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Resource: Often-used units

Often-used units converted into SI units. Units are listed for length, time, mass, quantity (amount of substance), temperature, angle, area, volume, velocity, energy and power. If you know of a unit that is not included, please leave a comment below.

SI units are emphasised. Symbols/abbreviations can vary.

Length units
Parsec$1\,\mathrm{pc}$$=3.086\times 10^{16}\,\mathrm m$How far away to be located before the Earth’s
distance to the Sun fills an angle of one arcsecond[7]
Lightyear$1\,\mathrm{ly}$$=9.46\times 10^{15}\,\mathrm m$How far light moves in a year[6]
Astronomical unit$1\,\mathrm{AU}$$=149.6\times 10^9\,\mathrm m$Distance from Earth to the Sun[5]
Nautical mile$1\,\mathrm{nmi}$$=1852\,\mathrm m$An arcminute (angle) along the Earth’s surface[3]
Mile$1\,\mathrm{mi}$Country-specificOriginally 500 feet[3]
Fathom$1\,\mathrm{fath}$$=1.883\,\mathrm m$Six feet[3]
Metre$\boldsymbol{1\,\mathrm{m}}$SI unit
Yards$1\,\mathrm{yd}$$=0.9144\,\mathrm m$Three feet[3]
Cubit$1\,\mathrm{cubit}$$=0.508\,\mathrm m$Length of a forearm (20 inches)[3]
Foot$1\,\mathrm{ft}$$=0.3048\,\mathrm m$Length of a foot[3]
Inch$1''$ or $1\,\mathrm{in}$$=0.3048\,\mathrm m$Length of a thump[3]
Ångström$1\,\mathrm{Å}$$= 10^{-10}\,\mathrm m$Size of a typical atom[3,4]
Planck length$1\,\ell_p$$= 1.616\times10^{-35}\,\mathrm m$The smallest length of physical meaning[1,2]
Time units
Year or annum$1\,\mathrm{yr}$$=31.56\times 10^{6}\,\mathrm s$ 3Duration of Earth’s orbit around the Sun
Quarter$1\,\mathrm{qrt}$$=7.889\times 10^{6}\,\mathrm s$A quarter of a year
Month$1\,\mathrm{mth}$$=2.630\times 10^{6}\,\mathrm s$ 2Duration of the Moons’ orbit (a twelfth of a year)
Day$1\,\mathrm{day}$$=86\,400\,\mathrm s$ 124 hours
Hour$1\,\mathrm{hr}$$=3600\,\mathrm s$60 minutes
Minutes$1\,\mathrm{min}$$=60\,\mathrm s$Minute pointer's round on a clock
Second$\boldsymbol{1\,\mathrm{s}}$SI unit
Planck time$1\,\mathrm{t_p}$$=5.39\times 10^{-44}\,\mathrm s$Time for light to move 1 Planck length[2]
Mass units
Tonne or ton6$1\,\mathrm{ton}$$=1000\,\mathrm{kg}$
(but country-specific)
The weight of a large wine barrel[3]
Kilogram$\boldsymbol{1\,\mathrm{kg}}$SI unit
Pound$1\,\mathrm{lb}$ 5$=0.4536\,\mathrm{kg}$7000 grains[3]
Gramme or gram$1\,\mathrm{g}$$=10^{-3}\,\mathrm{kg}$A thousandth of a kilogram
Ounce$1\,\mathrm{oz}$$=2.835\times10^{-4}\,\mathrm{kg}$A sixteenth of a pound[3]
Carat$1\,\mathrm{ct}$$=2\times10^{-4}\,\mathrm{kg}$Mass of a portion of grains[3]
Grain$1\,\mathrm{grain}$$=6.5\times10^{-5}\,\mathrm{kg}$Mass of a portion of wheat kernels[3]
Atomic mass unit$1\,\mathrm{AMU}$$=1.674\times10^{-27}\,\mathrm{kg}$A twelfth of the mass of a carbon-12 atom[9] 4
Quantity (amount of substance) units
Mole$\boldsymbol{1\,\mathrm{mol}}$$\boldsymbol{=6.022\times 10^{23}}$SI unit[10]
Dozen[11]$1\,\mathrm{dozen}$$=12$
Temperature units
Kelvin$\boldsymbol{\mathrm K}$SI unitZero point: $\boldsymbol{0\,\mathrm K}$
Freezing point: $\boldsymbol{273.15\,\mathrm K}$
