Science · Inquiry & data skills
Units of Measurement and SI Conversions
Reading a result correctly means knowing its unit, and being able to switch between units without changing what the measurement actually says.
On the CAEC Science test you will not be asked to memorize science facts. Instead you will read experiments, tables, and graphs and answer questions about them. Almost every one of those questions comes with a unit attached, centimetres, grams, seconds, degrees Celsius. If you misread or mismatch a unit, a perfectly good calculation can give a wrong answer.
This lesson teaches a transferable inquiry skill: how to choose a sensible unit for a measurement and how to convert within the SI (metric) system. A calculator is permitted on the Science test, so the arithmetic is easy, the skill is knowing which way to move and by how much.
The SI units you will actually see
SI stands for the International System of Units, the shared measuring language scientists use so everyone reports results the same way. You do not need to memorize all of them. These are the ones that show up in test scenarios:
| What it measures | Unit | Symbol |
|---|---|---|
| Length / distance | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Volume (liquids) | litre | L |
| Temperature (everyday) | degree Celsius | °C |
Metric prefixes: kilo, centi, milli
The metric system is built on tens, so converting is just sliding a decimal point. A prefix in front of a base unit tells you how big or small it is. These three cover almost everything on the test:
| Prefix | Symbol | Means | Example |
|---|---|---|---|
| kilo− | k | 1,000 of the unit | 1 km = 1,000 m |
| (base unit) | N/A | 1 of the unit | 1 m = 1 m |
| centi− | c | one hundredth (1⁄100) | 1 m = 100 cm |
| milli− | m | one thousandth (1⁄1000) | 1 m = 1,000 mm |
A small conversion reference
Keep this handful in your back pocket. Once you know these, most test conversions are a single multiply or divide on the calculator.
| Quantity | Conversion |
|---|---|
| Length | 1 km = 1,000 m · 1 m = 100 cm · 1 cm = 10 mm |
| Mass | 1 kg = 1,000 g · 1 g = 1,000 mg |
| Volume | 1 L = 1,000 mL · 1 mL = 1 cm³ |
Worked example #1: metres to centimetres
A student investigating plant growth records that a seedling grew 0.42 m over four weeks. The data table she is filling in uses centimetres. What value should she write?
We are going from a larger unit (metres) to a smaller one (centimetres), so the number should get bigger. From the reference, 1 m = 100 cm, so we multiply by 100.
- Set up: 0.42 m × 100 cm per m.
- Calculate: 0.42 × 100 = 42.
Worked example #2: grams to kilograms
In a chemistry experiment, a group measures the mass of a salt sample as 250 g on a balance. To compare it with another team whose results are in kilograms, they need the same mass in kilograms. What is it?
Now we are going from a smaller unit (grams) to a larger one (kilograms), so the number should get smaller. Since 1 kg = 1,000 g, we divide by 1,000.
- Set up: 250 g ÷ 1,000 g per kg.
- Calculate: 250 ÷ 1,000 = 0.25.
The trap: moving the decimal the wrong way
The most common slip is multiplying when you should divide (or vice versa). Watch the same conversion done two ways. A team measures a reaction time of 1,500 milliseconds and wants it in seconds (1 s = 1,000 ms).
"Milliseconds are smaller, so I multiply: 1,500 × 1,000 = 1,500,000 seconds."
This makes the number bigger when going to a larger unit, backwards. A 1.5 second reaction is not a million seconds.
Seconds are bigger than milliseconds, so the number shrinks: 1,500 ÷ 1,000 = 1.5 seconds.
A reaction time of about a second and a half is sensible, the sanity check confirms the direction.
The fix is always the same: before you trust a converted number, ask "did it move in the direction I expected?" Smaller unit, bigger number; larger unit, smaller number.
Habits that prevent unit mistakes
- Read the unit in the question and the unit in the answer choices. If they differ, a conversion is part of the problem, do not skip it.
- Predict the direction first. Decide whether the number should get bigger or smaller before you touch the calculator. It catches most errors instantly.
- Keep the unit attached to the number. Writing "0.25 kg" instead of just "0.25" keeps you from comparing grams to kilograms by accident.
- Match units before calculating. If a table mixes cm and m, convert everything to one unit first, then add, subtract, or compare.
Your turn: practice problems
For each one, first predict whether the number gets bigger or smaller, then convert. A calculator is fine.
- A wire is measured as 3.5 m long. How many centimetres is that?
- A sample of water has a volume of 750 mL. What is its volume in litres?
- A rock has a mass of 2.4 kg. How many grams is that?
Tap to reveal the answers
- 1. Metres to centimetres is a larger-to-smaller move, so multiply by 100: 3.5 × 100 = 350 cm.
- 2. Millilitres to litres is a smaller-to-larger move, so divide by 1,000: 750 ÷ 1,000 = 0.75 L.
- 3. Kilograms to grams is a larger-to-smaller move, so multiply by 1,000: 2.4 × 1,000 = 2,400 g.
Why this matters for the CAEC
The CAEC Science test is 35 questions in 90 minutes, and it rewards inquiry and data skills rather than memorized facts. Handling units confidently lets you read tables, compare results between groups, and check whether an answer is even reasonable, all of which come up again and again across biology, chemistry, physics, and earth-science scenarios.
Want more practice like this? Browse the Science lessons, grab the CAEC Ready Workbook, or start with a free sample to test yourself.
Disclaimer
This article is a general study lesson. CAEC Ready is an independent study resource and is not affiliated with or endorsed by any government, ministry of education, or official CAEC testing provider.