Gauge to millimetres for steel, aluminium and stainless, plus the die opening, natural inside radius, minimum flange and air bend tonnage that go with each thickness. One page, no sign-up.
Die openings follow the common workshop rule of roughly 6–8 × thickness. Tonnage is mild steel at Rm 410 N/mm², air bending.
| Thickness | Usual V-die | V ÷ t | Natural inside radius | Minimum flange | Air bend kN/m | Air bend t/m |
|---|---|---|---|---|---|---|
| 1 mm | V6 | 6.0× | 0.96 mm | 4.9 mm | 97 | 9.9 |
| 1.5 mm | V10 | 6.7× | 1.60 mm | 8.0 mm | 131 | 13.4 |
| 2 mm | V12 | 6.0× | 1.92 mm | 9.8 mm | 194 | 19.8 |
| 2.5 mm | V16 | 6.4× | 2.56 mm | 12.9 mm | 227 | 23.2 |
| 3 mm | V22 | 7.3× | 3.52 mm | 17.3 mm | 238 | 24.3 |
| 4 mm | V26 | 6.5× | 4.16 mm | 20.9 mm | 358 | 36.5 |
| 5 mm | V32 | 6.4× | 5.12 mm | 25.8 mm | 455 | 46.4 |
| 6 mm | V40 | 6.7× | 6.40 mm | 32.0 mm | 524 | 53.4 |
| 8 mm | V50 | 6.2× | 8.00 mm | 40.5 mm | 745 | 76.0 |
| 10 mm | V63 | 6.3× | 10.08 mm | 51.0 mm | 924 | 94.2 |
| 12 mm | V80 | 6.7× | 12.80 mm | 64.0 mm | 1048 | 106.8 |
Your tooling list, the radius the drawing calls for, and the material all move die choice. Treat this as a starting point, not a specification — and check the rated load of the tooling you pick.
Gauge is not one scale — the same number means a different thickness depending on the metal. Steel follows the Manufacturers' Standard Gauge (MSG), aluminium follows Brown & Sharpe, and stainless has its own sheet standard. That is why a 16 gauge job is 1.519 mm in steel but 1.290 mm in aluminium.
| Gauge | Steel (mm) | Steel (in) | Aluminium (mm) | Stainless (mm) |
|---|---|---|---|---|
| 7 ga | 4.554 | 0.1793 | 3.665 | 4.763 |
| 8 ga | 4.176 | 0.1644 | 3.264 | 4.366 |
| 9 ga | 3.797 | 0.1495 | 2.906 | 3.969 |
| 10 ga | 3.416 | 0.1345 | 2.588 | 3.571 |
| 11 ga | 3.038 | 0.1196 | 2.304 | 3.175 |
| 12 ga | 2.657 | 0.1046 | 2.052 | 2.779 |
| 13 ga | 2.278 | 0.0897 | 1.829 | 2.382 |
| 14 ga | 1.897 | 0.0747 | 1.628 | 1.984 |
| 15 ga | 1.709 | 0.0673 | 1.450 | 1.786 |
| 16 ga | 1.519 | 0.0598 | 1.290 | 1.588 |
| 17 ga | 1.367 | 0.0538 | 1.151 | 1.429 |
| 18 ga | 1.214 | 0.0478 | 1.024 | 1.270 |
| 19 ga | 1.062 | 0.0418 | 0.912 | 1.113 |
| 20 ga | 0.912 | 0.0359 | 0.813 | 0.953 |
| 21 ga | 0.836 | 0.0329 | 0.724 | 0.874 |
| 22 ga | 0.759 | 0.0299 | 0.643 | 0.795 |
| 23 ga | 0.683 | 0.0269 | 0.574 | 0.714 |
| 24 ga | 0.607 | 0.0239 | 0.511 | 0.635 |
| 25 ga | 0.531 | 0.0209 | 0.455 | 0.556 |
| 26 ga | 0.455 | 0.0179 | 0.404 | 0.478 |
| 27 ga | 0.417 | 0.0164 | 0.361 | 0.437 |
| 28 ga | 0.378 | 0.0149 | 0.320 | 0.396 |
| 29 ga | 0.343 | 0.0135 | 0.287 | 0.358 |
| 30 ga | 0.305 | 0.0120 | 0.254 | 0.318 |
Steel: Manufacturers' Standard Gauge for uncoated sheet. Aluminium: Brown & Sharpe (B&S/AWG). Stainless: stainless sheet gauge standard. Galvanised sheet is thicker than the equivalent uncoated gauge because the coating is included — do not use the steel column for galv. Mill tolerance applies to all of them, so measure the actual coil before calculating a flat pattern.
In air bending the inside radius is set mainly by the die opening — roughly 16% of it — because the sheet spans the die and forms a natural arc. The punch tip only dictates the radius when you bottom or coin. So the die you choose determines the radius you get, which in turn determines the K-factor and the flat pattern. Choose the die first, then calculate the development, not the other way around.
Below about 6 × t the tonnage climbs steeply and you risk marking or cracking the material on a radius tighter than it likes. Above about 10 × t you get low tonnage but a soft, large radius and more springback, and short flanges stop reaching across the shoulder. The 6–8 × band is where most shops live because it balances all three.
A flange must reach across the die shoulder or the bend never forms — the part lifts out of the die. The practical limit is about 0.65 × the die opening plus one thickness. If a drawing asks for a flange shorter than that, you need a narrower die, special tooling, or a different bend sequence, and it is far cheaper to find that out before the sheet is cut.
3.416 mm, on the Manufacturers' Standard Gauge for uncoated sheet steel. In aluminium, 10 gauge is 2.588 mm — a completely different thickness for the same gauge number.
Commonly V22, which is about 7 × thickness and gives a natural inside radius near 3.5 mm. If the drawing needs a tighter radius you will need a narrower die and more tonnage; check the tooling's rated load before you do.
In air bending, the die — roughly 0.16 × the opening. The punch tip governs only when bottoming or coining. This surprises a lot of people who have swapped punches trying to change a radius.
No. Galvanised gauge thicknesses include the zinc coating, so they run slightly thicker than the equivalent uncoated gauge. Use your supplier's galvanised chart, or measure it.
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