Convert teaspoon to gallon (imperial)

Recipes give ingredients either by volume (millilitres, cups, spoons) or by weight (grams, ounces), and the two cannot be converted directly: the weight of a given volume depends on the ingredient. A cup of water weighs about 240 g, a cup of flour only about 125 g and a cup of sugar about 200 g. The link between the two is the density of the ingredient.

A cup is not the same size everywhere: the US customary cup is 236.6 ml (US food labels round it to 240 ml), the metric cup used in Australia, New Zealand and Canada is 250 ml, and the old imperial cup was 284 ml, half an imperial pint. A tablespoon is 15 ml in the US, UK and Europe but 20 ml in Australia; a teaspoon is 5 ml. The fluid ounce here is the 30 ml of nutrition labels; the US fluid ounce is strictly 29.57 ml and the imperial one 28.41 ml.

The UK and the US kept different gallons after 1824. The imperial gallon is 4.546 l and the US gallon 3.785 l, so a US gallon is only five-sixths of a British one; the same applies to their quarts and pints, and a British pint of beer (568 ml, 20 fl oz) is bigger than an American one (473 ml, 16 fl oz). The US also has a separate dry gallon of 4.405 l for grain and produce. Fuel is sold by the litre in Britain today but by the US gallon in America, which matters for fuel-economy figures.