Unit Conversion Reference: Metric and US Customary

Every conversion factor on this page is given to full precision, with the exact ones marked as exact. That distinction matters more than it sounds: a handful of "imperial" units are defined in metric terms and convert without any rounding at all, while the rest are derived and will always carry a remainder. Below: the factors, the ones people get wrong, and why temperature is not like the others.

How to read these tables

Each category has a base unit. Multiply by the factor to get the base unit; divide to go back. So 12 inches × 0.0254 = 0.3048 meters, and 5 meters ÷ 0.0254 = 196.85 inches. The unit converter does this interactively across all six categories if you would rather not do it by hand.

"Exact" means the relationship is fixed by definition rather than measured. Since the international yard and pound agreement of 1959, an inch is defined as exactly 2.54 centimetres and a pound as exactly 0.45359237 kilograms. Those are not approximations that a better measurement could improve — they are the definitions. "Derived" means the number is the reciprocal of an exact one and therefore repeats or truncates: 1 metre is 3.280839895… feet, and the digits never stop.

Length

The ones people look up most

The mile is the unit that trips people up, because there are several. The statute mile above (1,609.344 m) is the everyday US and UK road mile. A nautical mile is 1,852 m exactly — about 15% longer — and is used in aviation and at sea because it corresponds to one minute of latitude. Speeds quoted in knots are nautical miles per hour, not statute.

Weight and mass

The ones people look up most

Strictly, these are conversions of mass, not weight; weight is a force and depends on gravity. In everyday use on Earth the distinction never matters, and every kitchen scale and bathroom scale in the world ignores it. Where it does matter: the ounce here is the avoirdupois ounce used for ordinary goods. Precious metals are sold in troy ounces of 31.1034768 g — about 10% heavier — so a "1 oz" gold coin is not the same mass as 1 oz of flour.

Volume

The ones people look up most

This is the category with the most expensive mistake in it: US gallons and imperial gallons are different sizes. A US gallon is 3.785411784 L; an imperial (UK) gallon is 4.54609 L, about 20% larger. Fuel economy figures in miles per gallon are therefore not comparable between the two systems unless you check which gallon is meant — the same car is roughly 20% more efficient on paper in UK figures. US and imperial fluid ounces differ too, though in the opposite direction: a US fl oz is 29.57 mL and an imperial one is 28.41 mL.

American recipe measures are their own hazard. A US cup is 236.588 mL, but a "metric cup" used in Australia and New Zealand is 250 mL, and a Japanese cup is 200 mL. For baking, where ratios matter, weighing ingredients avoids the problem entirely.

Area

The ones people look up most

The acre has an odd definition with a practical origin: it was one chain by one furlong, the area a team of oxen could plough in a day. That leaves it as exactly 43,560 square feet, a number with no obvious pattern. The hectare, by contrast, is simply 100 m × 100 m. One hectare is about 2.47 acres, so a "10-acre" plot and a "4-hectare" plot are within a few percent of each other.

Speed

The ones people look up most

Two shortcuts worth knowing while driving: mph → km/h is close to "add 60%," and km/h → mph is close to "take 60%." At 70 mph the shortcut gives 112 km/h against a true 112.65, which is near enough for reading a foreign speed limit sign.

Temperature: the one that is not a factor

Every conversion above is a multiplication, because the scales share a zero — zero metres is zero feet. Temperature scales do not. 0 °C is 32 °F, so converting requires both a scale change and an offset:

• °C to °F: °F = °C × 9/5 + 32
• °F to °C: °C = (°F − 32) × 5/9
• °C to K: K = °C + 273.15

Multiplying by 1.8 alone is the most common temperature mistake, and it is wrong by 32 degrees at every point on the scale. Two anchors worth memorising: −40 is the same number in both scales (the only point where they cross), and 16 °C ≈ 61 °F, which is the digit-reversal coincidence that makes it easy to remember.

Kelvin is different again. It is an absolute scale, so it has no degree sign and no negative values, and a change of one kelvin is the same size as a change of one degree Celsius. That is why scientific work quotes temperature differences in kelvin without ambiguity.

Sources

The exact definitions above are set by standards bodies, not by convention:

• NIST — Unit conversion — the US authority on customary-to-metric factors.
• NIST Special Publication 811 — the full conversion tables and the rules for significant digits.
• BIPM — The International System of Units (SI Brochure) — the definitions of the base units themselves.