Systems Of Units
The most commonly used unit systems include the International SI system of units, the US system of units, and the Imperial (mostly UK) system of units. Some of the most important units are used to measure weight, length, area, volume, temperature, pressure, etc.
Most Popular Weight Units
SI weight unit for weight is a kilogram (or often just 'kilo'), while non-SI units are Imperial Ton, Tonne, 1 US Ton, Stone, Pound, and Ounce.
Other units may be used as well, but they are not used that often, like grain, drachm, slug, etc.
| Non-SI Weight Units |
SI Weight Units |
| 1 Imperial Ton = 2240 Pounds | 1016.05 kg |
| 1 Tonne = 2204.62 Pounds | 1000 kg |
| 1 US Ton = 2000 Pounds | 907.185 kg |
| 1 Quarter = 28 Pounds | 12.70058 kg |
| 1 Stone = 14 Pounds | 6.35029 kg |
| 1 Pound | 0.45359237 kg |
| 1 Ounce = 1/16 Pound = 0.0625 Pounds | 0.0283495 kg = 28.3495g |
Most Popular Units of Length, Area, and Volume
The most popular non-SI units for length are Nautical Mile, Mile, Yard, Foot, and Inch. Again, there are other units as well, but these are the most popular and still in frequent use.
| Non-SI Lenght Units | SI Length Units |
| 1 Nautical Mile = 2025.37 Yards | 1852 m |
| 1 Mile = 1760 Yards | 1609.34 m |
| 1 Yard = 3 feet = 36 inches | 0.9144 m = 91.44 cm |
| 1 Foot = 0.3333 Yard = 12 Inches | 0.3048 m = 30.48 cm |
| 1 Inch | 0.0254 m = 2.54 cm |
The units for volume are often derived from the units of length, but not necessarily, for example:
| Non-SI Volume Units | SI Volume Units |
| 1 Cubic Yard = 27 Cubic Feet | 0.764554857984 m3 = 764.554857984 liters |
| 1 Cubic Foot = 1728 Cubic Inches = 957.506 US Fluid Ounces | 0.0283168 m3 = 28.3168 liters |
| Dry Quart (US) = 2 Dry Pints (US) = 67.2 Cubic Inches | 1101.22 milliliters = 1.10122 liters |
| Dry Pint (US) = 33.6 Cubic Inches | 550.6 milliliters = 0.5506 liters |
| 1 Cubic Inch = 0.554113 US Fluid Ounces | 0.0163871 liters = 16.3871 milliliters |
Note: 1 Cubic Meter (m3) = 1000 liters = 1000 dm3
Also, sometimes, the Imperial (British) system uses one unit (quart, pint) for liquid and dry measures, while the US system uses two units (dry quart, liquid quart, dry pint, liquid pint).
Probably the largest source of confusion among the units is the conversion between the US measurement system and the Imperial (UK) measurement system:
| US Units (Volume) |
Imperial Units (Volume) |
| 1 US Liquid Gallon = 128 US Fluid Ounces = 3.78541 liters | 1 Imp Liquid Gallon = 160 Imp Fluid Ounces = 4.54609 liters |
| 1 US Liquid Gallon = 0.832674 Imp Liquid Gallon | 1 Imp Liquid Gallon = 1.20095 US Liquid Gallon |
| 1 US Liquid Quart = 32 US Fluid Ounces = 0.946353 liters | 1 Imp Liquid Quart = 40 Imp Fluid Ounces = 1.13652 liters |
| 1 US Liquid Quart = 0.832674 Imp Liquid Quart | 1 Imp Liquid Quart = 1.20095 US Liquid Quart |
| 1 US Liquid Pint = 16 US Fluid Ounces = 0.473176 liters | 1 Imp Liquid Pint = 20 Imp Fluid Ounces = 0.568261 liters |
| 1 US Liquid Pint = 0.832674 Imp Liquid Pint | 1 Imp Liquid Pint = 1.20095 US Liquid Pint |
| 1 US Customary Cup = 8 US Fluid Ounces = 0.2365882365 liters | 1 Imp Cup = 10 Imp Fluid Ounces = 0.284131 liters |
