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Weight and Mass

Weight is the measure of how heavy an object is. Weight is measured in standard customary units.

The mass of an object is the amount of material it contains. Mass is measured in standard metric units.

For everyday purposes, when you're on the surface of the earth, the difference is not important. But if you measure something on another planet, its mass will be the same as it is on earth—but its weight will be different. (Weight depends on gravity, and gravity is different on other planets! This is why when you're floating in space, you're weightless. You still have mass, though,)

Metric Units

The gram and kilogram are two units used to measure mass in the metric system .

From the gram, we get the rest of the metric units using the standard metric prefixes.


0.001 gram or 1 1000 gram


0.01 gram or 1 100 gram

Decigram (dg)

0.1 gram or 1 10 gram


1,000 milligrams

Dekagram (dag)

10 grams

Hectogram (hg)

100 grams

Kilogram (kg)

1,000 grams

Metric ton(t)

1,000 kilograms

A paper clip has a mass of about 1 gram.

one thousandth

A 1 liter bottle of water has a mass of 1 kilogram (plus a little bit extra for the plastic).

one thousandth

U.S. Customary Units

The system for measuring weight (not mass) in the customary system is based on ounce, pound, and ton. The basic unit of weight is a pound(lb).

1 pound ( lb ) = 16 ounces ( oz ) 1 ton ( T ) = 2 , 000 pounds

An ounce is the smallest unit of weight. A strawberry has a weight of about 1 ounce.

one thousandth

A car has a weight of about 2 tons.

one thousandth

To convert from one unit to another, we can use unit analysis . For example, a truck weights 6,000 pounds. How many tons is this?

We know that 1 ton = 2 , 000 pounds .

To convert pounds to tons, you can divide 6 , 000 by 2 , 000 .

6 , 000 ÷ 2 , 000 = 3 tons .


(a) To convert from larger units to smaller units, multiply by the appropriate unit ratio.

(b) To convert from smaller units to larger units, multiply by the reciprocal of the appropriate unit ratio.

(c) Multiplying or dividing by a unit ratio is mathematically equivalent to using a proportion to convert between units of measure .