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.
Before 1824, units like the gallon and the pound varied confusingly from town to town and trade to trade across Britain; the Weights and Measures Act of 1824 finally standardised them into the single official “imperial” system used across the British Empire, which is where the name comes from. The United States actually passed its own Metric Conversion Act in 1975, intending a voluntary switch to metric, but the change never fully took hold with the public, so the US remains the largest country still using customary (imperial-derived) units for everyday life such as miles, pounds, and gallons. Interestingly, even fully metric countries keep one imperial holdout: aircraft altitude is measured in feet almost everywhere in the world, a historical convention from early aviation that stuck because switching now would require every pilot and air traffic controller worldwide to retrain at the same time.
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
5 ft = 5 × 30.48 = 152.4 cm. 6 in = 6 × 2.54 = 15.24 cm. Total = 167.64 cm.
10 × 0.454 = 4.54 kg.
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).