Compute bend allowance, bend deduction, and flat pattern length for sheet metal bends — with K-factor presets by material and a downloadable DXF flat strip.
Material & K-Factor
K-factor range: 0 (sharp inside edge) to 0.5 (neutral axis at mid-thickness). Typical range 0.33–0.50.
Bend Geometry
Bend Allowance (BA)
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Bend Deduction (BD)
—
Outside Setback (OSSB)
—
Flat Pattern Length (optional legs)
Enter the two leg lengths (from the tangent point to each edge) to compute the total flat pattern blank size.
Flat pattern blank
Leg1 + Leg2 + BA − 2×OSSB =—mm
This is the total length of the unfolded blank sheet strip.
Enter both leg lengths above to compute flat pattern and enable DXF export.
How it works
When sheet metal is bent, the inside of the bend compresses and the outside stretches. Somewhere through the thickness is a neutral axis that neither stretches nor compresses — the K-factor describes where it sits as a fraction of the thickness (K = 0 means fully at the inner surface; K = 0.5 means dead centre).
Bend Allowance (BA)
BA = (π ÷ 180) × angle × (radius + K × thickness)
Example check: thickness = 1 mm, radius = 2 mm, angle = 90°, K = 0.33
BA = (π/2) × (2 + 0.33×1) = 1.5708 × 2.33 = 3.660 mm
OSSB = tan(45°) × (2+1) = 1 × 3 = 3.000 mm
BD = 2×3 − 3.660 = 2.340 mm
Frequently asked questions
What K-factor should I use for mild steel?
A K-factor of 0.33 is the industry default for mild steel and cold-rolled steel (CRS) bent on a standard air-bending press brake. It corresponds to the neutral axis sitting approximately one-third of the way through the material thickness. For bottom-bending or coining you should measure the actual springback and iterate toward 0.38–0.42; for sharp bends below 1× thickness radius, some shops use values as low as 0.20. The presets in this calculator use conservative air-bend values that are accurate for most workshop sheet metal work.
What is the difference between bend allowance and bend deduction — which one do I use?
Bend Allowance (BA) is the arc length of material consumed by the bend itself — the length you add to your flat pattern when working from the inside tangent lines. Bend Deduction (BD) is the shortcut: it tells you how much to subtract from the sum of the two outside-face leg lengths (measured to the theoretical sharp corner). Most CAD and press-brake controllers accept either value, but they describe the same geometry from different reference points. Use BA when you measure from inside tangent lines; use BD when you measure from the outer mold line (OML) or virtual sharp corner.
How do I verify the flat pattern on a real part?
Cut a test strip at the calculated flat length using scrap material from the same stock (same heat, same thickness). Bend it on the same tooling at the same tonnage setting. Measure the resulting leg lengths with calipers. If the legs are long, your effective K-factor is too high — reduce it by 0.01–0.02 and recalculate. If the legs are short, increase K. Iterate until the measured legs match your drawing targets. Record the confirmed K-factor per material/tooling combination in your shop's bend database so you never need to test the same combination twice.