree bend using the listed K-factor. Different tooling, bottoming or material conditions require a measured K-factor.
For 2 mm mild steel, K = 0.33, R = 3 mm and a 90° bend: BA = (π/180) × 90 × (3 + 0.33 × 2) = 5.76 mm. OSSB = (3 + 2) × tan 45° = 5.00 mm. BD = 2 × 5.00 − 5.76 = 4.24 mm. For 50 mm + 30 mm mold-line flanges, the flat length is 80.00 − 4.24 = 75.76 mm.
Bend deduction is the amount removed from the sum of two mold-line flange lengths to obtain the flat blank length. It accounts for the material consumed by the bend radius and the geometric overlap created when flanges are dimensioned to the theoretical bend intersection.
BD = 2 × OSSB − BA. OSSB is the outside setback, (R + t) × tan(α/2), and BA is the bend allowance, (π/180) × α × (R + Kt). The flat length for a single bend is flange 1 + flange 2 − BD when both flanges are measured to the mold line.
Bend allowance is the neutral-axis arc length through the bend. Bend deduction is the correction subtracted from mold-line flange lengths. They describe different steps in the same flat-pattern calculation: BA describes the arc; BD converts mold-line dimensions into a blank length.
Typical air-bending starting values are about 0.33 for mild steel and aluminium and 0.35 for stainless steel, but the actual value depends on material, radius-to-thickness ratio, tooling and bending method. Measure a trial part and reverse-calculate K when the flat pattern is critical.
The most common causes are flange dimension convention, K-factor, inside versus outside radius, bend angle definition and tooling method. Check whether the drawing dimensions stop at tangent points or extend to the theoretical mold-line intersection. The two conventions need different formulas.
Include the drawing, material, tooling, K-factor and flange convention so the blank can be checked against the intended process. Send the drawing to shawn@balford.net. We normally reply within 24 hours, China Standard Time.