## Saturday, 4 June 2011

### Cubic feet and freezers

The imperial unit, cubic foot, as a measure of capacity is preserved in fridges and freezers, judging by the advertising leaflet from an electrical goods store that I have been perusing. It’s interesting to reflect on why this particular imperial unit survives in this context when all the other dimensions of the appliance are in metric units.

The Hotpoint RFA52P fridge freezer, for example, has 5.5 cubic feet fridge capacity and 3.7 cubic feet freezer capacity. Why not give these capacities in metric units, say, cubic metres or cubic millimetres?

The answer is clear once you try to do it!

The SI unit for length is the metre. A foot is about 0.305 of a metre. So, a cubic foot is about (0.305 × 0.305 × 0.305) m3, which is about 0.0284 m3.

So, the fridge capacity of the appliance above would be about 0.16 cubic metres (5.5 × 0.0284) and the freezer capacity would be about 0.105 cubic metres (3.7 × 0.0284). These are inconveniently small numbers to work with and for many people difficult to visualize or to use for comparison. No domestic fridge or freezer will have a capacity in cubic metres that is anything other than a decimal number less than 1 unit.

So, the cubic metre is too large a unit to use in this context. By contrast, the cubic millimetre would be ridiculously small. The 5.5 cubic feet would be about 160,000,000 cubic millimetres! [Yes, there are 1,000,000,000 cubic millimetres in 1 cubic metre!] Even cubic centimetres would be too small. The fridge with 5.5 cubic feet capacity would be about 160,000 cm3.

A sensible metric unit would be the cubic decimetre. A decimetre (dm) is a tenth of a metre, so 1 m3 = 1000 dm3. This gives the capacities of our fridge and freezer as 160 dm3 and 105 dm3, respectively. Now those are very handy numbers for comparing capacities of different appliances.

Unfortunately, the decimetre is a rather neglected metric unit and may appear a bit inaccessible. But, a cubic decimetre is neither neglected nor inaccessible! It has another name: the litre! Of course, we usually associate litres with measures of liquid volume. But it would work well for fridge and freezer capacity. You tell me the capacity of the fridge is 160 litres and I can easily imagine 160 litre-cartons of fruit juice stacked inside. The litre as a unit is accessible and familiar and can be easily visualized. It’s perfect for the job.