Measure mass & choose appropriate units
Theory
Mass is how heavy something is. Choose the unit that gives a tidy number: mg, g, kg or t. To read a scale, work out what one mark is worth, then read the pointer.
Mass is how heavy something is. The right unit depends on how heavy the object is.
Use milligrams (mg) for tiny masses like a vitamin tablet, grams (g) for everyday objects like an apple, kilograms (kg) for a school bag or a person, and tonnes (t) for very heavy loads like a truck.
Each step up is a thousand: \(1\text{ g} = 1000\text{ mg}\), \(1\text{ kg} = 1000\text{ g}\) and \(1\text{ t} = 1000\text{ kg}\).
First work out what one mark is worth, then read where the pointer sits.
| Object | Best unit | Rough mass |
|---|---|---|
| Vitamin tablet | mg | a few hundred mg |
| Apple | g | \(150\) g |
| Bag of flour | kg | \(1\) kg |
| Small car | t | \(1\) t |
On a dial marked every \(250\) g, the marks read \(0, 250, 500, 750, 1000\), so the mark after \(500\) is \(750\) g.
Choosing a unit and reading a scale
- Estimate how heavy the object is and pick the unit that gives an easy number.
- Find the step size on the scale by looking at two numbered marks.
- Read the pointer against the marks, not by counting them.
- Check the unit: mass is grams, kilograms or tonnes, never millilitres or metres.
An apple is about \(150\) g, a tidy number.
The best unit is grams.
A loaded truck is about \(20\) t.
The best unit is tonnes.
Each step is \(250\) g.
The mark after \(500\) is \(750\) g.
Kilograms measure mass; litres measure capacity.
5 kg of potatoes is the mass.
Common pitfalls
Frequently asked questions
What is mass?
Mass is how heavy something is. It is measured in milligrams, grams, kilograms or tonnes, depending on how heavy the object is.
How do I choose the best unit?
Pick the unit that gives a number you can say easily. An apple is \(150\) g, a bag of flour is \(1\) kg, and a truck is about \(20\) t.
How do I read a scale?
Look at two numbered marks to find the step size, then read where the pointer sits. If the marks jump by \(200\), the second mark is \(400\) g.
Is 5 kg a mass or a capacity?
A mass. Kilograms measure how heavy something is. Capacity, how much a container holds, is measured in millilitres and litres.
How are the mass units related?
Each step up is a thousand: \(1\text{ g} = 1000\text{ mg}\), \(1\text{ kg} = 1000\text{ g}\), and \(1\text{ t} = 1000\text{ kg}\).