Metric vs Imperial vs US Customary Units

• The metric system, in its modern form the International System of Units (SI), uses the meter, kilogram, second and other units scaled by powers of ten. It is the standard in science and in nearly every country for trade and daily life.
• US customary units (inch, foot, mile, pound, US gallon) are the everyday system in the United States. US science, medicine and much of US industry also use SI.
• British imperial units grew from the same older English units as the US system. The United Kingdom now uses metric units officially, but miles on road signs and pints for draft beer and milk are still common.

US customary and imperial units share many names, and most lengths and masses are identical. Volumes are where they part ways.

Since the international yard and pound agreement of 1959, the yard and the pound have exact metric definitions used by the United States, the United Kingdom and other Commonwealth countries.

• 1 yard = 0.9144 m exactly, so 1 foot = 0.3048 m and 1 inch = 0.0254 m (25.4 mm) exactly.
• 1 mile = 1760 yards = 1609.344 m exactly.
• 1 pound (avoirdupois) = 0.45359237 kg exactly, and 1 ounce = 1/16 pound.

So an inch is not "about" 2.54 cm, it is exactly 2.54 cm. Going the other way, 1 m is about 3.28084 ft and 1 kg is about 2.20462 lb.

The United States also kept a slightly different "survey foot" for land surveying. NIST deprecated it at the end of 2022, so the international foot of 0.3048 m is now the foot for all uses.

Liquid volumes have the same names in both systems but different sizes. The US gallon is defined as 231 cubic inches. The imperial gallon is defined as exactly 4.54609 liters.

• Gallon: 1 US gallon = 3.785411784 L; 1 imperial gallon = 4.54609 L. One imperial gallon is about 1.201 US gallons.
• Pint: 1 US pint = 473.176 mL; 1 imperial pint = 568.261 mL.
• Fluid ounce: 1 US fl oz = 29.5735 mL; 1 imperial fl oz = 28.4131 mL. Here the imperial unit is the smaller one.
• Ton: the US (short) ton is 2000 lb = 907.18474 kg; the British (long) ton is 2240 lb = 1016.046909 kg. The metric tonne is 1000 kg, about 2204.62 lb.

The pint puzzle comes from how the units are built. A US pint holds 16 US fluid ounces, while an imperial pint holds 20 imperial fluid ounces. The imperial ounce is a little smaller, but there are more of them in a pint.

Metric units are related by powers of ten, so converting within the system only moves the decimal point: 1.25 km is 1250 m. The prefixes (kilo, centi, milli and so on) mean the same thing for every unit.

Customary units use many different ratios: 12 inches to the foot, 3 feet to the yard, 1760 yards to the mile, 16 ounces to the pound. Each step needs its own factor.

• When a recipe, fuel figure or product size says "gallon", "pint" or "fluid ounce", check whether it is US or imperial. A UK recipe and a US recipe can use the same word for different amounts.
• Fuel economy in miles per gallon is not the same in the US and the UK, because the gallons differ.
• "Ton" can mean a short ton, a long ton or a metric tonne. In international trade, the tonne (t) is the safe choice.
• Lengths and masses (inch, foot, mile, pound, ounce) are the same in both systems, so no adjustment is needed there.