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By aeiffert, 9/8/2018

SI Units


If you've looked at some of the experiments or facts on Experimonkey, you may have noticed that we don't use some of the measurements you're used to, like pounds, inches, or gallons. Instead, we use measurements from something called the International System of Units, or SI for short. (Why not "IS"? Well, "SI" comes from the french Système international.)

SI is the modern, updated version of the metric system, the most common measurement system in the world. It is unlike the United States Customary System, which uses different conversion factors for each unit:

US Customary Unit Conversions
1 mile 880 fathoms 1,760 yards 5,280 feet 63,360 inches
1 US ton 17.857 long hundredweights 20 hundredweights 2,000 pounds 32,000 ounces
1 gallon 4 quarts 8 pints 16 cups 128 ounces

Sure we're used to understanding things like, "I ran a mile," or "add a cup of flour," but what happens when we need to convert say 2/3 cups to ounces, or calculate someone's running speed in inches/second? And what if we want to discuss our findings with a scientist from a different country?:

SI to US Customary Unit Conversions
1 kilometer .621 miles 1,093.61 yards 3,280.84 feet 39,370.1 inches
1 metric ton 1.102 US tons 2204.62 pounds 19.684 hundredweights 35,274 ounces
1 liter .264 gallons 1.057 quarts 4.227 cups 33.814 ounces

Believe it or not, US Engineers still have to deal with such complicated conversions. Confused? It would be surprising if you weren't! That's where SI comes into play. SI starts off with 7 base units:

  • The Ampere (A) - Measures electric current
  • The Kelvin (K) - Measures temperature
  • The Second (s) - Measures time
  • The meter (m) - Measures length
  • The Kilogram (kg) - Measures mass (not weight!)
  • The Candela (A) - Measures the strength of light
  • The Mole (A) - Amount of a substance

And also gives 22 more derived units, or units that can be expressed from the base units. Click here to see them.

Then comes the best part about SI units: to convert between them, we use multiples, or bases of 10. Each base of 10 has a corresponding prefix and abbreviation. For example, "Kilo-" or "kg" means 1,000. So, a "Kilogram" means 1,000 grams. A "Kilo-monkey" means 1,000 monkeys! (That's just a joke, don't worry.) This system is much easier to understand and remember than the US units (which have different conversions for every unit)! Some of the most commonly used prefixes are:

Common SI Conversions
Prefix Abbreviation Base 10 Full number
Nano- n 10-9 .000000001
Milli- m 10-3 .001
Centi- c 10-2 .01
Kilo- k 103 1,000

Because of the prefix system, SI is much easier to use than other systems like the US Customary System. And since it's silly to go back and forth between different systems (remember that second crazy table?), scientists typically choose to work with SI. And so, that's why it's very important to learn and get comfortable with the SI system!


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