Basic Units
Basic units in the metric system provide a standardised way to measure length, mass, and capacity. The Year 2 UK National Curriculum introduces six fundamental units: centimetres and metres for length, grams and kilograms for mass, and millilitres and litres for capacity. Choosing the appropriate unit depends on the size of the object being measured — a pencil measures about 15 cm, whilst a football pitch measures about 100 m.
Why it matters
Understanding basic units forms the foundation for all measurement in science, cooking, construction, and daily life. A recipe might call for 250 mL of milk and 500 g of flour — using the wrong units could ruin the dish. In medicine, precise dosages measured in milligrams can be life-critical. Construction workers must choose between millimetres for precision work and metres for room dimensions. Shop assistants weigh fruit in grams for small amounts or kilograms for larger purchases. Weather reports give temperatures in degrees Celsius and rainfall in millimetres. This knowledge prepares students for more advanced topics like area (square metres), volume (cubic metres), and unit conversions in GCSE mathematics and sciences.
How to solve basic units
Choosing Appropriate Units
- Length: mm (small), cm (hand-sized), m (room), km (distance).
- Mass: g (light), kg (everyday), tonnes (very heavy).
- Capacity: mL (spoon), L (bottle).
- Choose the unit that gives sensible numbers.
Example: A door is about 2 m tall (not 200 cm or 0.002 km).
Worked examples
What unit would you use to measure the weight of a coin?
Answer: g
- Think about the size of the thing → a coin is about 5 g — Picture a coin in your hand or in your mind. Is it something you can hold? Something that fits on a table? That tells you it's small.
- Choose the right unit: small things use small units → Best unit: g — Small lengths use cm (not metres -- that's for rooms). Light things use g (not kg -- that's for people). Small amounts of liquid use mL (not L -- that's for bottles). A coin is about 5 g, so g is perfect.
- State the answer → g — We measure a coin in g.
What unit would you use to measure the weight of a bag of potatoes?
Answer: kg
- Think about the size of the thing → a bag of potatoes is about 5 kg — Picture a bag of potatoes. Is it big enough to walk across? Heavy enough to carry with two hands? That tells you it's a medium-to-large thing.
- Big things use big units → Best unit: kg — Big lengths use m or km (imagine measuring a classroom in mm -- you'd get a huge number!). Heavy things use kg. Large volumes use L. Using the right-sized unit keeps the number manageable.
- State the answer → kg — We measure a bag of potatoes in kg. It's about 5 kg.
A builder measured 4 km. Express this in m.
Answer: 4000
- Remember: 1 km = 1000 m → 1 km = 1000 m — This is the key fact. Think of 1 km as a big box that contains 1000 smaller m boxes inside it.
- Bigger to smaller = multiply → 4 x 1000 = ? — We have 4 big units. Each one 'unpacks' into 1000 small units. More small pieces means multiply. Like opening 4 bags of 1000 sweets -- you get lots of sweets!
- Calculate → 4 x 1000 = 4000 — 4 x 1000 = 4000. So 4 km = 4000 m.
Common mistakes
- Choosing metres to measure a pencil gives an impractical answer like 0.15 m instead of the sensible 15 cm
- Using grams for a person's weight results in an unwieldy 70,000 g instead of the manageable 70 kg
- Measuring a teaspoon of medicine in litres produces 0.005 L rather than the clearer 5 mL
- Converting 3 km to metres by dividing gives 3 ÷ 1000 = 0.003 instead of multiplying to get 3000 m