Canadian Measurement Math: A Friendly Guide to Metric Units

Start with the unit

A measurement has two parts: a number and a unit. Five metres is not the same as five centimetres, even though both numbers are 5. The unit tells you what the number is counting.

For length, the base unit is the metre (m). For liquid volume, the litre (L) is common. For mass, the gram (g) and kilogram (kg) appear often. Temperature is commonly described in degrees Celsius (°C).

Use prefixes as scale clues

The prefix tells you how many smaller pieces make one larger unit. That is why the same number can become larger when the unit becomes smaller: 2 m is 200 cm.

1. Remember that centi- means one hundredth, so 1 m = 100 cm.
2. Remember that milli- means one thousandth, so 1 L = 1,000 mL.
3. Remember that kilo- means one thousand, so 1 kg = 1,000 g.
4. When moving to a smaller unit, multiply. When moving to a larger unit, divide.

Make a conversion table

Suppose a school garden bed is 3.5 m long. To write that length in centimetres, multiply by 100: 3.5 × 100 = 350 cm. To go back, divide 350 by 100 to get 3.5 m.

Write the unit beside every number while you work. The labels act like a guardrail and make it easier to notice when a conversion is headed in the wrong direction.

Temperature needs a different question

Celsius is a temperature scale, not a prefix conversion. A change from 5°C to 15°C is an increase of 10 degrees. A temperature of −8°C is 8 degrees below zero, so it helps to place it on a number line before comparing it with a positive temperature.

For a weather report, ask whether you are comparing the temperatures themselves or the size of the change. From −8°C to 2°C is a 10-degree increase, even though the final temperature is only 2°C.