By Zencalcy Editorial TeamLast reviewed About us
When sheet metal bends, the inside compresses and the outside stretches; one surface in between keeps its length. The K-factor is where that neutral axis sits as a fraction of thickness. It sets the bend allowance, which is what a flat pattern must include.
Neutral-axis position and bend allowance for sheet metal.
BA = θ × (R + K × T) BD = 2 × (R + T) × tan(θ/2) − BA
θ is the bend angle in radians, R the inside radius and T the thickness. K ranges from 0.33 for a large radius-to-thickness ratio to 0.50 for a tight bend. The bend deduction is what you subtract from the sum of the flange lengths to get the flat length.
| Symbol | Meaning |
|---|---|
| K | Neutral-axis position as a fraction of thickness. |
| R | Inside bend radius. |
| θ | Bend angle. |
BA = 1.571 × (3 + 0.84) = 6.03 mm; BD = 2 × 5 × 1 − 6.03 = 3.97 mm.
Continue with the Beam Deflection Calculator, Beam Load Calculator, Bending Stress Calculator, or the Wood Beam Span Calculator — or browse more structural & workshop and all construction calculators.
0.33–0.38 for soft materials and large radii, 0.42–0.45 for cold-rolled steel, and up to 0.50 for tight bends.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
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Content updated: September 2026