Metric Conversion Charts
Fastener Metric Conversion Charts
Quick-reference tables for converting between US customary (inch) and metric (mm) fastener sizes. Includes bolt diameter, wrench size, and length conversions.
Bolt Diameter — Inch to Metric Equivalents
| US Size | Decimal (in) | Metric (mm) | Nearest Metric Bolt | Metric Decimal (mm) |
|---|---|---|---|---|
| #4 | 0.112 | 2.845 | M3 | 3.000 |
| #6 | 0.138 | 3.505 | M3.5 | 3.500 |
| #8 | 0.164 | 4.166 | M4 | 4.000 |
| #10 | 0.190 | 4.826 | M5 | 5.000 |
| #12 | 0.216 | 5.486 | M5.5 | 5.500 |
| 1/4" | 0.250 | 6.350 | M6 | 6.000 |
| 5/16" | 0.3125 | 7.938 | M8 | 8.000 |
| 3/8" | 0.375 | 9.525 | M10 | 10.000 |
| 7/16" | 0.4375 | 11.112 | M12 | 12.000 |
| 1/2" | 0.500 | 12.700 | M12 | 12.000 |
| 9/16" | 0.5625 | 14.288 | M14 | 14.000 |
| 5/8" | 0.625 | 15.875 | M16 | 16.000 |
| 3/4" | 0.750 | 19.050 | M20 | 20.000 |
| 7/8" | 0.875 | 22.225 | M22 | 22.000 |
| 1" | 1.000 | 25.400 | M24 | 24.000 |
| 1-1/8" | 1.125 | 28.575 | M30 | 30.000 |
| 1-1/4" | 1.250 | 31.750 | M30 | 30.000 |
| 1-1/2" | 1.500 | 38.100 | M36 | 36.000 |
Wrench & Socket Size — Inch to Metric
Common wrench sizes and their closest metric equivalents. Sizes within 0.5mm may interchange but are not recommended for regular use — always use the correct wrench to prevent rounding.
| Inch Size | Metric (mm) | Difference | Inch Size | Metric (mm) | Difference |
|---|---|---|---|---|---|
| 1/4" | 6mm (6.350) | +0.350mm | 3/4" | 19mm (19.050) | +0.050mm |
| 5/16" | 8mm (7.938) | −0.062mm | 13/16" | 21mm (20.638) | −0.362mm |
| 3/8" | 10mm (9.525) | −0.475mm | 7/8" | 22mm (22.225) | +0.225mm |
| 7/16" | 11mm (11.112) | +0.112mm | 15/16" | 24mm (23.812) | −0.188mm |
| 1/2" | 13mm (12.700) | −0.300mm | 1" | 25mm (25.400) | +0.400mm |
| 9/16" | 14mm (14.288) | +0.288mm | 1-1/16" | 27mm (26.988) | −0.012mm |
| 5/8" | 16mm (15.875) | −0.125mm | 1-1/8" | 29mm (28.575) | −0.425mm |
| 11/16" | 17mm (17.463) | +0.463mm | 1-1/4" | 32mm (31.750) | −0.250mm |
Length Conversions
Inches to Millimeters
Multiply inches × 25.4 = mm
| 1/4" | 6.35mm |
| 3/8" | 9.53mm |
| 1/2" | 12.70mm |
| 5/8" | 15.88mm |
| 3/4" | 19.05mm |
| 1" | 25.40mm |
| 1-1/2" | 38.10mm |
| 2" | 50.80mm |
| 3" | 76.20mm |
| 4" | 101.60mm |
Millimeters to Inches
Multiply mm × 0.03937 = inches
| 5mm | 0.197" |
| 6mm | 0.236" |
| 8mm | 0.315" |
| 10mm | 0.394" |
| 12mm | 0.472" |
| 16mm | 0.630" |
| 20mm | 0.787" |
| 25mm | 0.984" |
| 30mm | 1.181" |
| 50mm | 1.969" |
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