Boiling point: $\boldsymbol{373.15\,\mathrm K}$
[15,10]
Degree Celsius$^\circ \mathrm C$$=[\mathrm K]-273.15$Zero point: $-273.15\,\mathrm{^\circ C}$
Freezing point: $0\,\mathrm{^\circ C}$
Boiling point: $100\,\mathrm{^\circ C}$ [15,10]
Degree Fahrenheit$^\circ \mathrm F$$=1.8\left([\mathrm K]-273.15\right)+32$Zero point: $-459.67\,\mathrm{^\circ F}$
Freezing point: $32\,\mathrm{^\circ F}$
Boiling point: $212\,\mathrm{^\circ F}$ [15]
Degree Rankine$^\circ \mathrm R$$=\frac95 [\mathrm K]$Zero point: $0\,\mathrm{^\circ R}$
Freezing point: $491.67\,\mathrm{^\circ R}$
Boiling point: $671.67\,\mathrm{^\circ R}$ [13,14]
Degree Rømer[3]$^\circ \mathrm{Rø}$$=\left([\mathrm K]-273.15\right)\frac{21}{40}+7.5$Zero point: $-135.90\,\mathrm{^\circ Rø}$
Freezing point: $7.5\,\mathrm{^\circ Rø}$
Boiling point: $60\,\mathrm{^\circ Rø}$ [12]
Angle units
Radian$\boldsymbol{1\,\mathrm{rad}}$ or no unitSI unit[10]
Degree$1^\circ$$=\frac{\pi}{180}$One 360th of a circle[10,16]
Arcminute$1'$$=\frac{\pi}{10800}$One 60th of a degree[3]
Arcsecond$1''$$=\frac{\pi}{648000}$One 60th of an arcminute[3,7]
Area units
Hectar[3]$1\,\mathrm{ha}$$=10\,000\,\mathrm{m^2}$
Square metre$\boldsymbol{1\,\mathrm{m^2}}$SI unit
Barn$1\,\mathrm{b}$$=10^{-28}\,\mathrm{m^2}$Cross-sectional area of an atom nucleus[3]
Volume units
Cubic metre$\boldsymbol{1\,\mathrm{m^3}}$SI unit
Barrel$1\,\mathrm{ba}$$=0.1590\,\mathrm{m^3}$An oil barrel[3]
Bushel$1\,\mathrm{bush.}$$=0.036375\,\mathrm{m^3}$How much you can carry unaided (8 gallons)[3]
Peck$1\,\mathrm{peck}$$=0.009096\,\mathrm{m^3}$8 quarts[3]
Gallon$1\,\mathrm{gal}$$=0.004546\,\mathrm{m^3}$Volume of 10 pounds pure (distilled) water[3]
Quart$1\,\mathrm{qt}$$=0.001137\,\mathrm{m^3}$One fourth gallon[3]
Litre$1\,\mathrm{L}$$=0.001\,\mathrm{m^3}$Volume of 1 kg cold,7 pure water[3]
Pint$1\,\mathrm{pt}$$=5.685\times 10^{-4}\,\mathrm{m^3}$Half a quart[3]
Velocity units
Mach number$\mathrm{Mach}\,1$$=340.3\,\mathrm{m/s}$The speed of sound in air[17]
Cubic metre$\boldsymbol{1\,\mathrm{m/s}}$SI unit
Knot$1\,\mathrm{kn}$$=0.514\,\mathrm{m/s}$Used at sea. A nautical mile-per-second[3]
Energy units
Tonne oil equivalent$1\,\mathrm{toe}$$=4.187\times 10^{10}\,\mathrm{J}$Energy in one tonne oil[19]
Kilowatt-hour$1\,\mathrm{kWh}$$=3.6\times 10^6\,\mathrm{J}$Energy spent by a kilowatt in 1 hour[18]
British thermal unit$1\,\mathrm{BTU}$$=1056\,\mathrm{J}$Energy to raise the temperature of 1 pound water by $1\,^\circ \mathrm C$[3]
Calorie$1\,\mathrm{cal}$$=4.19\,\mathrm{J}$Energy to raise the temperature of 1 gramme water by $1\,^\circ \mathrm C$[3]
Joule$\boldsymbol{1\,\mathrm{J}}$$\boldsymbol{=\mathrm{1\,kg\cdot m^2/s^2}}$SI unit[10]
Electron volt$1\,\mathrm{eV}$$=1.602\times 10^{-19}\,\mathrm{J}$Energy to move an electron through 1 volt[3,10]
Power units
Horse power$1\,\mathrm{HP}$$=735.5\,\mathrm{W}$What a horse can achieve[3]
Watt$\boldsymbol{1\,\mathrm{W}}$$\boldsymbol{=\mathrm{1\,J/s}}$SI unit[10]

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