12 gauge mild steel bend allowance and bend deduction
At a 90° bend with an inside radius of one thickness (0.1046 in / 2.66 mm) and K = 0.43, 12 gauge mild steel (0.1046 in / 2.66 mm) has a bend allowance of 0.2350 in (5.97 mm) and a bend deduction of 0.1834 in (4.66 mm).
| Bend angle | Included angle | BA (in) | BD (in) | BA (mm) | BD (mm) |
|---|---|---|---|---|---|
| 30° | 150° | 0.0783 | 0.0338 | 1.99 | 0.86 |
| 45° | 135° | 0.1175 | 0.0558 | 2.98 | 1.42 |
| 60° | 120° | 0.1566 | 0.0849 | 3.98 | 2.16 |
| 90° | 90° | 0.2350 | 0.1834 | 5.97 | 4.66 |
| 120° | 60° | 0.3133 | 0.4114 | 7.96 | 10.45 |
| 135° | 45° | 0.3524 | 0.6577 | 8.95 | 16.70 |
| Inside radius | R / T | K-factor | BA (in) | BD (in) | BA (mm) | BD (mm) |
|---|---|---|---|---|---|---|
| 0.0523 in | 0.5 | 0.38 | 0.1446 | 0.1692 | 3.67 | 4.30 |
| 0.1046 in | 1 | 0.43 | 0.2350 | 0.1834 | 5.97 | 4.66 |
| 0.1569 in | 1.5 | 0.43 | 0.3171 | 0.2059 | 8.05 | 5.23 |
| 0.2092 in | 2 | 0.43 | 0.3993 | 0.2283 | 10.14 | 5.80 |
| 0.3138 in | 3 | 0.43 | 0.5636 | 0.2732 | 14.31 | 6.94 |
Practical notes
The angle in these formulas is the bend angle — how far the flange turns away from flat. The included angle between the two flanges is 180° minus the bend angle, so a 90° bend is also 90° included, but a 135° bend leaves a 45° included angle.
K-factor values here come from a generic air-bending table that depends on R/T (about 0.38 below R = T, 0.43 from T to 3T, 0.50 above 3T for mild steel). They are approximations: tooling, die width, grain and temper all shift the neutral axis. Bend a test strip, measure the flanges and back-calculate K before cutting a production run.
A V-die opening of about 8 × T (0.8368 in / 21.25 mm) is the usual starting point. In air bending the inside radius forms at roughly 15–17% of the die opening for mild steel, so the radius you actually get depends on the die more than the punch — recompute with that radius if it differs from the R = T used above.
Springback for mild steel at modest radii is typically about 0.5–2°, increasing as R/T grows; overbend to compensate. It changes the angle, not the bend allowance calculation.
Cold-rolled mild steel bends well in any direction at R ≈ T on thin sheet; about 1 × T is a common minimum inside radius, and thin sheet will often go tighter. On hot-rolled plate, bend across the rolling direction and expect a less consistent K-factor.
FAQ
What is the bend deduction for 12 gauge mild steel at 90°?
With R = T (0.1046 in / 2.66 mm) and K = 0.43, the bend deduction is 0.1834 in (4.66 mm). For a part with two 2 in flanges measured to the outside, the flat blank is 2 in + 2 in − 0.1834 in = 3.8166 in.
What K-factor should I use for 12 gauge mild steel?
For air bending mild steel at R = T, a K-factor of about 0.43 is a typical starting value; below R = T it drops to about 0.38, and above 3 × T it approaches 0.50. Real values depend on tooling and material lot, so confirm with a test bend.
How thick is 12 gauge steel?
Under the Manufacturers' Standard Gauge, 12 gauge sheet steel is 0.1046 in (2.66 mm) thick. Galvanized and stainless gauges differ slightly at the same number.
Is the angle in the bend allowance formula the bend angle or the included angle?
It is the bend angle, the amount the flange is turned from flat. If your drawing gives the included angle between flanges, use 180° minus that value: a 45° included angle is a 135° bend.
Sources: Manufacturers' Standard Gauge · Bend allowance formula BA = A·(π/180)·(R + K·T); bend deduction BD = 2·tan(A/2)·(R + T) − BA (Machinery's Handbook, sheet metal bending) · Generic air-bending K-factor table by R/T (approximate)
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