Measuring Weight & Mass

Every scale measures either weight or mass — two words people use interchangeably but that mean different things. Learn what makes them different, the customary units (ounces, pounds, tons) and metric units (grams, kilograms) used to measure them, how to convert between units, and practice with the activities below.

Weight vs. Mass

These two words get swapped for each other constantly — even by adults — but they're measuring two different things.

Mass

The amount of matter — of actual "stuff" — that makes up an object. Mass doesn't change no matter where that object goes.

Weight

A measure of how hard gravity is pulling on an object's mass. Because gravity can be stronger or weaker depending on where you are, weight can change even when mass doesn't.

Example — an astronaut on the Moon

An astronaut's mass on Earth: 70 kilograms

The Moon's gravity is about 1/6 as strong as Earth's

The astronaut's mass on the Moon: still 70 kilograms — same "stuff," nothing was removed

But their weight on the Moon is only about 1/6 of their weight on Earth

Why the mix-up happens: here on Earth, gravity is about the same everywhere you'd ever stand, so weight and mass change together — heavier things have more mass, and more mass means more weight. Since they always seem to move together on Earth, most people never need to tell them apart, and the words get used as if they mean the same thing.

It even shows up in which units we use. Customary units like ounces, pounds, and tons are technically measurements of weight — the pull of gravity on an object. Metric units like grams and kilograms are technically measurements of mass — the amount of matter itself. A kitchen scale reading "1 pound" and a science scale reading "454 grams" are both describing the very same block of butter, just through two different lenses.

Customary Units

Customary units are the system most commonly used for everyday weight measurement in the United States.

Unit	Abbreviation	About the weight of
Ounce	oz	A slice of bread, or a handful of pretzels.
Pound	lb	A loaf of bread, or a football.
Ton	T	About the weight of a small car.

Metric Units

The metric system is used by scientists everywhere and by most countries in the world. Its biggest advantage: every unit is related to every other unit by a power of 10, so converting is just a matter of shifting a decimal point — no awkward numbers like 16 or 2,000 to remember.

Unit	Abbreviation	About the mass of
Milligram	mg	A grain of sand, or a single eyelash.
Gram	g	A paperclip, or a raisin.
Kilogram	kg	A liter bottle of water, or a textbook.

What the Prefixes Mean

Every metric mass unit is built from the same base unit — the gram — with a prefix in front that tells you how it's scaled up or down. Once you know what a prefix means, you know how that unit relates to a gram.

Prefix	Meaning	Relative to a gram
milli-	one thousandth	1 mg = 0.001 g
(none)	the base unit itself	1 g = 1 g
kilo-	one thousand	1 kg = 1,000 g

Think of it like money: a gram is like a dollar. A milligram is tiny — it takes 1,000 of them to make a dollar. A kilogram is like a $1,000 bill — it takes 1,000 dollars (grams) to add up to one.

You may notice there's no "centigram" in everyday use — unlike length, where centimeters are common, mass almost always jumps straight from milligrams to grams to kilograms.

1 g = 1,000 mg

1 kg = 1,000 g

Converting Between Units

Converting within a system just means multiplying or dividing by the relationship between the two units.

1 lb = 16 oz

1 T = 2,000 lb

1 g = 1,000 mg

1 kg = 1,000 g

The rule: going from a bigger unit to a smaller unit, multiply. Going from a smaller unit to a bigger unit, divide.

Example — bigger to smaller

How many ounces are in 3 pounds?

3 lb × 16 = 48

3 pounds = 48 ounces

Example — smaller to bigger

How many kilograms are in 4,500 grams?

4,500 g ÷ 1,000 = 4.5

4,500 grams = 4.5 kilograms

Practice

Four ways to practice what you've learned.

Pick the Best Unit

Choose the unit that makes the most sense for each measurement.

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Conversion Practice

Type your answer as a whole number and check it.

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Which Is Heavier?

Two measurements, two different units. Which one actually weighs more — or are they equal?

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Read the Scale

Where is the pointer? Count the small steps past the last whole pound.

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