US Customary System Conversions

US Customary Imperial Conversions to Metric NIST
The bushel (bu) is a measurement of dry volume in the US Customary System

Two systems of weights and measures are derived from English Units: the US Customary System and the British Imperial System. Neither is dominant worldwide, but they do retain significant usage. For commercial and everyday use, the US Customary System is used in the United States, and the Imperial System still applies to volume and vehicle speed in the United Kingdom.

Therefore, it is important to understand US Customary and Imperial units and their conversions to metric. In fact, there have been some substantial errors in the past associated with unit-based misunderstandings. Most notably, in 1999, due to a mishap with one engineering team using English units while another used SI units, a $125 million Mars orbiter was lost to the great expanse of space.

English units and their International System of Units (SI) equivalents are listed below. Please note that the National Institute of Standards and Technology (NIST) produces Weights and Measures Publications for areas of legal metrology, laws, regulations, measurement devices, and specifications and tolerances. Much of this information was derived from NIST’s website, as well as some other sources on US Customary Units.

Length

US Customary/Imperial UnitEquivalent SI (Metric) Unit
1 inch (in)25.4 millimeters (mm)
1 in2.54 centimeters (cm)
1 foot (ft)30.48 cm
1 yard (yd)0.91 meter (m)
1 mile (mi)1.61 kilometer (km)
1 rod5 m
1 furlong (fur)201.16 m

Area

US Customary/Imperial UnitEquivalent SI (Metric) Unit
1 square inch (in2)6.45 square centimeters (cm2)
1 square foot (ft2)0.09 square meter (m2)
1 square yard (yd2)0.84 m2
1 square mile (mi2)2.59 square kilometer (km2)
1 acre0.41 hectare (ha)

Mass (Weight)

US Customary/Imperial UnitEquivalent SI (Metric) Unit
1 ounce (oz)28.35 grams (g)
1 pound (lb)0.45 kilogram (kg)
1 short ton (2,000 lbs)1 metric ton

While these three remain the common units of weight under the Imperial and US Customary Systems, there are actually three systems of weight in English units. The common weight/mass units, as listed above, actually are part of the avoirdupois system, and there is also troy and apothecaries’ weight.

Avoirdupois Weight

US Customary/Imperial UnitEquivalent SI (Metric) Unit
1 grain (gr)64.79891 milligrams (mg)
1 dram (dr)1.772 g
1 hundredweight (cwt) (100 lbs)45.359 kg

Apothecaries’ Weight

US Customary/Imperial UnitEquivalent SI (Metric) Unit
1 scruple (s ap)1.296 g
1 dram apothecaries (dr ap)3.888 g
1 ounce apothecaries (oz ap)31.103 g
1 pound apothecaries (lb ap)373.242 g

Troy Weight

US Customary/Imperial UnitEquivalent SI (Metric) Unit
1 pennyweight (dwt)1.555 g
1 ounce troy (oz t)31.103 g
1 pound troy (lb ap)373.242 g

Volume

US Customary UnitEquivalent SI (Metric) Unit
1 teaspoon (tsp)4.93 milliliters (mL)
1 tablespoon (Tbsp)14.79 mL
1 fluid ounce (fl oz)29.57 mL
1 cup (c)0.24 liter (L)
1 cubic inch (in3)16.39 mL
1 pint (pt)0.47 liter (L)
1 quart (qt)0.95 L
1 gallon (gal)3.79 L
1 cubic foot (ft3)0.03 cubic meter (m3)
1 cubic yard (yd3)0.76 m3

Volume is one of the primary areas in which the US Customary and British Imperial Systems differ. Imperial units for volume are listed below.

Imperial UnitEquivalent SI (Metric) Unit
1 fl oz28.41 mL
1 pt0.57 L
1 qt1.13 L
1 gal4.54 L

Units of Dry Volume

US Customary UnitEquivalent SI (Metric) Unit
1 dry pint (pt)0.5506 L
1 dry quart (qt)1.1012 L
1 dry gallon (gal)4.4048 L
1 peck (pk)8.8098 L
1 bushel (bu)35.2391

Units of Pressure

US Customary/Imperial UnitEquivalent SI (Metric) Unit
1 pound-force/foot2 (psf)47.880 Pascals (Pa)
1 pound-force/inch2 (psi)6,894.76 Pa
1 poundal/foot2 (pdl/ft2)1.4992 Pa
1 atmosphere (atm)101,325 Pa

Units of Energy and Work

US Customary/Imperial UnitEquivalent SI (Metric) Unit
1 British thermal unit (Btu)1055 joules (J)
1 therm (thm)1055.056 x 105 J
1 watt-hour (Wh)3600 J
1 foot-pound-force (ft*lbf)1.3559 J
1 foot-poundal (ft*pdl)0.04214 J

Units of Power

US Customary/Imperial UnitEquivalent SI (Metric) Unit
1 British thermal unit/hour (Btu/h)0.29307 watt (W)
1 mechanical horsepower (hp)745.69 W
1 boiler horsepower (hp)9,809.50 W
1 foot-pound-force/second (ft*lbf/s)1.3558 W

Temperature

Temperature can be converted from Fahrenheit to Celsius, and vice versa, by following the two equations below:

T(°C) = (T(°F) – 32) × 5/9

T(°F) = T(°C) × 9/5 + 32

Other US Customary Units

Did we miss anything? Are there any other units you’d like to add to this list? Comment below, and we’ll add anything accordingly.

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