Units of Measurement and Conversion
What Is This?
Measurement assigns a number and a unit to a quantity — how long, how heavy, how much, how long in time. Conversion is changing a measurement from one unit to another (like metres to centimetres) without changing the actual amount. The key skill: knowing the conversion facts and whether to multiply or divide.
Why Does It Matter?
Recipes use grams and millilitres, road signs use kilometres or miles, doctors measure in centimetres and kilograms, and schedules run on hours and minutes. Converting between units correctly prevents real mistakes — a medicine dose, a recipe, or a journey time depends on it.
The Golden Rule of Conversion
Big unit → small unit: MULTIPLY Small unit → big unit: DIVIDE
Think about it: 1 metre contains 100 centimetres, so converting metres (big) to centimetres (small) gives a bigger number — multiply. Going the other way gives a smaller number — divide.
Part 1 — Metric Length
| Conversion | Fact |
|---|---|
| 1 kilometre (km) | = 1,000 metres (m) |
| 1 metre (m) | = 100 centimetres (cm) |
| 1 centimetre (cm) | = 10 millimetres (mm) |
Examples:
- 3 km = 3 × 1,000 = 3,000 m
- 250 cm = 250 ÷ 100 = 2.5 m
- 4.5 m = 4.5 × 100 = 450 cm
- 80 mm = 80 ÷ 10 = 8 cm
Part 2 — Metric Mass
| Conversion | Fact |
|---|---|
| 1 kilogram (kg) | = 1,000 grams (g) |
| 1 gram (g) | = 1,000 milligrams (mg) |
| 1 tonne (t) | = 1,000 kilograms (kg) |
Examples:
- 2 kg = 2 × 1,000 = 2,000 g
- 3,500 g = 3,500 ÷ 1,000 = 3.5 kg
- 0.75 kg = 0.75 × 1,000 = 750 g
Part 3 — Metric Capacity (Volume)
| Conversion | Fact |
|---|---|
| 1 litre (L) | = 1,000 millilitres (mL) |
Examples:
- 2 L = 2 × 1,000 = 2,000 mL
- 500 mL = 500 ÷ 1,000 = 0.5 L
- 1.25 L = 1,250 mL
Part 4 — Time
| Conversion | Fact |
|---|---|
| 1 minute | = 60 seconds |
| 1 hour | = 60 minutes |
| 1 day | = 24 hours |
| 1 week | = 7 days |
| 1 year | = 12 months = 52 weeks ≈ 365 days |
⚠️ Time is NOT base-10! 1.5 hours = 90 minutes (not 150). Always use 60 for hours↔minutes and minutes↔seconds.
Examples:
- 3 hours = 3 × 60 = 180 minutes
- 240 seconds = 240 ÷ 60 = 4 minutes
- 2.5 hours = 2 × 60 + 30 = 150 minutes
- 100 minutes = 1 hour 40 minutes (100 ÷ 60 = 1 R40)
Part 5 — Customary (US) Units
| Type | Facts |
|---|---|
| Length | 1 foot = 12 inches; 1 yard = 3 feet; 1 mile = 5,280 feet |
| Weight | 1 pound (lb) = 16 ounces (oz); 1 ton = 2,000 pounds |
| Capacity | 1 cup = 8 fluid ounces; 1 pint = 2 cups; 1 quart = 2 pints; 1 gallon = 4 quarts |
Examples:
- 4 feet = 4 × 12 = 48 inches
- 6 yards = 6 × 3 = 18 feet
- 2 pounds = 2 × 16 = 32 ounces
- 3 gallons = 3 × 4 = 12 quarts
- 24 inches = 24 ÷ 12 = 2 feet
Capacity Chain
1 gallon = 4 quarts = 8 pints = 16 cups
Choosing the Right Unit
Pick a unit that gives sensible numbers:
| Measuring… | Sensible unit |
|---|---|
| Width of a fingernail | mm |
| Height of a person | cm or m |
| Distance between cities | km (or miles) |
| Mass of an apple | g |
| Mass of a person | kg |
| Medicine dose | mL or mg |
| A bath full of water | L (or gallons) |
Multi-Step Conversions
Some conversions need two steps. Convert through the middle unit.
Example: How many millimetres in 2 metres?
- Step 1: 2 m = 200 cm
- Step 2: 200 cm = 2,000 mm ✓ (Or directly: 1 m = 1,000 mm, so 2 m = 2,000 mm.)
Example: How many cups in 2 gallons?
- 2 gallons = 8 quarts = 16 pints = 32 cups ✓
Worked Examples
Worked Example 1: Metric length
A ribbon is 3.2 m long. How many centimetres is that?
- Big → small: multiply. 3.2 × 100 = 320 cm ✓
Worked Example 2: Mass comparison
Which is heavier: 2.4 kg or 2,350 g?
- Convert to the same unit: 2.4 kg = 2,400 g
- 2,400 g > 2,350 g → 2.4 kg is heavier ✓
Worked Example 3: Time
A movie lasts 2 hours 15 minutes. How many minutes in total?
- 2 × 60 + 15 = 120 + 15 = 135 minutes ✓
Worked Example 4: Adding mixed units
Two boards are 1.35 m and 85 cm long. What is their total length in metres?
- 85 cm = 0.85 m
- 1.35 + 0.85 = 2.2 m ✓
Worked Example 5: Customary capacity
A recipe needs 6 cups of water. How many pints is that?
- 1 pint = 2 cups → 6 ÷ 2 = 3 pints ✓
Worked Example 6: Word problem
A bottle holds 1.5 L. Glasses hold 250 mL each. How many glasses can be filled?
- 1.5 L = 1,500 mL
- 1,500 ÷ 250 = 6 glasses ✓
Common Mistakes
Mistake 1: Multiplying when you should divide (or vice versa) Going from small units to big units (cm → m), the number must get smaller — divide. Check: does the answer make sense?
Mistake 2: Treating time as base-10 1.5 hours is 90 minutes, not 150. Half an hour = 30 minutes. Always use 60.
Mistake 3: Comparing different units directly You cannot compare 2.4 kg with 2,350 g until both are in the same unit.
Mistake 4: Mixing up conversion facts 1 m = 100 cm but 1 km = 1,000 m. The metric prefixes: kilo = 1,000, centi = 1/100, milli = 1/1,000.
Mistake 5: Skipping the middle step in chain conversions Gallons → cups must go through quarts and pints (×4, ×2, ×2 = ×16) unless you remember 1 gallon = 16 cups directly.
Key Takeaways
- Big → small unit: multiply. Small → big unit: divide.
- Metric: km/m/cm/mm (length), kg/g/mg (mass), L/mL (capacity) — all powers of 10
- Time uses 60s and 24s — never base-10
- Customary: 12 inches/foot, 3 feet/yard, 16 oz/lb, and the gallon→quart→pint→cup chain (×4, ×2, ×2)
- Always convert to the same unit before comparing or adding measurements
Practice and Resources
Ready to practise? Try our Grade 3-5 Units of Measurement and Conversion Worksheet covering metric, customary, time, and multi-step conversions. Test yourself with the Grade 3-5 Math Practice Test.