Conversions use exact defined factors (1 in = 25.4 mm, 1 lbf = 4.448222 N). For lengths, the nearest fractional-inch equivalent is shown to the closest 1/64″. Thread "equivalents" below are nearest-size guides for design substitution studies only — metric and unified threads are never mechanically interchangeable.
Nearest thread size equivalents (reference)
| Metric coarse | Major dia. | Nearest UNC | Major dia. | Difference |
|---|---|---|---|---|
| M3 × 0.5 | 3.00 mm | #4-40 | 2.85 mm (0.112″) | −0.15 mm |
| M4 × 0.7 | 4.00 mm | #8-32 | 4.17 mm (0.164″) | +0.17 mm |
| M5 × 0.8 | 5.00 mm | #10-24 | 4.83 mm (0.190″) | −0.17 mm |
| M6 × 1.0 | 6.00 mm | 1/4″-20 | 6.35 mm | +0.35 mm |
| M8 × 1.25 | 8.00 mm | 5/16″-18 | 7.94 mm | −0.06 mm |
| M10 × 1.5 | 10.00 mm | 3/8″-16 | 9.53 mm | −0.47 mm |
| M12 × 1.75 | 12.00 mm | 1/2″-13 | 12.70 mm | +0.70 mm |
| M16 × 2.0 | 16.00 mm | 5/8″-11 | 15.88 mm | −0.12 mm |
| M20 × 2.5 | 20.00 mm | 3/4″-10 | 19.05 mm | −0.95 mm |
| M24 × 3.0 | 24.00 mm | 1″-8 | 25.40 mm | +1.40 mm |
Imperial screw gauges → metric
| Gauge | Major dia. (in) | Major dia. (mm) | Common threads (TPI) | Nearest metric | Difference |
|---|---|---|---|---|---|
| #0 | 0.060″ | 1.52 mm | 80 UNF | M1.6 | +0.08 mm |
| #1 | 0.073″ | 1.85 mm | 64 UNC · 72 UNF | M2 | +0.15 mm |
| #2 | 0.086″ | 2.18 mm | 56 UNC · 64 UNF | M2 | −0.18 mm |
| #3 | 0.099″ | 2.51 mm | 48 UNC · 56 UNF | M2.5 | −0.01 mm |
| #4 | 0.112″ | 2.85 mm | 40 UNC · 48 UNF | M3 | +0.15 mm |
| #5 | 0.125″ | 3.18 mm | 40 UNC · 44 UNF | M3 | −0.18 mm |
| #6 | 0.138″ | 3.51 mm | 32 UNC · 40 UNF | M3.5 | −0.01 mm |
| #8 | 0.164″ | 4.17 mm | 32 UNC · 36 UNF | M4 | −0.17 mm |
| #10 | 0.190″ | 4.83 mm | 24 UNC · 32 UNF | M5 | +0.17 mm |
| #12 | 0.216″ | 5.49 mm | 24 UNC · 28 UNF | M5 / M6 (between) | −0.49 / +0.51 mm |
| #14 | 0.242″ | 6.15 mm | rare — see 1/4″ | M6 | −0.15 mm |
Gauge major diameter follows the ASME B18 numbered-size rule: dia (in) = 0.060 + 0.013 × gauge №. "Nearest metric" is the closest standard ISO 261 coarse size by major diameter — a stock-substitution guide, not a thread compatibility claim. Above #14, imperial machine screws are specified fractionally (1/4″ and up; see the table above). Wood/self-tapping screw gauges follow the same diameter rule but have no thread-count equivalence.