It is said that measurement is the basis of all natural sciences. And true enough – since ancient times, humans felt the need to measure everything they see, and to express the measured quantities in units that can be easily understood and communicated by other people.

These units are therefore called **units of measurement**. They are basically terms in which we express quantities – and everything can be a unit of measurement. For example, if you have a friend named Bob, and…. let’s say, 10 big bags of candy. If we weigh them both, and their weight is equal, we can say that Bob weighs 10 big bags of candy. In this case, one big bag of candy is a unit of measurement. We can also say that a big bag of candy weighs 0.1 Bobs. In that case, Bob is actually a unit of measurement. Since not everybody knows Bob or buys the same candy, people had to come up with some units that could be easily compared and communicated because everybody knew what they are.

Sounds funny, right? Well, yes, but… throughout history, people have used some really strange things as units of measurement. In these couple of lessons, however, we’ll concentrate on the most current and most commonly used units of measure. In particular, on the units of measurement organized into the metric system and U.S. version of the imperial system.

The **metric system** stems from late 18^{th} century France, and it has been revised quite a few times. Almost every country in the world currently uses the metric system. The only exceptions are the United States of America, Liberia, and Myanmar. It’s a base-10 system, which means that every unit is ten, hundred, a thousand and more times larger or smaller than another unit used to measure the same thing.

For example, a kilogram (an SI unit for measuring mass) is a thousand times lighter than a ton, which is then a thousand times lighter than a kiloton etc. At the same time, a kilogram is thousand times heavier than a gram, which is a thousand times heavier than a milligram, which is a thousand times heavier than a microgram etc. Do you see the pattern? The units in the metric system can be converted into each other using multiplication or division by an integer <power> of ten, and applying the right prefixes.

The SI system recognizes base and derived units of measurement. The base units of measurement are the ones from which all other units are derived, and they are the meter, kilogram, second, ampere, kelvin, mole and candela. Each unit is defined using a reference standard, some of which are naturally occurred phenomena, while others are physical prototypes.

The U.S. version of the imperial system (or the U.S. customary measurement system) stems from the old English measurement system, from which the British imperial system was also created. It is a bit more complicated since conversion doesn’t follow a simple pattern. Keeping that in mind, we’ve organized related lessons using the SI system, but those measures will be expressed using the U.S. system as well.

We’ve prepared lessons and worksheets so you can learn more about different units of measurement and practice what you’ve learned. Just click on the links below and enjoy!