4th Grade Math: Kilograms and Other Metric Units

The Metric System

The metric system is a system of measurement that is used in almost every nation in the world and is also the internationally accepted system of measurement in science. It is different than the measurement system used in the U.S., which is referred to as the customary system. The key difference between the two systems is how length, weight and quantity are measured. While the U.S. customary system uses units expressed as feet, pounds and gallons, the metric system uses meters, kilograms and liters.

Units of Mass

To measure how heavy an object is (its mass), you'll use units like grams and kilograms. One gram (g) is about as heavy as a paper clip, and 1,000 grams equal one kilogram (kg). A kilogram weighs a little over two pounds. You can express the mass of a smaller item, like a piece of fruit or a cell phone, in grams. For larger items, like a TV or a bag of groceries, you usually use kilograms. To convert a measurement that's in kilograms into grams, multiply by 1,000. For instance, if you weigh 40 kilograms, that means you weigh 40,000 grams, since 40 x 1,000 = 40,000.

Metric Lengths

The main unit of measure for length in the metric system is the meter (m). A meter is slightly longer than one yard or three feet. Meters are useful for measuring medium distances, lengths and heights, like the size of a classroom or the height of a building. To measure smaller things, like the length of a pencil or the height of a plant, you can use centimeters (cm). A dime is a little less than one centimeter across, and one meter equals 100 centimeters.

For measuring longer distances, you can use kilometers (km). A kilometer is a little over half a mile, so you might use this unit to measure the distance between two cities or the length of a river. Since one kilometer is equal to 1,000 meters, you multiply a distance in kilometers by 1,000 if you want to convert it to meters. For instance, two kilometers is equal to 2,000 meters because 2 x 1,000 = 2,000.

Measuring Volume

You can also use metric units to measure volume, which is the amount of space something takes up. The standard metric unit for volume is the liter (l), which is half the size of a large bottle of soda pop. You'll typically use liters to measure things that don't have a solid shape, like water, flour or soil. To measure substances like these, you may use a 1-liter beaker or a measuring cup.

For smaller amounts, you can use milliliters (ml). One liter equals 1,000 milliliters, so you convert liters to milliliters the same way that you convert kilograms to grams and kilometers to meters: multiply by 1,000. For example, if you have five liters of water, it would be equal to 5,000 milliliters, since 5 x 1,000 = 5,000.