What is the SI System?
Every day, scientists, engineers, and mathematicians around the world need to communicate measurements clearly. Imagine if one country measured length in feet and another in metres — the confusion would be enormous! To solve this, the world agreed on a single system called the Système International (SI).
The Système International (SI) is the international standard system of measurement used in science and most of the world. It is built around base units and uses prefixes to scale those units up or down.
The most important SI base units you'll use in MYP Sciences are:
| Measurement | Standard Unit | Symbol |
|---|---|---|
| Length | Metre | m |
| Mass | Kilogram | kg |
| Capacity | Litre | L |
The SI system is sometimes called the metric system. Almost every country in the world uses it for science, medicine, and everyday life. The United States is a notable exception in everyday use, but even American scientists use SI units.
SI Prefixes — The Language of Scale
One of the most powerful features of the SI system is its use of prefixes. A prefix is a word part placed before a unit name that tells you how many times bigger or smaller that unit is compared to the base unit.
A prefix is a letter or group of letters added to the beginning of a unit name to indicate a power of ten multiplier. For example, "kilo" means 1000, so 1 kilometre = 1000 metres.
Here are the most important prefixes you need to know for MYP:
| Prefix | Symbol | Meaning (multiply base unit by…) |
|---|---|---|
| mega | M | 1 000 000 |
| kilo | k | 1 000 |
| hecto | h | 100 |
| base unit | — | 1 |
| centi | c | 0.01 |
| milli | m | 0.001 |
| micro | μ | 0.000 001 |
| nano | n | 0.000 000 001 |
Think of prefixes like zoom levels on a map. "Kilo" zooms way out (showing 1000 times more distance), while "milli" zooms way in (showing only 1/1000 of the distance). The base unit is your normal street-level view.
A handy memory trick for the most common prefixes in order from large to small: "King Henry Died By Drinking Cold Milk" → Kilo, Hecto, Deca, Base, Deci, Centi, Milli.
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