International System of Units (SI)
Sistema Internacional de Unidades (SI)
Magnitude/MagnitudUnit/Unidad
Length / Longitud (L)metre / metro (m)
Mass / Masa (M) kilogram / kilogramo (kg)
Time / Tiempo (T) second / segundo (s)
Electric current / Intensidad de corriente eléctrica (I) ampere / amperio (A)
Thermodynamic temperature / Temperatura (Θ) kelvin / kelvin (k)
Amount of substance / Cantidad de sustancia (N)mole / mol (mol)
luminous intensity / Intensidad luminosa (J)candela / candela (cd)

Standard prefixes for the SI units / Prefijos SI
yotta (Y) zetta (Z) exa (E) peta (P)tera (T)giga (G)mega (M)kilo (k)hecto (h)deca (da)
10241021101810151012109106103102101
deci (d)centi (c)milli (m)micro (μ)nano (n)pico (p)femto (f)atto (a)zepto (z)yocto (y)
10-110-210-310-610-910-1210-1510-1810-2110-24

Imperial units / Unidades Inglesas
Length
inch (in) ( '' ) pulgada25.4 mm
foot (ft) ( ' )12 inchespie30.480 cm
yard (yd)3 feetyarda0.91440 m
mile1760 yardsmilla1.609344 km
nautical mile  1.852 km
Mass *avoirdupois
pound (lb) 16 ounceslibra0.453 592 37 kg
ounce (oz)  onza28.3495231 g
Volume or capacity*Imperial
gallon (gal) 8 pintsgalón4.54609 litres
pint (pt)  pinta0.56826125 litres
Pressure *avoirdupois
psi pound-force per square inch6.894757 kPa 
Temperature
Fahrenheit (°F) Fahrenheit °F = °C 9 / 5 + 32Celsius °C = (°F - 32) 5 / 9