Calculate bend allowance, bend deduction, flat blank length, and outside setback for sheet metal bending. Supports multiple materials and K-factors.
The **Bend Allowance Calculator** solves one of the most common sheet-metal fabrication problems: how long should the flat blank be so that, after bending, the finished part has the correct flange dimensions? Bend allowance (BA) is the arc length of the neutral axis through the bend zone. Combined with the bend deduction (BD) and outside setback (OSSB), it determines the flat-pattern layout.
Accurate flat-pattern calculations save material, reduce scrap, and eliminate trial-and-error on the press brake. The K-factor — the ratio of the neutral-axis offset to thickness — varies by material, temper, bend method, and grain direction. This calculator includes presets for mild steel, stainless steel, aluminum alloys, copper, and brass.
Enter the material thickness, bend angle, inside radius, K-factor, and flange lengths. The calculator instantly returns the bend allowance, bend deduction, flat blank length, outside setback, and neutral-axis radius. Use the angle and material tables to plan multiple bends or compare materials. The result keeps the neutral-axis geometry visible so the flat pattern can be checked against the bend setup before metal is cut.
Use this calculator when you need a reliable flat-pattern estimate before the part reaches the press brake.
It is useful for quoting, programming, fixture planning, and reducing trial-and-error when thickness, inside radius, or K-factor change from one job to the next. It also helps keep the bend assumptions consistent when different operators or shops use different setup conventions.
Bend Allowance: BA = A × (Ri + K × t) Bend Deduction: BD = 2 × OSSB − BA Outside Setback: OSSB = (Ri + t) × tan(A/2) Flat Length: L = Flange_A + Flange_B + BA − 2 × OSSB where A = bend angle (rad), Ri = inside radius, t = thickness, K = K-factor.
Result: BA = 6.09 mm, flat blank = 76.09 mm
A 2 mm mild steel sheet bent 90° with a 3 mm inside radius (K = 0.44) gives BA = (pi/2) x (3 + 0.44 x 2) ≈ 6.09 mm. With 50 mm and 30 mm mold-line flanges, the flat blank is 50 + 30 + 6.09 - 10 = 76.09 mm.
Flat-pattern work is easiest when the bend assumptions are tied to the real process. Material temper, tooling, air bending versus bottoming, and grain direction can all shift the neutral axis enough to matter, so a stored shop K-factor or bend table is often more valuable than a generic handbook number.
Most fabrication errors come from mixing dimension conventions. If flange lengths are inside, outside, or mold-line dimensions, the blank-length math changes. Another common problem is assuming the inside radius equals the punch radius; the formed radius depends on material and tooling behavior, not just the punch label.
K-factor is the ratio of the distance from the inner surface to the neutral axis to the total thickness. It ranges from 0 to 1, with 0.33–0.50 being typical for most metals.
It depends on material ductility, bend radius relative to thickness, grain direction, tooling, and bending method. That is why many shops rely on tested bend tables instead of a single universal value.
BD is the amount subtracted from the sum of mold-line flange dimensions to get the flat blank length. It accounts for the material that is effectively used up in the bend zone.
OSSB is the distance from the bend tangent line to the apex of the outside mold line. It is one of the geometric pieces used to convert flange dimensions into a flat pattern.
Springback is significant in high-strength or thick materials. Over-bend by 2–5° to compensate, and confirm the final bend with a test piece when the fit is critical.
A common rule is Ri ≥ 1 × t for ductile metals. Harder alloys may need 2–4 × t. Shop bend charts or supplier data should override the rule of thumb when the material is sensitive to cracking.