Unit converter: one value, every unit

Every unit it knows

Because one value fills the whole list, there is no “to” unit to pick. Type 5 miles and you read 8.04672 kilometers, 8,800 yards and 26,400 feet in the same glance, which is handy when a recipe, a plan or a spec sheet mixes systems. Nautical miles and knots are there for sailing and flying: a nautical mile is exactly 1,852 meters, and a knot is one nautical mile per hour.

Not included: US short tons, pints and quarts, imperial fluid ounces, square inches, fuel economy and currency.

The factors behind the answers

Most everyday conversions rest on a handful of exact definitions. Since 1959 the inch has been exactly 2.54 centimeters and the pound exactly 0.45359237 kilograms, so these are not rounded guesses:

Two worked examples: a height of 5 ft 9 in is 69 inches, and 69 × 2.54 = 175.26 cm. A marathon of 42.195 km is 42.195 ÷ 1.609344 = 26.22 miles. The converter works every answer out from your value with the full factor and rounds only what it shows, so rounding errors do not pile up.

Temperature needs a formula, not a factor

Length and weight scales start at zero, so one number converts them. Temperature scales start in different places, which is why the converter uses formulas:

• °F = °C × 9/5 + 32, so 20 °C is 68 °F and 37 °C is 98.6 °F.
• °C = (°F − 32) × 5/9, so a 350 °F oven is 176.67 °C, which recipes usually round to 180.
• K = °C + 273.15, so water freezes at 273.15 K.

The two scales meet at −40, where Celsius and Fahrenheit read the same. The classic mistake is converting a change in temperature as if it were a reading. A rise of 10 °C is a rise of 18 °F, because only the 9/5 part applies to a difference. Entering 10 °C in the converter gives 50 °F, which is the reading on a thermometer, not the size of the rise. Multiply differences by 1.8 in the calculator instead.

US, UK and kitchen measures

Several units share a name but not a size, and that is where most conversion errors come from:

• Gallons. A UK gallon is 4.54609 liters and a US gallon 3.785411784, so the UK one is about 1.2 US gallons. Miles per gallon figures from the two countries are not comparable for the same reason. Octoolo lists both gallons on separate lines.
• Ounces. An ounce of weight (28.35 g) and a fluid ounce of volume (29.57 ml in the US) are different units. A US cup of water, 236.59 ml, weighs about 8.35 ounces, not 8.
• Cups. The converter uses the US customary cup of 236.59 ml. US nutrition labels round it to 240 ml, and recipes from Australia or Canada often mean a 250 ml metric cup.
• Tons. The converter's tonne is metric: 1,000 kg, or about 2,204.6 pounds. A US ton is 2,000 pounds and a UK long ton 2,240.

Turning cups of flour into grams needs the flour's density, so that one is outside what any unit converter can do.