| 1 US Customary Cup = 0.832673 Imp Cup | 1 Imp Cup = 1.200951 US Customary Cups |
| 1 US Legal Cup = 240 milliliters | 1 Imp Cup = 1.183879 US Legal Cups |
| 1 US Fluid Ounce = 29.5735 milliliters | 1 Imp Fluid Ounce = 28.4131 milliliters |
| 1 US Fluid Ounce = 1.04084 Imp Fluid Ounces | 1 Imp Fluid Ounce = 0.96076 US Fluid Ounces |
| 1 US Tablespoon = 1/2 US Fluid Ounces = 14.7868 milliliters | 1 Imp Tablespoon = 0.625 Imp Fluid Ounces = 17.7582 milliliters |
| 1 US Tablespoon = 0.832674 Imp Tablespoon | 1 Imp Tablespoon = 1.20095 US Tablespoon |
| 1 US Teaspoon = 1/6 US Fluid Ounces = 4.92892 milliliters | 1 Imp Teaspoon = 0.208333 Imp Fluid Ounces = 5.91939 milliliters |
| 1 US Teaspoon = 0.832674 Imp Teaspoon | 1 Imp Teaspoon = 1.20095 US Teaspoon |
International System (SI) of Units Official Prefixes
To write extra small and extra large values more easily, the International System (SI) of Units uses prefixes that range from yocto (y, 10-24) to yotta (Y, 1024).
The following chart lists the International System of Units (SI) official prefixes:
| Prefix | Base 10 | Decimal | English Word | ||
| Name | Symbol | Short Scale | Long Scale | ||
| quetta | Q | 1030 | 1 000 000 000 000 000 000 000 000 000 000 | nonillion | quintillion |
| ronna | R | 1027 | 1 000 000 000 000 000 000 000 000 000 | octillion | quadrilliard |
| yotta | Y | 1024 | 1 000 000 000 000 000 000 000 000 | septillion | quadrillion |
| zetta | Z | 1021 | 1 000 000 000 000 000 000 000 | sextillion | trilliard |
| exa | E | 1018 | 1 000 000 000 000 000 000 | quintillion | trillion |
| peta | P | 1015 | 1 000 000 000 000 000 | quadrillion | billiard |
| tera | T | 1012 | 1 000 000 000 000 | trillion | billion |
| giga | G | 109 | 1 000 000 000 | billion | milliard |
| mega | M | 106 | 1 000 000 | million | |
| kilo | k | 103 | 1 000 | thousand | |
| hecto | h | 102 | 100 | hundred | |
| deca | da | 101 | 10 | ten | |
| - | - | 100 | 1 | one | |
| deci | d | 10-1 | 0.1 | tenth | |
| centi | c | 10-2 | 0.01 | hundredth | |
| milli | m | 10-3 | 0.001 | thousandth | |
| micro | μ | 10-6 | 0.000 001 | millionth | |
| nano | n | 10-9 | 0.000 000 001 | billionth | milliardth |
| pico | p | 10-12 | 0.000 000 000 001 | trillionth | billionth |
| femto | f | 10-15 | 0.000 000 000 000 001 | quadrillionth | billiardth |
| atto | a | 10-18 | 0.000 000 000 000 000 001 | quintillionth | trillionth |
| zepto | z | 10-21 | 0.000 000 000 000 000 000 001 | sextillionth | trilliardth |
| yocto | y | 10-24 | 0.000 000 000 000 000 000 000 001 | septillionth | quadrillionth |
| ronto | r | 10-27 | 0.000 000 000 000 000 000 000 000 001 | octillionth | quadrilliardth |
| quecto | q | 10-30 | 0.000 000 000 000 000 000 000 000 000 001 | nonillionth | quintillionth |
Note: the 'Short Scale' is in use, for example, in the United States, while the 'Long Scale' is in use, for example, in Europe - this may be the largest source of mistakes and errors in everyday speech and conversation.
Personally, the use of the SI system simplifies math greatly, but tradition is tradition. Nonetheless, just in case, it is good to know relations between various units and how to convert from to another and vice versa.