What is the difference between mass and weight?

Mass is the amount of matter in an object and stays the same everywhere. Weight is the force of gravity pulling on that mass and changes depending on location. On the Moon, your weight would be about one-sixth of your weight on Earth, but your mass would not change at all.

What units do you use to measure mass?

In the metric system, mass is measured in milligrams (mg), grams (g), and kilograms (kg). A paperclip has a mass of about 1 g, and a textbook has a mass of about 1 kg. In the U.S. customary system, people sometimes use pounds and ounces, though these technically measure weight.

Does mass change in space?

No. Mass is an intrinsic property of matter, so it remains the same whether you are on Earth, on the Moon, or floating in space. What changes in space is your weight, because the gravitational pull is different.

mass — Definition, Formula & Examples

Mass is the amount of matter (stuff) in an object. It is measured in metric units such as grams (g) and kilograms (kg) and stays the same no matter where the object is located.

Mass is a scalar quantity that represents the total amount of matter contained in a body. Unlike weight, which depends on gravitational pull, mass is an intrinsic property of an object and remains constant regardless of location. The SI base unit of mass is the kilogram (kg), with the gram (g) and milligram (mg) as commonly used smaller units.

Key Formula

1\,\text{kg} = 1{,}000\,\text{g}

Where:

$\text{kg}$ = kilograms — the larger metric unit of mass
$\text{g}$ = grams — the smaller metric unit of mass

How It Works

You measure mass using a balance or a scale. A balance compares an unknown object to known masses — when both sides are level, the masses are equal. In everyday metric units, lighter objects like a paperclip are measured in grams, while heavier objects like a person are measured in kilograms. The key conversion is that 1 kilogram equals 1,000 grams. To convert from kilograms to grams, multiply by 1,000. To convert from grams to kilograms, divide by 1,000.

Worked Example

Problem: A bag of apples has a mass of 3 kg. What is its mass in grams?

Step 1: Write down the conversion factor: 1 kg = 1,000 g.

1\,\text{kg} = 1{,}000\,\text{g}

Step 2: Multiply the number of kilograms by 1,000.

3 \times 1{,}000 = 3{,}000

Step 3: Attach the correct unit.

3\,\text{kg} = 3{,}000\,\text{g}

Answer: The bag of apples has a mass of 3,000 g.

Another Example

This example adds two masses together and converts from smaller units (grams) to larger units (kilograms), the reverse direction from the first example.

Problem: A pencil has a mass of 8 g and an eraser has a mass of 15 g. What is their combined mass in grams? What is that in kilograms?

Step 1: Add the two masses together to find the total mass in grams.

8\,\text{g} + 15\,\text{g} = 23\,\text{g}

Step 2: To convert grams to kilograms, divide by 1,000.

23 \div 1{,}000 = 0.023

Step 3: Write the answer with the correct unit.

23\,\text{g} = 0.023\,\text{kg}

Answer: The combined mass is 23 g, which equals 0.023 kg.

Visualization

Why It Matters

Understanding mass is essential in elementary and middle school science when you study states of matter, density, and simple machines. In cooking and baking, recipes in many countries list ingredients by mass in grams, so knowing how to read and convert these units is a practical life skill. Later courses in physics and chemistry rely heavily on mass for calculations involving force, energy, and chemical reactions.

Common Mistakes

Mistake: Saying mass and weight are the same thing.

Correction: Mass measures how much matter an object contains (in kg), while weight measures the gravitational force on that object (in newtons). They are related but not identical.

Mistake: Multiplying when you should divide (or vice versa) during unit conversion.

Correction: Going from a larger unit to a smaller unit (kg → g), multiply by 1,000. Going from a smaller unit to a larger unit (g → kg), divide by 1,000. Ask yourself: should my answer be a bigger or smaller number?

Mistake: Forgetting to convert to the same unit before comparing or adding masses.

Correction: You cannot directly compare 2 kg and 500 g without converting first. Change both values to the same unit, then compare or add.

Check Your Understanding

Convert 5,200 g to kilograms.

Hint: Divide the number of grams by 1,000.

Answer: 5.2 kg

A watermelon has a mass of 4 kg and a pineapple has a mass of 1,500 g. Which is heavier?

Hint: Convert to the same unit before comparing.

Answer: The watermelon (4 kg = 4,000 g, which is more than 1,500 g).

If you travel to the Moon, does your mass increase, decrease, or stay the same?

Hint: Think about what mass actually measures.

Answer: It stays the same. Only your weight decreases.

Related Terms

Measurement — Parent concept — mass is a type of measurement
Metric Length Units (mm, cm, m, km) — Sibling metric system units for length
Centimeter — Another commonly used metric unit
Kilometre (Kilometer) — Large metric unit used for distance
Cubic Centimeter — Volume unit linked to mass through density
Measurement Error — Accuracy concerns when measuring mass