An older system still in use
Canada officially uses the metric system, but the imperial system is still everywhere: people give their height in feet and inches, buy lumber as "2 by 4s", weigh themselves in pounds, and order a pizza by its size in inches. The United States uses a very similar system, often called US customary units.
Length
| Unit | Symbol | Equals |
|---|---|---|
| Inch | in or ″ | about the width of a thumb |
| Foot | ft or ′ | 12 inches |
| Yard | yd | 3 feet = 36 inches |
| Mile | mi | 5,280 feet = 1,760 yards |
Weight
| Unit | Symbol | Equals |
|---|---|---|
| Ounce | oz | about the weight of a slice of bread |
| Pound | lb | 16 ounces |
| Ton (short ton) | t | 2,000 pounds |
Capacity (US customary)
| Unit | Equals |
|---|---|
| fluid ounce (fl oz) | about 30 mL |
| cup | 8 fluid ounces |
| pint | 2 cups |
| quart | 2 pints |
| gallon | 4 quarts |
Note: the British imperial gallon, pint and quart are about 20% larger than the US ones. Older Canadian recipes and gas pumps used imperial gallons, so always check which is meant.
Converting within the system
The numbers aren’t all tens, so you need to know the relationships.
- To go to a smaller unit, multiply: 4 ft = 4 × 12 = 48 in.
- To go to a bigger unit, divide: 48 oz = 48 ÷ 16 = 3 lb.
Mixed units: 5 ft 7 in = 5 × 12 + 7 = 67 in.
Comparing with metric
| Imperial | Metric (approx.) |
|---|---|
| 1 inch | 2.54 cm (exact) |
| 1 foot | 30.5 cm |
| 1 yard | 0.91 m |
| 1 mile | 1.61 km |
| 1 pound | 0.45 kg |
| 1 ounce | 28 g |
| 1 US gallon | 3.79 L |
Quick estimates: a metre is a little more than a yard; a kilogram is about 2.2 pounds; a kilometre is about 0.6 miles.
Worked examples
Example 1: How many inches in 3 feet?
3 × 12 = 36 in.
Example 2: A baby weighs 120 oz. How many pounds?
120 ÷ 16 = 7.5 → 7 lb 8 oz.
Example 3: About how many centimetres tall is someone who is 5 ft (60 in)?
60 × 2.54 ≈ 152 cm.
Common mistakes
- Treating imperial like metric and moving the decimal point. There are 12 inches in a foot, not 10.
- Writing 5.7 ft for 5 ft 7 in. These aren’t the same: 5 ft 7 in is about 5.58 ft.
- Mixing up fluid ounces (volume) and ounces (weight).
Try it yourself
- How many feet in 4 yards?
- Convert 6 ft 2 in to inches.
- How many ounces in 2.5 lb?
- How many cups in 1 quart?
Answers: 1) 12 ft 2) 74 in 3) 40 oz 4) 4 cups
Quick recap
- Imperial system length: inch, foot (12 in), yard (3 ft), mile (5,280 ft).
- Weight: ounce and pound (16 oz).
- Multiply to go to smaller units; divide to go to bigger ones.