Sheet Metal Bend Calculator

How It Works

When sheet metal is bent, the material on the outside of the bend stretches and the inside compresses. The neutral axis — an imaginary line where the material neither stretches nor compresses — shifts inward during bending. The K-factor describes where this neutral axis sits relative to the material thickness.

Bend Allowance (BA): The arc length along the neutral axis through the bend. Added to flat segment lengths (measured to the inside mold line) to calculate total flat pattern.

Bend Deduction (BD): The difference between the outside setback and the bend allowance. Subtracted from flat segment lengths (measured to the outside mold line) to calculate total flat pattern.

Both methods produce the same physical flat pattern — they just use different measurement references. BA is more common in manual fabrication; BD is often used in CAD/CAM systems.

How to Use

1. Select material — the K-factor auto-fills based on material type (override if you have tested values)
2. Enter thickness — measure your actual sheet; nominal thickness can differ from actual
3. Add bend segments — for each bend, enter the preceding flat length, bend angle, and inside radius
4. Enter tail flat — the last flat portion after the final bend
5. Read the total flat pattern length — this is what you cut before bending

Understanding K-Factor

K-factor ranges from 0 (neutral axis at the inside) to 1 (neutral axis at the outside). In practice:

• 0.33 — soft materials (annealed copper, soft aluminum) and air bending
• 0.40 — medium materials (brass, cold-rolled steel) with standard tooling
• 0.44 — typical for mild steel with a standard V-die
• 0.50 — hard materials or tight radius bends (neutral axis near center)

K-factor increases with: harder material, larger radius-to-thickness ratio, and bottom bending (coining). It decreases with: softer material, tight radius, and air bending.

Tips for Metal Workers

• Test before production — cut a test piece at the calculated length, bend it, and measure. Adjust K-factor up or down to match reality. Your tooling, material batch, and process affect the actual result.
• Grain direction matters — bending perpendicular to the grain allows tighter radii without cracking. Bending parallel to grain requires larger minimum radii (typically 1-2× thickness more).
• Minimum bend radius — most materials crack if bent too tightly. General rule: minimum inside radius = 1× material thickness for ductile metals. Aluminum and stainless may need 1.5-2×.
• Springback compensation — this calculator gives the geometric flat pattern. In practice, you need to overbend slightly because the material springs back. Springback angle depends on material yield strength and radius/thickness ratio.
• Order of operations — when making multiple bends, consider which bend to make first to avoid tool interference. Work from the inside out when possible.

FAQ

Which method should I use — BA or BD?

Use BA (bend allowance) if you measure flat segments to the inside of the bend (most common in manual work). Use BD (bend deduction) if your dimensions reference the outside mold line (common in engineering drawings and CAD). The cut length will be the same either way.

My part is too short/long after bending. What went wrong?

Most likely your K-factor is wrong for your specific material and tooling. Make a test bend: cut a known length, bend it, measure the resulting flange lengths, and back-calculate the actual K-factor. Even small K-factor changes (±0.02) accumulate across multiple bends.

What inside radius should I use?

As a starting point, use 1× material thickness. Your brake's V-die opening determines the natural radius: typical radius ≈ die opening ÷ 8. Check your die manufacturer's specifications for the actual radius produced.

Does this calculator handle springback?

No. This calculator gives the flat pattern for the desired geometry. Springback (the material's tendency to partially unbend) must be compensated separately by overbending. Typical springback for mild steel at 90° is 1-3°.

Can I use this for aluminum?

Yes. Aluminum alloys (6061-T6, 5052-H32) work well with K-factors around 0.33-0.36. Note that tempered aluminum (T6) requires larger minimum bend radii than annealed (O temper) to avoid cracking.

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Last reviewed: June 2026