NIST SP 811
Convert metric and imperial length, mass and force values with exact, traceable conversion factors.
NIST SP 811 · Current source · unit: SI / inch
Factors are exact where an international agreement defines them. Mass and force are different quantities, so they are listed separately.
| Length unit | Symbol | In metres (exact) | In inches (exact) | In millimetres | Common use | Note |
|---|---|---|---|---|---|---|
| Metre | m | 1 | 39.37007874 | 1000 | SI base unit | Exact by definition |
| Kilometre | km | 1000 | 39370.0787402 | 1000000 | Road and site distances | Exact by definition |
| Centimetre | cm | 0.01 | 0.3937007874 | 10 | Short metric sizes | Exact by definition |
| Millimetre | mm | 0.001 | 0.03937007874 | 1 | Engineering default | Exact by definition |
| Micrometre | µm | 0.000001 | 0.000039370079 | 0.001 | Tolerance and coating thickness | Exact by definition |
| Nanometre | nm | 0.000000001 | 0.00000003937007874 | 0.000001 | Surface finish and coatings | Exact by definition |
| Inch | in | 0.0254 | 1 | 25.4 | Imperial drawings and fasteners | Exact: 1 in = 25.4 mm |
| Foot | ft | 0.3048 | 12 | 304.8 | Imperial drawings | Exact: 1 ft = 12 in |
| Yard | yd | 0.9144 | 36 | 914.4 | Imperial drawings | Exact: 1 yd = 3 ft |
| Mile | mi | 1609.344 | 63360 | 1609344 | Imperial road lengths | Exact: 1 mi = 5280 ft |
| Nautical mile | NM | 1852 | 72913.3858268 | 1852000 | Marine and aviation | Exact by international agreement |
NIST SP 811 · Current source · unit: kg / lbf / N
Force and mass are different quantities. These factors make the difference explicit.
| Mass / force unit | Symbol | In kg | In lbf | In newton (N) | Common use | Note |
|---|---|---|---|---|---|---|
| Kilogram | kg | 1 | 2.2046226218 | 9.80665 | SI base unit | Exact by definition |
| Gram | g | 0.001 | 0.0022046226 | 0.00980665 | Small parts and powder | Exact by definition |
| Tonne (metric) | t | 1000 | 2204.6226218 | 9806.65 | Shipment and press capacity | Exact by definition |
| Pound mass | lb | 0.45359237 | 1 | 4.4482216153 | Imperial drawings | Exact: 1 lb = 0.45359237 kg |
| Ounce | oz | 0.028349523125 | 0.0624279606 | 0.2780138508 | Small quantities | 1 lb = 16 oz |
| Kilonewton | kN | — | 224.8089438 | 1000 | Press and clamping forces | 1 kN = 1000 N |
| Newton | N | — | 0.2248089438 | 1 | General force unit | SI derived unit |
| Kilogram-force | kgf | — | 2.2046226218 | 9.80665 | Legacy tooling and fixtures | 1 kgf = 9.80665 N exactly |
| Pound-force | lbf | — | 1 | 4.4482216153 | Imperial tooling | 1 lbf = 4.4482216153 N exactly |
The conversion factors are exact where an international agreement defines them, so 1 inch is exactly 25.4 mm and 1 pound-force is exactly 4.4482216153 newtons. Mass and force are different quantities, so the table keeps them apart and shows the newton equivalent of every mass and force unit.
Reference source
NIST SP 811
Yes, by international agreement. The inch was defined as exactly 25.4 millimetres in 1959, so the conversion is exact and not a rounded working value. The foot is exactly 12 inches and the yard exactly 3 feet.
They are force units, not mass units. One kilogram-force is the force produced by one kilogram under standard gravity, which is exactly 9.80665 newtons. One pound-force is exactly 4.4482216153 newtons. A drawing written in kgf is describing a force, even though the unit looks like a mass.
Because the Celsius and Fahrenheit scales have different zero points, so the relationship is a straight line rather than a straight line through the origin. The formula is F equals C multiplied by 1.8 and then add 32. Kelvin and Rankine are absolute scales and do convert by a simple factor.
No, and the difference is larger than most people expect. A pound-mass is 0.45359237 kilograms, so a drawing written in pounds describes a part roughly 2.2 times lighter than the same number of millimetres would suggest. Convert the values, not just the unit labels.
Use newton-metres as the working unit. One pound-force foot is 1.3558179 newton-metres and one kilogram-force metre is 9.80665 newton-metres. Torque is a moment, so the length unit and the force unit both change the number, which is where most conversion errors come from.
The SI units and the inch are exact by definition, so those factors never change. The pound and the pound-force are also exact by international agreement. Values quoted for pound-force come from the standard gravity definition and are constant as well.
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