Any quantity that is measurable needs a **unit** to describe how large or small it is relative to another quantity. For example, 1 meter is a unit of length, 1 second is a unit of time, and 1 m/s (one meter per second) is a **unit rate**.

To convert from one unit to another, a **conversion factor **(also known as *conversion ratio*) is required; a *conversion factor* is used to express a quantity in different units of measurement. For example, to convert from 150 seconds to minutes, you use the known conversion factor 60 seconds = 1 minute. For more complicated unit conversions, conversion factors are provided by your teacher.

The video below demonstrates how to use a known **conversion factor** to go from one unit to another. You’ll notice that in example (3), sometimes a conversion factor may not be given. For example, you may be given:

2.54 cm = 1 in

But the question may ask you to go from cm² to inches². In this case, you will have to modify your conversion factor to look the way you want it to look.

Oftentimes you may need to convert from **metric units** to **imperial units**. The basic *metric units are* **meters** (for length), **grams** (for mass or weight), and **liters** (for volume), while their equivalents imperial counterparts are **inches, feet, and miles** (for length), **pounds** (for mass or weight), and **gallons** (for volume), respectively. Unlike the imperial system, the metric system is based on joining one of a series of prefixes, including *kilo-*, *hecto-*, *deka-*, *deci-*, *centi-*, and *milli-*, with a base unit of measurement mentioned above – meter, liter, or gram. Luckily, to go from metric to imperial, all that’s needed is a **conversion factor**, so the process doesn’t change.

Let’s watch a few examples of this in action:

As mentioned above, the base unit of measurement in the metric system can be modified by adding **prefixes** to unit. Each prefix has a specific meaning when applied to the base unit that indicates multiples or fractions of the units. In other words, the prefixes of the metric system, such as *kilo-* and *milli-*, represent multiplication by powers of ten. Once again, if you know how to convert units, converting from one prefix to another is just as simple. Here are a few examples of this in action: