The units of volume
Volume is where the metric and imperial systems diverge most awkwardly, because the imperial units were never harmonised between Britain and the United States. A US gallon is 3.785 litres and an imperial gallon is 4.546, a 20% gap that survives in pints, quarts and fluid ounces too. The litre itself is not an SI base unit but is accepted alongside it, defined as exactly one cubic decimetre.
| Unit | Symbol | Equals |
|---|---|---|
| Millilitres | ml | 0.001 l |
| Centilitres | cl | 0.01 l |
| Litres | l | 1 l |
| Cubic metres | m³ | 1,000 l |
| Cubic centimetres | cm³ | 0.001 l |
| US gallons | gal | 3.78541178 l |
| Imperial gallons | gal imp | 4.54609 l |
| US fluid ounces | fl oz | 0.02957353 l |
| US pints | pt | 0.47317647 l |
| US cups | cup | 0.23658824 l |
How to convert by hand
Every volume conversion follows the same procedure: take the amount to the base unit, then from there to the target unit. In practice it is a single multiplication, because the two steps combine into one factor.
Multiply by the source unit's factor to reach litres, then divide by the target's. For cubic units, remember the factor is the length factor cubed: a cubic metre is 1,000 litres because a metre is 10 decimetres and 10 cubed is 1,000.
The equivalents people look up most
| Conversion | Result |
|---|---|
| A can of drink | 330 ml · 11.2 US fl oz |
| A bottle of wine | 750 ml · 25.4 US fl oz |
| A car fuel tank | 50 l · 13.2 US gal |
| A cubic metre | 1,000 l · 264 US gal |
| A US cup | 237 ml · 8 US fl oz |
Common mistakes
- Mixing US and imperial gallons. They differ by 20%, which is the difference between a full tank and a very empty one.
- Treating a fluid ounce as an ounce. One is volume, the other is mass, and they only coincide roughly for water.
- Using the length factor for cubic units. A cubic metre is 1,000 litres, not 10.
- Assuming a cup is a fixed size. A US cup is 237 ml, a metric cup is 250 ml and a UK recipe cup is often 284 ml.