Sheet Metal Bend Allowance, K-Factor & Bend Deduction Calculator
To calculate a sheet metal flat pattern for press brake bending, compute the Bend Allowance along the neutral axis as BA = (π / 180) × A × (R + K × T) and the Outside Setback as OSSB = tan(A / 2) × (R + T), then subtract the Bend Deduction BD = 2 × OSSB - BA from the sum of outside flange lengths.
Interactive Sheet Metal Bend Allowance (BA), K-Factor & Bend Deduction Calculator
Calculate developed neutral-axis arc length (BA), outside setback (OSSB), bend deduction (BD), flat blank length, and air-bending V-die tonnage.
How Does the Neutral Axis Shift (K-Factor = t / T) Govern Sheet Metal Flat Patterns?
When a flat sheet of metal of thickness T is formed over a press brake V-die, the inner surface adjacent to the punch nose is compressed while the outer surface is stretched in tension. Separating these two zones is an imaginary plane called the Neutral Axis that neither stretches nor compresses—its arc length remains identical before and after bending. Under ISO International Metrology Standard, DIN 6935, and Machinery's Handbook (Industrial Press) plastic deformation mechanics, as the outer fibers yield and thin out, the neutral axis migrates inward toward the inside bend radius R to a distance t from the inside face. The K-Factor is simply the dimensionless ratio of that neutral-axis depth to the total sheet thickness: K = t / T.
In elastic theory for very large sweep radii (R > 4T), the neutral axis sits near the center of the sheet (K ≈ 0.50). However, during standard press brake air bending where the inside radius equals the sheet thickness (R ≈ 1.0T), plastic radial compression shifts the neutral axis inward to K = 0.38 to 0.44 depending on material ductility and V-die opening. Leaving your CAD software at a generic default of K = 0.50 overestimates the Bend Allowance and underestimates the Bend Deduction, causing every 90° formed flange to grow oversized by 0.25 to 0.60 mm per bend.
| Sheet Gauge & Thickness (T) | Inside Radius (R = 1T) | K-Factor | Bend Allowance (BA) | Setback (OSSB) | Bend Deduction (BD) | Rec. V-Die (8T) | Air Force (CRS) |
|---|---|---|---|---|---|---|---|
| 16 GA (0.0598" / 1.52 mm) | 0.060" (1.52 mm) | 0.38 | 0.1299" (3.30 mm) | 0.1198" (3.04 mm) | 0.1097" (2.79 mm) | 0.472" (12 mm) | 4.8 Tons/ft |
| 14 GA (0.0747" / 1.90 mm) | 0.075" (1.90 mm) | 0.38 | 0.1624" (4.12 mm) | 0.1497" (3.80 mm) | 0.1370" (3.48 mm) | 0.630" (16 mm) | 5.9 Tons/ft |
| 12 GA (0.1046" / 2.66 mm) | 0.105" (2.67 mm) | 0.40 | 0.2307" (5.86 mm) | 0.2096" (5.32 mm) | 0.1885" (4.79 mm) | 0.787" (20 mm) | 9.2 Tons/ft |
| 11 GA (0.1196" / 3.04 mm) | 0.120" (3.05 mm) | 0.40 | 0.2636" (6.70 mm) | 0.2396" (6.09 mm) | 0.2156" (5.48 mm) | 0.984" (25 mm) | 10.1 Tons/ft |
| 10 GA (0.1345" / 3.42 mm) | 0.135" (3.43 mm) | 0.42 | 0.3008" (7.64 mm) | 0.2695" (6.85 mm) | 0.2382" (6.05 mm) | 1.102" (28 mm) | 11.5 Tons/ft |
| 3/16" (0.1875" / 4.76 mm) | 0.188" (4.78 mm) | 0.42 | 0.4190" (10.64 mm) | 0.3755" (9.54 mm) | 0.3320" (8.43 mm) | 1.575" (40 mm) | 15.8 Tons/ft |
| 1/4" (0.2500" / 6.35 mm) | 0.250" (6.35 mm) | 0.44 | 0.5655" (14.36 mm) | 0.5000" (12.70 mm) | 0.4345" (11.04 mm) | 2.000" (50 mm) | 21.8 Tons/ft |
What Is the Difference Between Bend Allowance (BA) and Bend Deduction (BD)?
