How heavy is a bag of rice, a newborn baby or a truck of sand? To answer, we measure mass in grams and kilograms. This lesson explains the difference between mass and weight, the instruments used to measure them, how to write measurements in different ways, how to convert between grams and kilograms, and how to solve everyday problems. It works just like the metric length conversions in Units of Measurement.
Mass or weight?
Mass is the amount of matter (stuff) in an object. It is measured in grams (g) and kilograms (kg), and it is the same everywhere – on Earth, on the Moon or in space. Weight is the pull of gravity on that mass. Scientists measure weight in newtons, and it changes with gravity: on the Moon, gravity is about one sixth as strong, so a 60 kg astronaut would weigh about as much as a 10 kg object does on Earth, even though their mass is still 60 kg. In everyday life people say “weight” when they mean mass (“I weigh 45 kg”), and kitchen and bathroom scales are marked in kilograms.
Units and instruments
| Unit | Symbol | Size | Good for |
|---|---|---|---|
| milligram | mg | 1,000 mg = 1 g | medicines, a grain of sand |
| gram | g | about the mass of a paper clip | letters, food packets, coins |
| kilogram | kg | 1 kg = 1,000 g; a litre of water | people, bags of rice, fruit |
| tonne | t | 1 t = 1,000 kg | cars, trucks, cargo |
Instruments include a beam balance (compare an object with known masses), a kitchen scale, a bathroom scale, a spring balance (a hook and spring – this really measures weight) and a digital scale. Read a dial scale with your eye level with the pointer, and check that the scale shows 0 before you start.
Writing and converting measurements
A mass can be written in several equivalent ways: 3 kg 100 g = 3,100 g = 3.1 kg = 3110 kg.
- kg → g: multiply by 1,000 (move the decimal point 3 places right).
- g → kg: divide by 1,000 (move the decimal point 3 places left).
- Always change to the same unit before adding, subtracting or comparing.
Worked examples
Example 1: kg to g
Question: Convert 2.45 kg to grams.
- Grams are smaller, so multiply by 1,000.
- 2.45 × 1,000 = 2,450.
Answer: 2,450 g.
Example 2: g to kg
Question: Convert 750 g to kilograms.
- Kilograms are bigger, so divide by 1,000.
- 750 ÷ 1,000 = 0.75.
Answer: 0.75 kg (three quarters of a kilogram).
Example 3: Mixed units
Question: Write 5 kg 60 g in grams and in kilograms.
- 5 kg = 5,000 g, so 5 kg 60 g = 5,060 g. (Careful: 60 g, not 600 g.)
- 5,060 ÷ 1,000 = 5.06 kg.
Answer: 5,060 g = 5.06 kg.
Example 4: Adding masses
Question: A shopping bag holds 1.2 kg of flour, 750 g of sugar and two 450 g tins. What is the total mass?
- Change everything to grams: 1.2 kg = 1,200 g.
- 1,200 + 750 + 2 × 450 = 1,200 + 750 + 900 = 2,850 g.
Answer: 2,850 g = 2.85 kg.
Example 5: Sharing
Question: A 25 kg sack of rice is packed into 500 g bags. How many bags are filled?
- 25 kg = 25,000 g.
- 25,000 ÷ 500 = 50.
Answer: 50 bags.
Example 6: Cost
Question: Tomatoes cost $18 per kg. How much do 2.5 kg cost? And if 1.5 kg of cheese costs $30, what is the price per kg?
- 2.5 × 18 = 45, so $45.
- 30 ÷ 1.5 = 20, so $20 per kg.
Answer: $45; $20 per kg.
Common mistakes
- Multiplying by 100 instead of 1,000. There are 1,000 g in a kilogram.
- Writing 5 kg 60 g as 5.6 kg. 60 g is 0.06 kg, so it is 5.06 kg.
- Adding grams to kilograms without converting first.
- Forgetting to zero the scale (or to subtract the mass of the container).
Practice questions
Try these yourself first, then open each answer to check your working.
1. Convert:
(a) 4.3 kg to g
(b) 650 g to kg
(c) 2,005 g to kg.
Show answer
(a) 4,300 g
(b) 0.65 kg
(c) 2.005 kg.
2. Write 7 kg 80 g in grams.
Show answer
7,080 g.
3. Put in order, lightest first: 1.2 kg, 950 g, 1,025 g, 1 kg.
Show answer
950 g, 1 kg, 1,025 g, 1.2 kg.
4. A biscuit packet has a mass of 250 g. What is the mass of 12 packets, in kg?
Show answer
3,000 g = 3 kg.
5. Three parcels of 275 g each are put on a scale that can hold 1 kg. How much more can be added?
Show answer
1,000 − 825 = 175 g.
6. 3.6 kg of beef is cut into 8 equal pieces. What is the mass of each piece in grams?
Show answer
3,600 ÷ 8 = 450 g.
7. Rice costs $3.50 per kg. What do 12 kg cost?
Show answer
12 × 3.50 = $42.
8. Which unit would you use for
(a) a mango
(b) a bus
(c) a vitamin tablet?
Show answer
(a) grams
(b) tonnes
(c) milligrams.
Quick check
1. How many grams are in 1 kg?
Show answer
1,000.
2. Does your mass change on the Moon?
Show answer
No – your mass stays the same; your weight is less.
3. Is 0.5 kg more or less than 450 g?
Show answer
More (500 g).
Next: Time: 12- and 24-Hour Clocks, Converting Units and Working Out Durations
Curriculum link: This post is relevant to the Trinidad and Tobago National Lower Secondary (NCSE) Mathematics Curriculum – Form 1, Term 2, Strand 1.5 Measurement, Topic 1.5.5 Mass and Weight – Learning Outcomes 1.5.5.1 (measuring mass and weight in kilograms and grams), 1.5.5.2 (converting between grams and kilograms) and 1.5.5.3 (solving problems involving mass and weight).
Attributions to media used in this post:
Balance scale IMGP9755.jpg – Nikodem Nijaki – CC BY-SA 3.0 (via Wikimedia Commons)
Medals weighed in digital scale.jpg – Kpubkalanami – CC BY-SA 4.0 (via Wikimedia Commons)
Updates
2026/10/25
– Put each lettered part of the questions and answers on its own line.
© 2026 Vedesh Kungebeharry. All rights reserved.
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