Imperial Units - Length, Mass and Capacity
Imperial units are a system of measurement that developed in Britain and were once used throughout the British Empire. Today they are still used in everyday life in the UK alongside metric, and are the primary system in the USA.
Imperial Units of Length
| Unit | Abbrev. | Relation | Metric approx. |
|---|---|---|---|
| Inch | in | 12 in = 1 ft | 1 in = 2.54 cm |
| Foot | ft | 3 ft = 1 yd | 1 ft = 30.48 cm |
| Yard | yd | 1,760 yd = 1 mi | 1 yd ≈ 0.914 m |
| Mile | mi | — | 1 mi ≈ 1.609 km |
Imperial Units of Mass
| Unit | Abbrev. | Relation | Metric approx. |
|---|---|---|---|
| Ounce | oz | 16 oz = 1 lb | 1 oz ≈ 28.35 g |
| Pound | lb | 14 lb = 1 stone | 1 lb ≈ 0.454 kg |
| Stone | st | 2,240 lb = 1 ton | 1 st ≈ 6.35 kg |
| Ton | — | Imperial ton | 1 ton ≈ 1,016 kg |
Imperial Units of Capacity
| Unit | Relation | Metric approx. |
|---|---|---|
| Fluid ounce (fl oz) | 20 fl oz = 1 pint | 1 fl oz ≈ 28.4 ml |
| Pint | 8 pints = 1 gallon | 1 pt ≈ 568 ml |
| Gallon | — | 1 gal ≈ 4.546 l |
Worked Examples
Convert 5 feet 6 inches to centimetres.
5 ft = 5 × 30.48 = 152.4 cm. 6 in = 6 × 2.54 = 15.24 cm. Total = 167.64 cm.
Convert 10 pounds to kilograms.
10 × 0.454 = 4.54 kg.
Convert 3 gallons to litres.
3 × 4.546 = 13.638 l.
Key Takeaways
- Imperial units are based on historical conventions, not powers of ten.
- Key conversions: 1 in = 2.54 cm; 1 mi ≈ 1.609 km; 1 lb ≈ 0.454 kg; 1 pt ≈ 568 ml.
- US customary units differ slightly from UK imperial (e.g. US gallon = 3.785 l, not 4.546 l).
Practice Questions
- How many inches are in 4 feet 3 inches?
- Convert 8 miles to kilometres.
- A person weighs 11 stone 4 pounds. How many pounds is that in total?
- Convert 6 pints to litres. (1 pint ≈ 568 ml.)
- Convert 3.5 oz to grams. (1 oz ≈ 28.35 g.)