Fabricators and CAD designers frequently confuse Bend Allowance (BA) and Bend Deduction (BD) because both parameters adjust flat blank length, but they measure from opposite geometric references. Bend Allowance is the true arc length of the neutral axis through the bend zone: BA = (π / 180) × A × (R + K × T), where A is the bend angle swept by the flange (90° for a right-angle bracket). You add BA only when summing the flat straight leg lengths measured up to the tangent points where the radius begins: L_flat = Leg_1(tangent) + Leg_2(tangent) + BA.
Because engineering blueprints almost universally dimension sheet metal brackets to their virtual sharp outside corners (Flange_1 and Flange_2), adding the two outside flange dimensions counts the corner thickness twice. To find how much material to subtract from the sum of outside flange dimensions, you first compute the Outside Setback—the tangent distance from the virtual sharp apex to the bend tangent line: OSSB = tan(A / 2) × (R + T)—and then calculate the Bend Deduction: BD = (2 × OSSB) - BA. Your laser or turret-punch flat blank length is then L_flat = Flange_1 + Flange_2 - BD.
| Sheet Metal Alloy | R < 1T (Tight Air / Bottoming) | R = 1T to 2T (Standard Air Bend) | R > 2T (Large Radius Bend) | Recommended V-Die | Press Brake Shop Notes |
|---|---|---|---|---|---|
| Aluminum 5052-H32 (Soft) | 0.33 – 0.36 | 0.38 – 0.41 | 0.42 – 0.45 | 6T – 8T | Low springback (~1.0°–1.5°) |
| Aluminum 6061-T6 (Tempered) | 0.35 – 0.38 | 0.40 – 0.43 | 0.44 – 0.47 | 8T – 10T (Min R ≥ 1.5T) | Cracks if R < 1T! Use 5052 for tight bends |
| Cold Rolled Steel (CRS 1008/1018) | 0.36 – 0.39 | 0.40 – 0.44 | 0.45 – 0.48 | 8T Standard | Baseline press brake reference (1.5°–2.0° springback) |
| Hot Rolled P&O Steel (A36) | 0.38 – 0.41 | 0.42 – 0.45 | 0.46 – 0.49 | 8T – 10T | Grain direction sensitive on heavy plate |
| 304 / 316 Stainless Steel | 0.40 – 0.43 | 0.44 – 0.47 | 0.47 – 0.50 | 8T – 10T (1.5x Tonnage) | High work hardening & 2.5°–4.0° springback |
| C260 Cartridge Brass (Half-Hard) | 0.35 – 0.38 | 0.39 – 0.42 | 0.43 – 0.46 | 6T – 8T | Smooth forming, minimal galling |
How Do You Reverse-Calculate the Exact K-Factor from a Physical Press Brake Test Coupon?
In precision sheet metal job shops audited under ASME Geometric & Mechanical Standards and NIST (National Institute of Standards and Technology) quality systems, relying on textbook K-factor tables is secondary to empirical shop-floor coupon testing because in air bending, the actual inside bend radius R is governed by the V-die opening width (typically R ≈ 0.16 × V for mild steel, 0.14 × V for aluminum, and 0.20 × V for 304 stainless) rather than the punch tip radius! To calibrate your CAD K-factor table for a specific press brake die, shear a precision rectangular test coupon of known flat length L_flat and thickness T, bend it to 90°, and measure the outside flange lengths F_1, F_2 and inside radius R using radius gauges.
Next, compute your measured Bend Deduction BD_exp = F_1 + F_2 - L_flat and measured Bend Allowance BA_exp = 2 × (R + T) - BD_exp, then plug BA_exp into the inverse K-factor equation: K = [(180 × BA_exp) / (π × A) - R] / T. When laying out fastener holes near a bend line, always keep the hole edge at least 2.0 × T + R away from the bend tangent to prevent oval hole distortion, or size clearance and countersunk holes using our ISO 273 & ASME Clearance Hole Chart, 82° & 100° Flat Head Screw Countersink Chart, CNC Countersink Depth Calculator, and Roll Form Tap Drill Chart.