US Customary vs. British Imperial Fluid Units Explained

One of the most confusing areas of measurement is liquid volume. While length and mass were standardized internationally in 1959, volume was not. The British Weights and Measures Act of 1824 standardized the British Empire around the imperial gallon (4.54609 liters), defined as the volume of 10 pounds of distilled water at 62 °F. Meanwhile, the United States retained the earlier 1706 Queen Anne wine gallon of 231 cubic inches (3.78541 liters).

Because the British gallon was divided into 160 fluid ounces and the US gallon into 128 fluid ounces: • 1 US Fluid Ounce = 29.5735 mL • 1 Imperial Fluid Ounce = 28.4131 mL Thus, an individual US fluid ounce is about 4% larger than a British fluid ounce. However, an Imperial pint contains 20 Imperial ounces (568.26 mL), whereas a US pint contains only 16 US fluid ounces (473.18 mL)—making the British pint 20% larger overall!

When following British, Australian, or Canadian recipes: be careful with pints, cups, and gallons. In modern Australian, New Zealand, and UK metric cooking, a metric cup is defined as 250 mL, whereas in the US, a customary cup is 236.59 mL (and a legal FDA nutrition cup is 240 mL). For delicate baking, weighing ingredients in grams is always superior to measuring volume in cups.