K-Factor Calculator — Sheet Metal Bend Allowance & Deduction
Calculate the exact bend allowance (BA), bend deduction (BD) and flat blank length for any sheet-metal bend. Enter material thickness, inside radius, bend angle and the K-factor for your material — and get the flat layout dimensions you need before the first cut.
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Arc length of the neutral axis through the bend zone
Neutral axis radius = IR + K × T
Bend allowance BA = (π/180) × A × (IR + K × T)
Outside setback OSSB = tan(A/2) × (IR + T)
Bend deduction BD = 2 × OSSB − BA
- 1
Neutral axis offset
0.44 × 2 = 0.88K-factor × thickness locates the neutral axis from the inside face of the bend. - 2
Neutral axis radius
3 + 0.88 = 3.88 - 3
Angle in radians
90° × π ÷ 180 = 1.570796 - 4
Bend allowance BA
1.570796 × 3.88 = 6.095
How does this calculator work?
Sheet metal K-factor (0.33–0.50) locates the neutral axis at K × T from the inside face. Bend allowance BA = (π/180) × A × (IR + K × T) gives the arc length of that axis. Subtract the bend deduction BD = 2 × tan(A/2) × (IR + T) − BA from the sum of outside leg dimensions, or add BA to the sum of flat leg dimensions, to get the flat blank length.
Formula
How this is calculated
When sheet metal is bent, the material on the inside of the bend compresses while the outside stretches. The neutral axis — the imaginary surface that neither compresses nor stretches — sits at a fraction K of the total thickness T from the inside face. The K-factor encodes this fraction: K = 0.35 for soft aluminum or copper, around 0.38–0.42 for harder aluminum alloys, 0.44 for mild steel, and 0.50 for stainless steel or very tight radii. Typical values range from 0.3 (very soft material, large radius) to 0.5 (hard material, sharp bend).
The bend allowance (BA) is the arc length of the neutral axis through the bend zone: BA = (π/180) × A × (IR + K × T). For a 90° bend in 2 mm mild steel with a 3 mm inside radius: BA = (π/2) × (3 + 0.44 × 2) = (π/2) × 3.88 ≈ 6.09 mm. The outside setback (OSSB) is the distance from the tangent of the outer radius to the virtual apex: OSSB = tan(A/2) × (IR + T). Bend deduction BD = 2 × OSSB − BA tells you how much shorter the flat blank is compared to adding the two leg lengths measured to the outer apex.
These formulas are industry-standard but assume a uniform K-factor throughout the bend — an approximation that holds well for press-brake work with conventional tooling. Very sharp bends (IR < 0.5 × T), extremely thick material, or exotic alloys may require empirical K-factor measurement from trial bends.
Frequently asked questions
Typical values: soft copper 0.33–0.35, soft 3003 aluminum 0.38, 5052 aluminum 0.40–0.42, mild steel 0.44, 304 stainless steel 0.48–0.50. The sharper the bend (IR/T ratio closer to zero), the lower the K-factor tends to be. For critical work, make a test bend and measure BA empirically, then back-calculate K.
Bend allowance (BA) is the actual material length consumed by the bend zone — you add it to your flat leg lengths to get the total blank. Bend deduction (BD) is the amount to subtract when working from outside mold-line dimensions: Flat blank = Leg1_outside + Leg2_outside − BD. Both methods give the same flat blank length if your leg dimensions are measured consistently.
A wrong K-factor shifts the neutral axis assumption, so your flat blank comes out too short or too long. For mild steel (K ≈ 0.44), using K = 0.50 instead adds about 0.12 mm per mm of thickness per bend — small for thin sheet, significant for heavy plate or multi-bend parts. Always verify with a test piece before cutting the full run.
Also known as
TG we-Calculate Editorial Team. (2026). K-Factor Calculator — Sheet Metal Bend Allowance & Deduction [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/k-factor-calculator
TG we-Calculate Editorial Team. "K-Factor Calculator — Sheet Metal Bend Allowance & Deduction." TG we-Calculate. 2026. https://we-calculate.com/calculator/k-factor-calculator.
TG we-Calculate Editorial Team, "K-Factor Calculator — Sheet Metal Bend Allowance & Deduction," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/k-factor-calculator
@misc{wecalculate_k_factor_calculator, title = {K-Factor Calculator — Sheet Metal Bend Allowance & Deduction}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/k-factor-calculator}}, year = {2026}, note = {TG we-Calculate} }
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