E. Spatial Sense · Angles, Rectangles, Grids and Measurement

Mass: Grams and Kilograms

E2.1About 35 minutes

Learning goals

I can explain that 1 kilogram is the same as 1000 grams.
I can use benchmarks to estimate mass in grams and kilograms.
I can choose grams or kilograms to measure the mass of an object.
I can change kilograms to grams, and grams to kilograms, for simple amounts.

Words to know

mass
How much matter is in an object. We find mass by weighing.

gram (g)
A small metric unit of mass. A small paper clip has a mass of about 1 g.

kilogram (kg)
A larger metric unit of mass. 1 kg = 1000 g.

benchmark
A familiar object that helps you estimate a measurement.

scale
A tool that measures mass, like a kitchen scale or a bathroom scale.

Remember from Grade 3

In Grade 3 you compared masses with a pan balance and measured with units like cubes or bricks. You noticed a problem: different people used different units, so their numbers didn't match.

That is why we use standard units. In Canada, we measure mass in grams and kilograms.

Let's start in the grocery store

Look at food packages in your kitchen. You might see "454 g" on a block of butter or "2 kg" on a bag of sugar. The g stands for grams and the kg stands for kilograms.

Grams and kilograms

A gram (g) is a very small mass. A small paper clip has a mass of about 1 g. We use grams for light things: a pencil, an apple, a letter, a slice of cheese.
A kilogram (kg) is much bigger. A 1 litre carton of milk or water has a mass of about 1 kg. We use kilograms for heavier things: a bag of potatoes, a pumpkin, a dog, you!

1 kilogram = 1000 grams

The word kilo means one thousand. So a kilogram is a thousand grams. It would take about 1000 small paper clips to balance a 1 L carton of milk.

Benchmark	About how much mass?
A small paper clip	1 g
A 1 L carton of milk or water	1 kg (1000 g)
A bag of sugar (check the label)	often 2 kg

Benchmarks for estimating mass. Labels on packages give the exact mass.

Grams from 0 to 1000. Half a kilogram is 500 g. A quarter of a kilogram is 250 g. 1000 g makes 1 kg.

Worked example

Example 1: Kilograms to grams

A bag of flour has a mass of 3 kg. How many grams is that?

Each kilogram is 1000 g.
3 kg is 3 groups of 1000 g: 1000 + 1000 + 1000 = 3000 g.

3 kg = 3000 g

Worked example

Example 2: Grams to kilograms

Four bags of trail mix each have a mass of 250 g. Do they make 1 kg altogether?

Add the masses: 250 + 250 + 250 + 250 = 1000 g.
1000 g = 1 kg.

Yes. Four 250 g bags have a mass of 1 kg, so they would balance a 1 kg bag on a pan balance.

A 1 kg bag on one side balances four 250 g blocks on the other side.

Worked example

Example 3: Mixed units

A pumpkin has a mass of 2500 g. Write it in kilograms and grams.

Find the thousands: 2500 = 2000 + 500.
2000 g = 2 kg. There are 500 g left over.
500 g is half a kilogram.

2500 g = 2 kg 500 g, or two and a half kilograms.

Worked example

Example 4: Use a benchmark to estimate

Is the mass of a backpack full of books closer to 4 g or 4 kg?

4 g is about the mass of 4 small paper clips. That is far too light.
4 kg is about the mass of 4 cartons of milk. That sounds right for a heavy backpack.

About 4 kg.

Choosing the unit

Ask yourself: "Is it light enough to lift easily with one hand, like a pencil or an apple?" If yes, grams are usually best. "Do I need two hands to carry it?" Then kilograms is probably better.

Math talk

When you estimate, say which benchmark you used: "I think it's about 2 kg because it feels like 2 cartons of milk." Then check with a scale or a label.

Just for fun

Riddle: Which is heavier, 1 kg of feathers or 1 kg of bricks?

Answer: Neither! They both have a mass of 1 kg. The feathers just need a much bigger bag.

Hands-on activity

Kitchen label hunt and benchmark test

You need: food packages from your cupboard, a 1 L carton of milk or water, a box of paper clips, and a kitchen scale if you have one.

Find 5 packages that show mass. Make a table: item, mass on the label, g or kg?
Order your packages from lightest to heaviest using the labels. Be careful: 2 kg is more than 900 g!
Hold the 1 L carton in one hand to feel about 1 kg. Hold a paper clip in the other hand to feel about 1 g.
Pick 3 objects (a shoe, a book, an apple). Estimate each one's mass using your benchmarks. If you have a kitchen scale, check.
Find something you think is close to 1 kg. Check it against the carton by holding one in each hand, or with a scale.

Grown-up note: keep paper clips away from younger children.

Math talk: "Which estimate was closest? What benchmark helped you most?"

Your turn

How many grams are in 1 kg?

Change 5 kg to grams.

Change 4000 g to kilograms.

Would you measure the mass of a pencil in grams or kilograms?

Would you measure the mass of a bag of potatoes in grams or kilograms?

Which is heavier: 1 kg of feathers or 900 g of rocks?

Write 1500 g in kilograms and grams.

More practice: warm-up

What does the g stand for on a food label?

What does kilo mean?

About how much mass does a small paper clip have?

About how much mass does a 1 L carton of water have?

Change 2 kg to grams.

More practice: on your own

Order from lightest to heaviest: 3 kg, 2500 g, 800 g.

Half a kilogram is how many grams?

A dog has a mass of about 20. Should the unit be g or kg? Explain.

Mo buys two bags of rice. One is 1 kg and one is 750 g. What is the total mass in grams?

How many 500 g bags of pasta make 3 kg?

Challenge

A pan balance has a 2 kg bag on one side. On the other side there are three 400 g cans. How many more grams are needed to balance?

A backpack has a mass of 3 kg 200 g. Lunch adds 450 g. What is the new mass? Write it two ways.

Jia says a small paper clip is about 1 g, so 1000 paper clips must be about 1 kg. Is her thinking reasonable? Explain.

Check your understanding

Quiz

How many grams are in 1 kilogram?

Which object has a mass of about 1 g?

Which unit would you use for the mass of a watermelon?

6000 g is the same as...

Which is the heaviest?

How many 250 g blocks balance a 1 kg bag?

For parents and teachers

Big idea: The Ontario curriculum (E2.1) asks Grade 4 students to explain the relationship between grams and kilograms (1 kg = 1000 g) and to use benchmarks to estimate mass. Common benchmarks: a small paper clip is about 1 g, and a 1 L carton of water or milk is about 1 kg.

Common difficulties:

Comparing numbers without units. Children may think 900 g is more than 2 kg because 900 is bigger than 2. Change to the same unit first.
Thinking bigger objects are always heavier. A big bag of chips can be lighter than a small can of beans.
Mixing up mass and capacity units. Grams and kilograms are for mass; litres and millilitres (next lesson) are for capacity.

How to help: Read package labels together at home and in stores. Let your child hold items and guess the mass before checking the label. If you have a kitchen scale, let them weigh fruit and vegetables for cooking.