The US volume ladder and its metric equivalents
US liquid measure doubles its way up from the teaspoon. Three teaspoons make a tablespoon, two tablespoons make a fluid ounce, eight fluid ounces make a cup, two cups make a pint, two pints make a quart, and four quarts make a gallon. Because the gallon is defined as exactly 231 cubic inches and the inch is exactly 2.54 cm, every one of these factors is exact: a gallon is 3.785411784 liters, a quart is 946.353 mL, a cup is 236.588 mL, a fluid ounce is 29.5735 mL, a tablespoon is 14.7868 mL and a teaspoon is 4.92892 mL.
One wrinkle: the FDA nutrition-labeling cup is rounded to 240 mL and the labeling fluid ounce to 30 mL, so a serving size on a package and a cup in a recipe differ by about 1.4 percent. This converter uses the customary 236.588 mL cup, which is what US measuring cups actually hold. Larger volumes use the cubic foot (28.3168 L, or 7.4805 gallons) and the cubic meter (1,000 L), while crude oil trades in barrels of exactly 42 US gallons, or 158.987 L.
Worked examples
Fuel and driving: a 15-gallon tank is 56.8 liters, and a 50-liter European tank is 13.2 gallons. Mileage figures do not convert as neatly, because 30 mpg is 7.84 L per 100 km. Cooking: 500 mL of stock is 2.11 cups, a 750 mL wine bottle is 25.4 fl oz or a little over three cups, and a 2-liter soda is 67.6 fl oz or 8.45 cups. A stick of butter is half a cup, 8 tablespoons, 118.3 mL.
Around the house: a standard bathtub holds roughly 40 to 60 gallons (151 to 227 L); a 40-gallon water heater is 151 L; a chest freezer advertised as 7 cubic feet holds 52.4 gallons or 198 L. Aquariums are sold in US gallons, so a 75-gallon tank is 284 L and weighs about 625 lb once filled, since water weighs 8.345 lb per gallon.
Imperial gallons, dry measure and other traps
The imperial gallon used in the UK and Canada is 4.54609 liters, about 20 percent larger than the US gallon, and the imperial fluid ounce is 28.413 mL, slightly larger than the US 29.574 mL ounce even though the imperial pint holds 20 ounces to the US pint's 16. That means a British recipe calling for a pint of milk wants 568 mL, not 473 mL, and a Canadian car rated 30 miles per imperial gallon only gets 25 US mpg.
The US also keeps a separate dry measure: a dry pint is 551 mL and a dry gallon is 4.405 L, used for berries and produce. Finally, fluid ounces measure volume, not weight. Eight fluid ounces of water weighs about 8.3 ounces, but eight fluid ounces of honey weighs 12 ounces and eight of flour weighs about 4.4, so switch to a kitchen scale whenever a recipe gives grams.