Constants, conversion factors and basic statistics

Data gathered unrigorously from various unreliable sources. E&OE.


An inch is 25.4 mm. A pound is 0.4536 kg.

A UK gallon is 4.54609 litres. A US gallon is 231 cubic inches or 3.78541 litres.

A litre is 61 cubic inches; 100 cubic feet is 2.83 cubic metres.

An acre is 4,046.86 square metres. Wales is 5.1 million acres.

The speed of light in a vacuum is 186 miles per millisecond.

The speed of sound in air is 12.4 miles per minute.

Energy conversions

Unless the context otherwise requires, a unit means a kilowatt hour (kWh), which is 3,412.14 BTU or 859.85 food calories (kcal).

A tonne of oil equivalent is 11,630 units (of heat). Food fat is not far off around 9 calories per gramme.

A tonne of coal equivalent is 8,141 units (of heat). Anthracite might give 10 units of heat per kg.

UK natural gas ranges from 10.55 to 11.39 units per cubic metre, which is around 15 units per kg.

Liquid fuels (these numbers seem surprisingly hard to pin down):

A cord of wood is 128 cubic feet or 3.62 cubic metres. Good seasoned wood might weigh around 2 tonne per cord and give 4 units per kg, so 8,000 units per cord.

Energy statistics

The UK consumes something like 1.6 trillion units of energy a year.

Electricity use is a little under 400 billion units. UK population is 65.64 million. Average electricity use per head is around 6,000 units a year.

Road transport accounts for almost 500 billion units a year, mainly liquid fuel. The mechanical energy produced might be of the order of 100 billion units.

Orders of magnitude

The kinetic energy of a 1.5-tonne car doing 50 mph is 0.1 unit (6 seconds of 60kW acceleration). For a two-tonne car doing 95 mph, kinetic energy is 0.5 unit (10 seconds of 180kW acceleration).

The kinetic energy of a 500-tonne train doing 191 mph is 500 units (4 minutes of 7,500kW acceleration).

A tonne of TNT equivalent is 1,163 units of explosive energy.

Solar radiation is 1kW a square meter at its best. A fixed surface in the UK might collect up to about 1,000 units a square meter in a year. In terms of electricity, it takes about seven square meters of photovoltaic array to get 1kW peak and 1,000 units a year. A 5,000kW solar farm producing around 5 million units a year uses 10-15 acres, displacing maybe 100 tonnes of potatoes a year.