Many aftermarket parts are still sized in inches, from bolts and spanners to AN fittings, hose inside and outside diameters (ID/OD) and exhaust pipe. Use this tool to convert a size to millimetres, or a metric measurement back to inches, before you order.
Millimetre figures are rounded to 2 decimal places. Decimal inches are shown to 3 or 4 places so the fractions stay accurate.
Inches to millimetres
Millimetres to inches
Fraction to millimetre chart
1/16″ to 2″ in 1/16″ steps. Sizes marked Common come up most often in car work.
| Inches | Decimal in | mm | Typical use |
|---|---|---|---|
| 1/16″ | 0.0625 | 1.59 | |
| 1/8″ | 0.1250 | 3.18 | |
| 3/16″ | 0.1875 | 4.76 | Common Brake line, small fuel/vac hose |
| 1/4″ | 0.2500 | 6.35 | Common Bolts, vacuum and fuel hose |
| 5/16″ | 0.3125 | 7.94 | Common Bolts, fuel hose, -5 fitting |
| 3/8″ | 0.3750 | 9.53 | Common Bolts, fuel/heater hose, -6 fitting |
| 7/16″ | 0.4375 | 11.11 | Common Bolts and spanner size |
| 1/2″ | 0.5000 | 12.70 | Common Bolts, -8 fitting, 1/2" drive |
| 9/16″ | 0.5625 | 14.29 | Common Bolts and spanner size |
| 5/8″ | 0.6250 | 15.88 | Common Heater hose, -10 fitting |
| 11/16″ | 0.6875 | 17.46 | |
| 3/4″ | 0.7500 | 19.05 | Common Heater hose, -12 fitting |
| 13/16″ | 0.8125 | 20.64 | |
| 7/8″ | 0.8750 | 22.23 | Common Spanner size |
| 15/16″ | 0.9375 | 23.81 | |
| 1″ | 1.0000 | 25.40 | Common Radiator/breather hose |
| Inches | Decimal in | mm | Typical use |
|---|---|---|---|
| 1-1/16″ | 1.0625 | 26.99 | |
| 1-1/8″ | 1.1250 | 28.58 | |
| 1-3/16″ | 1.1875 | 30.16 | |
| 1-1/4″ | 1.2500 | 31.75 | Common Radiator hose |
| 1-5/16″ | 1.3125 | 33.34 | |
| 1-3/8″ | 1.3750 | 34.93 | |
| 1-7/16″ | 1.4375 | 36.51 | |
| 1-1/2″ | 1.5000 | 38.10 | Common Radiator hose, intercooler pipe |
| 1-9/16″ | 1.5625 | 39.69 | |
| 1-5/8″ | 1.6250 | 41.28 | |
| 1-11/16″ | 1.6875 | 42.86 | |
| 1-3/4″ | 1.7500 | 44.45 | Common Radiator hose, exhaust |
| 1-13/16″ | 1.8125 | 46.04 | |
| 1-7/8″ | 1.8750 | 47.63 | |
| 1-15/16″ | 1.9375 | 49.21 | |
| 2″ | 2.0000 | 50.80 | Common Exhaust, intake, intercooler pipe |
Metric bolt sizes in inches
The M number is the thread diameter in millimetres. Inch figures are approximate, and metric and imperial bolts are not interchangeable even when the sizes look close. Spanner sizes vary by maker, so check before you buy tools.
| Size | Thread dia. | Approx. inches | Nearest fraction | Coarse pitch mm | Fine pitch mm | Spanner mm | Typical use |
|---|---|---|---|---|---|---|---|
| M5 | 5 mm | 0.197″ | just over 3/16″ | 0.8 | — | 8 | Trim, small brackets, sensors |
| M6 | 6 mm | 0.236″ | just under 1/4″ | 1.0 | — | 10 | Common Trim, covers, small brackets |
| M8 | 8 mm | 0.315″ | just over 5/16″ | 1.25 | 1.0 | 13 (12 on many Japanese cars) | Common Engine bay, intake, brackets |
| M10 | 10 mm | 0.394″ | just over 3/8″ | 1.5 | 1.25 | 16 or 17 (14 on many Japanese cars) | Common Brackets, mounts, brake calipers |
| M12 | 12 mm | 0.472″ | just under 1/2″ | 1.75 | 1.25 / 1.5 | 18 or 19 (17 on many Japanese cars) | Common Wheel studs and nuts, suspension |
| M14 | 14 mm | 0.551″ | just under 9/16″ | 2.0 | 1.5 | 21 or 22 | Common Wheel studs and bolts, suspension |
| M16 | 16 mm | 0.630″ | just over 5/8″ | 2.0 | 1.5 | 24 | Suspension and subframe bolts |
| M18 | 18 mm | 0.709″ | just over 11/16″ | 2.5 | 1.5 | 27 | Heavy suspension, 4WD |
| M20 | 20 mm | 0.787″ | just under 13/16″ | 2.5 | 1.5 | 30 | Heavy 4WD and towing |
How many millimetres are in an inch?
Exactly 25.4 mm. To convert, multiply inches by 25.4, or divide millimetres by 25.4.