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Mass: Kilograms & Grams

Maths · Y3 Measurement
Year 3
Lesson Guide
Explanation
Mass tells us how heavy something is. Light things are measured in grams (g) — a paperclip is about 1 gram, an apple about 150 g. Heavy things use kilograms (kg) — a litre of milk is about 1 kg, and you are probably 25 to 30 kg. The magic number: 1000 grams make 1 kilogram.
Your Turn
Grams or kilograms? A strawberry, a bicycle, a phone, an elephant. Check: g, kg, g (about 200 g!), kg (thousands of them).
Watch Out
'Kilo' means a thousand — kilogram = a thousand grams. That 1000 (not 100!) is the number to remember.
Explanation
Before numbers, we compare. Hefting means holding one object in each hand and feeling which is heavier. Balance scales do the same job fairly: the heavier side sinks, the lighter side rises, and level arms mean equal mass. A book versus a teddy? The book sinks — heavier.
Your Turn
Heft two things from your desk — a book and a pencil case. Which feels heavier? If you have kitchen scales, check your feeling!
Watch Out
Big doesn't mean heavy! A huge balloon hefts lighter than a small rock. Size and mass are different things.
Explanation
Kitchen scales point a needle at the answer — your job is to know what each mark is worth. Count the marks between two labelled numbers: if 0 to 1 kg has four gaps, each gap is 250 g. A needle halfway between 250 and 500 shows about 375 g. Work out the gap size FIRST, then read the needle.
Your Turn
A scale shows marks at 0, 500 g and 1 kg, with one small mark between each. What is each small mark worth? Check: 250 g.
Watch Out
Not all scales count in the same jumps! Always check what one gap is worth before reading.
Explanation
Converting is the ×1000 game. Kilograms to grams: multiply — 2 kg = 2000 g. Grams to kilograms: bundle each 1000 — 1500 g is one bundle of 1000 (1 kg) with 500 g left, so 1 kg 500 g. And the ones you'll hear at the shops: half a kilo is 500 g, a quarter is 250 g.
Your Turn
Convert: 3 kg to g, and 2500 g to kg-and-g. Check: 3000 g; and 2 kg 500 g.
Watch Out
Mass uses ×1000, not ×100 like metres and centimetres. Each measurement family has its own magic number!
Explanation
Mass problems use all your maths tools. Two 400 g bags of flour: 400 + 400 = 800 g. A 1 kg cereal box with 300 g eaten: convert first (1000 g), then subtract — 700 g left. Three 150 g apples versus a 500 g melon: 3 × 150 = 450 g, so the melon is heavier. Match the units, pick the operation, sense-check. Quiz and exercises await!
Your Turn
A puppy weighed 2 kg. It gained 500 g. How heavy now? Check: 2000 + 500 = 2500 g = 2 kg 500 g.
Watch Out
Never add kg and g directly — 1 kg + 300 g is NOT 301. Convert to the same unit first!
Step 1: Introduction
Step 1 of 5
Mass = how heavy something is 🪶 grams (g) — light things a feather, a coin, an apple 🎒 kilograms (kg) — heavy things a schoolbag, a dog, YOU 1000 g = 1 kg Handy anchors: a paperclip ≈ 1 g · an apple ≈ 150 g a litre of milk ≈ 1 kg · a Year 3 kid ≈ 25–30 kg
Balance scales tell the truth 📚 heavier — goes DOWN 🧸 lighter — goes UP Hefting: hold one thing in each hand and feel the difference. Balance scales do it fairly: the heavier side sinks. Level arms = equal mass!
What does the needle say? 0g 250g 500g 750g 1kg 1.25kg 1.5kg 1.75kg Count the jumps between marks: each mark here is 250 g. The needle points between 250 g and 500 g — about 375 g.
kg ↔ g 2 kg = 2 × 1000 = 2000 g 1500 g = 1 kg and 500 g half a kg = 500 g kg → g: multiply by 1000. g → kg: bundle each 1000 into a kg.
Mass maths at the shops 🛒 "A bag holds 400 g of flour. How much in 2 bags?" 400 + 400 = 800 g "A 1 kg box of cereal has 300 g eaten. How much is left?" 1 kg = 1000 g → 1000 − 300 = 700 g left "Which is heavier: 3 apples at 150 g each, or a 500 g melon?" 3 × 150 = 450 g < 500 g → the melon! Match the units, then add, subtract or multiply. Quiz time!
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