Weighing with Scales

What does weighing tell us?

When we weigh something, we find its mass — how heavy it is. Mass is measured in two everyday units:

• grams (g) for light things, like a strawberry, a letter or a pencil.
• kilograms (kg) for heavier things, like a bag of sugar, a cat or a person.

The link between them is simple and worth memorising:

1000 grams = 1 kilogram

You can revise these units in Units of Length, Mass and Capacity.

Two kinds of scales

There are two common tools for weighing, and they work in different ways.

Dial or digital scales (like kitchen scales) have a pointer or a screen. You place the object on top and read the mass directly off a scale.

Balance scales (like the old market scales) have two pans. You put the object in one pan and add known weights to the other until the two sides hang level. When the pans balance, the masses are equal.

Reading a dial scale

A dial scale is just a curved number line, so the first job is the same as for any scale: work out what one small mark is worth.

1. Find two numbered marks.
2. Subtract to find the gap.
3. Count the spaces between them.
4. Divide the gap by the number of spaces.

You can practise this skill further in Reading Scales and Measuring.

Worked example 1: kitchen scale

A scale is labelled 0, 500, 1000 grams, with 5 small spaces between each pair of numbers.

1. Gap: 500 − 0 = 500.
2. Spaces: 5.
3. One space: 500 ÷ 5 = 100 g.

If the pointer stops on the 3rd mark past 500, count on in hundreds: 500, 600, 700, 800. The mass is 800 g.

Worked example 2: into kilograms

The same scale shows the pointer on the mark for 1500 g. To give this in kilograms, divide by 1000:

1500 ÷ 1000 = 1.5 kg.

So 1500 g is the same as 1.5 kilograms.

Using a balance scale

A balance scale answers the question "are these the same mass?" or "how heavy is this?"

Worked example 3: balancing

An apple sits in the left pan. You add weights to the right pan: a 100 g weight, a 50 g weight and a 20 g weight. Now the pans are level. What is the apple's mass?

1. Add the known weights: 100 + 50 + 20 = 170 g.
2. Because the pans balance, the apple equals that total.
3. The apple's mass is 170 g.

Adding masses together

In real weighing — like following a recipe — you often add masses. As long as they are in the same unit, you add them like ordinary numbers.

A recipe needs 250 g of flour, 125 g of sugar and 75 g of butter. What is the total mass?

1. 250 + 125 = 375 g.
2. 375 + 75 = 450 g.

The total is 450 g. This is the same column addition you use in Column Addition with Carrying.

Choosing the right unit

Object	Sensible unit
A feather	g
A bar of chocolate	g
A bag of potatoes	kg
A bicycle	kg
A grain of rice	g
A child	kg

Pick the unit that gives a tidy number. A person is much easier to describe as 40 kg than as 40,000 g.

Why weighing matters

Weighing accurately is needed all over real life. Cooks weigh ingredients so a cake rises properly. Shops weigh fruit and vegetables to charge a fair price. Doctors weigh babies to check they are growing. Post offices weigh parcels to set the right postage. Behind all of it is the same maths: reading a scale by finding the value of one mark, and adding masses in matching units.

Try it yourself

If you have kitchen scales at home, try a weighing hunt:

1. Estimate the mass of three objects first — a book, an apple and a spoon.
2. Weigh each one and write down the real mass in grams.
3. How close were your estimates?
4. Add two of the masses together. Is the total more or less than 1 kg?

Well done!

You now know that mass is measured in grams and kilograms, that 1000 g make 1 kg, how to read a dial scale by finding one mark's value, and how a balance scale compares masses. Build your skills further in Reading Scales and Measuring or Units of Length, Mass and Capacity.