Introduction

The metric system is based on multiples of 10. This makes it very easy to use with decimal numbers. One of the principle advantages of this is that it makes doing math with metric measurements much easier, which is why metric measurements are much pretty much the standard in any sort of science or math class you will have. Once you learn the prefixes and base units, much of the work in the metric system is often just moving the decimal point around..

Also, the different types of measurement (distance, volume, mass and temperature) are related to each other in the metric system in part by using a very common substance — water. When you put all of these parts together, the metric system is very… well, it’s very systematic.

Metric Distance (Length)

A **meter is the base unit of distance** in the metric system. It is about the same length as a yard, or about three feet.

If a meter is divided into 100 smaller parts, each of those parts is called a centimeter. The word centimeter has the same base as the word cent or century. The prefix **centi means hundredth**. 100 cents is one dollar, 100 years is one century and **100 centimeters is one meter**.

1 meter = 100 centimeters |
1m = 100cm |

Prefixes are used in the metric system a lot. Each centimeter is divided into ten (metric system! units of ten!) smaller parts called millimeters. **Milli means thousandth**. There are **1,000 millimeters in a meter** (100cm x 10 = 1,000). Literally, millimeter means a thousandth of a meter. Millimeters are very small.

1 centimeter = 10 millimeters |
1cm = 10mm |

1 meter = 100 centimeters = 1,000 millimeters |
1m = 100cm = 1,000mm |

Notice how we use the same base word (“meter”) and the suffix changes to tell us what fraction or multiple of a meter the unit means. This same strategy is used for volume and mass.

A meter is bigger than a centimeter and a centimeter is bigger than a millimeter. You would measure a car or a room in meters. You would measure a pencil or a piece of paper in centimeters. You would measure something very small like a bug in millimeters. For measuring long distances (how far you drive in a car or fly in an airplane), we measure using kilometers. The prefix **kilo means thousand**.

1 kilometer = 1,000 meters |
1km = 1,000m |

Remember the metric system always, always, always deals in factors of ten (like 10mm=1cm, or 1,000m=1km). Traditional measurements use weird numbers (12 inches to a foot, 3 feet to a yard, 5280 feet to a mile, etc.). To convert from metric to traditional measurements, you have to use strange numbers as well. The basic conversion for distance is this…

1 inch = 2.54cm |
1 inch = 25.4mm |

You must memorize this! If you know this one fact, you can figure anything else out. For example, if 1 inch is 2.54cm, and a foot is 12 inches, 1 foot also equals 12 x 2.54cm = 30.48cm. You can do the same thing for a yard. A yard is 36 inches. 36 x 2.54cm = 91.44cm, so a yard is the same as 91.44cm.

Remember when we said one yard is close in size to a meter? We just showed how one yard is 91.44cm and we know one meter is 100cm! They really are pretty close!

Metric Volume (Fluids)

The **liter is the base unit of volume** in the metric system. It is about the same size as a quart in traditional measurements, and of course a two liter bottle of soda pop contains (you guessed it!) two liters.

If you had a tiny cube that was 1 centimeter on a side, it would be a bit smaller than a six sided die used to play a board game. This would be called a cubic centimeter. This is a measure of volume. A cubic centimeter has another special name — a milliliter.

1 cubic centimeter = 1 milliliter |
1cm |

Remember how **milli means thousandth**? And how 1,000mm equals one meter? It is similar for volume. **1 liter equals 1,000 milliliters**. A liter is about the size of a one quart bottle of chocolate milk. You could pour 1,000 teeny, tiny milliliter cups of chocolate milk from a one liter bottle. A milliliter is very, very small. Each of those tiny cups might be a nice drink… if you were a mouse!

1 liter = 1,000 milliliters |
1L = 1,000mL |

If a meter is 100 centimeters, a square meter has 100cm x 100cm = 10,000cm^{2} and a cubic meter has 10,000cm^{2} x 100cm = 1,000,000cm^{3}. We learned above that a cubic centimeter is the same as one milliliter (1cm^{3} = 1mL), so a cubic meter has one million milliliters. We also know each liter is 1,000mL, so 1,000,000mL / 1000 = 1,000L. So a cubic meter contains 1,000 liters. We could also call this a kiloliter, although this term is not often used.

Even though we can relate liters or milliliters to size measurements (cubic centimeters), we don’t ever say “cubic milliliter” or “cubic liter” because we already understand that “liter” means a measure of a three-dimensional volume. “Cubic liter” or anything similar would be redundant.

Metric Mass (Weight)

Remember how you visualized a milliliter as a tiny, tiny cup of chocolate milk, or a cube shape smaller than a board game die? If you filled that tiny, tiny cup up with water, you would have one milliliter of water. If you weighed it on a scale, the water would weigh exactly one gram. The **gram is the base unit of mass** in the metric system.

1cm |
1mL of water weighs 1g |

A gram is very, very light. A few paper clips might weigh a gram. But some things need even smaller units. Nutrients in food or doses of medicine are measured in units called milligrams. A few specks of sand might weigh a milligram. Remember that 1,000 millimeters is one meter? That 1,000 milliliters is one liter? Well…

1 gram = 1,000 milligrams |
1g = 1,000mg |

Remember how **kilo means thousand**? And how 1,000 meters is one kilometer? Well…

1 kilogram = 1,000 grams |
1kg = 1,000g |

A kilogram is very close 2.2 pounds in traditional measurement. Just like the conversion from inches to centimeters, you must memorize this!

1 kilogram = 2.2 pounds |
1kg = 2.2lb |

If you weigh 80 pounds, you can divide 80 by 2.2 to get about 36 kilograms. A kilogram is 1,000 grams, so if you weigh 36 kilograms, that’s 36 x 1000 or 36,000 grams. That’s the same as 36,000 of those little tiny 1mL cups of water, enough for a lot of thirsty mice!

Celsius Temperature

The temperature scale used in the metric measurements is usually the Celsius scale. It also uses water as a basis for its definition. Just like in the Fahrenheit temperature scale, the **unit of temperature measurement is called a degree** although values indicate different real temperatures between the two scales. The Celsius scale uses the freezing and boiling points of water a range and divides the temperature up into degrees.

0°C |
Temperature at which water freezes |

37°C |
Normal human body temperature |

100°C |
Temperature at which water boils |

Connections

The discussion above was long, but it also showed that the metric units are all related to each other, sometimes directly and sometimes by using water as a substance. Here’s a quick review.

A centimeter is a short distance, less than half an inch. A cube that is one centimeter on each side is called a cubic centimeter. This can hold a volume called a milliliter. A gram is the unit of mass defined as the amount of water in a milliliter. The Celsius temperature scale is also based on water, where zero is the freezing point of water and 100 is the boiling point of water.

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