Conversion Calculator

What this converter covers

Five quantities, one box: length, mass, volume, temperature and speed. Type a value in any unit for the quantity you picked and the tool returns the same amount in every other unit for it, so a single entry answers the whole question instead of one pair at a time.

The factors behind it are not approximations pulled from a table of rounded numbers. Most US and metric unit relations have been exact by definition since the 1959 international yard and pound agreement, and the metric side is anchored by the SI definitions in the BIPM brochure. That means a conversion here can be trusted to the digits shown, and the only decision left is how many of those digits you keep.

Two things sit outside this tool on purpose. Cooking weights, where a cup of flour and a cup of sugar weigh different amounts, belong to the cooking conversion calculator, which carries densities. Currencies belong nowhere near a static page, because rates move every day. Area and volume of physical objects get their own treatment, since those factors have to be squared or cubed.

Length: the inch has been exactly 25.4 millimeters since 1959

The whole US length system hangs off one exact number. Since the 1959 agreement, 1 inch = 25.4 millimeters exactly, which fixes everything above it: 1 foot = 0.3048 meters, 1 yard = 0.9144 meters, and 1 mile = 1,609.344 meters. There is no rounding error in those figures, only the rounding you choose to apply.

Worked examples:

• A person who is 6 feet 2 inches is 74 inches tall. In centimeters that is 74 x 2.54 = 187.96 cm, and in meters 1.8796 m.
• A 5 kilometer race is 5 / 1.609344 = 3.10686 miles, which is why a 5K is called a 3.1 mile race.
• A 1 meter shelf is 1 x 3.28084 = 3.28084 feet, or 3 feet 3.37 inches.

The nautical mile used at sea and in the air is a separate unit of 1,852 meters exactly, which is why a knot is 1.852 kilometers per hour rather than 1.6.

Household tape measures are marked in eighths and sixteenths of an inch, so a metric plan usually needs the decimal translated back to a fraction before anyone can cut wood. 187.96 cm is 74 inches exactly, and 187 cm is 73.62 inches, which is 73 5/8 inches to the nearest sixteenth.

Mass: pounds, ounces and kilograms

1 pound = 0.45359237 kilograms exactly, and 1 ounce = 28.349523125 grams, since an ounce is one sixteenth of a pound. Those two numbers cover groceries, luggage, body weight and shipping labels.

• 165 pounds is 165 x 0.45359237 = 74.84 kilograms.
• 8 ounces is 8 x 28.349523125 = 226.8 grams, close to the 227 grams printed on a bag of coffee.
• A US short ton is 2,000 pounds, which is 907.18474 kilograms. A metric tonne is 1,000 kilograms, so the two differ by about 10 percent.

British body weight still appears in stones, and 1 stone is 14 pounds, so 11 stone 7 pounds is 161 pounds or 73.03 kilograms.

One ounce needs a warning. The ounce on a kitchen scale is the avoirdupois ounce of 28.349523125 grams, but gold, silver and other precious metals trade in troy ounces of 31.1034768 grams. That is 9.7 percent heavier, enough to matter on a coin or a ring. If a listing says troy oz, the number 28.35 is the wrong factor.

Volume: US and imperial gallons are not the same gallon

1 US gallon = 3.785411784 liters, which is 231 cubic inches. The imperial gallon used in the UK and Canada before metrication is 4.54609 liters, so a British gallon is about 20 percent larger than an American one. The same split runs through pints and fluid ounces.

• 2.5 US gallons is 2.5 x 3.785411784 = 9.46353 liters.
• 1 US fluid ounce is 29.5735295625 milliliters, and 1 US cup is 8 fluid ounces, which is 236.5882365 milliliters.
• 3 cups is 3 x 236.5882365 = 709.76 milliliters, near the 710 milliliter mark on a blender jar.
• 1 imperial gallon is 4.54609 / 3.785411784 = 1.20095 US gallons.

Fluid ounces measure volume and ounces measure mass, so 8 fluid ounces of water weighs about 8.3 ounces on a scale while 8 fluid ounces of flour weighs about 4.4 ounces. The two numbers only match for water, and only roughly. The SI brochure from the BIPM defines the liter as exactly one cubic decimeter, which is the anchor for all of the above.

Temperature needs a formula, not a multiplier

The Celsius, Fahrenheit and Kelvin scales start at different points, so a single multiplier cannot convert them. Two formulas do the job:

• Fahrenheit to Celsius: C = (F - 32) x 5 / 9
• Celsius to Fahrenheit: F = C x 9 / 5 + 32
• Celsius to Kelvin: K = C + 273.15

A body temperature of 98.6 Fahrenheit is (98.6 - 32) x 5 / 9 = 37.0 Celsius. A 100 Celsius boil is 100 x 9 / 5 + 32 = 212 Fahrenheit. And -40 is the single reading where the two scales meet: -40 x 9 / 5 + 32 = -40.

Kelvin is the SI base unit and takes no degree sign, so 0 Kelvin is -273.15 Celsius and a normal body temperature is about 310 Kelvin. Nothing in everyday life touches those numbers except a physics class, but the offset is the reason a plain multiplier cannot work.

A temperature difference is a different job from a temperature. One degree Celsius of change equals 1.8 degrees Fahrenheit of change, with no 32 anywhere, because an interval has no starting point. Multiply a difference by 9 / 5 and leave the offset out; that mistake turns a 5 degree forecast swing into a 41 degree one.

Table: the exact factors worth keeping

Every factor in that table is exact under current definitions, and NIST prints the same values with more digits in its Guide to the SI, SP 811, which is the document to cite when a number has to survive an audit.

Rounding: how many digits are honest

A conversion is only as good as the digits you keep. Four or five significant digits cover every household and most engineering uses, and the trailing ones invite arguments that do not matter.

60 miles per hour is 60 x 1.609344 = 96.5606 kilometers per hour, which rounds to 96.6. A mile is 63,360 inches exactly, and 1 kilometer is 39,370.08 inches. Rounding in the middle of a chain is what breaks things: converting 1 kilometer to miles as 0.62 and then back to kilometers as 0.62 x 1.609 gives 0.998, not 1. Do the arithmetic in one step, then round once.

The 0.62 shortcut for miles to kilometers is the most common rounding problem in travel. It is off by 0.3 percent, which is 15 miles on a 5,000 mile flight, so use 0.621371 when the distance matters and keep the shortcut for estimating a drive.

Significant figures follow the weakest input. A room measured as 12 feet is 12 plus or minus half a foot, so its metric equivalent of 3.6576 meters should be reported as 3.7 meters. Precision in the digits does not create precision in the measurement.

Three conversions that go wrong in practice

Area and volume. A factor converts a length. Area needs the factor squared and volume needs it cubed, so 1 square foot is 144 square inches and 1 cubic foot is 1,728 cubic inches. The area calculator handles the squared case in several units at once; using a length factor for area makes an answer 3.3 times too big, and for volume 35 times too big.

US against imperial. A recipe from a British site using pints and gallons is not the same recipe in US units. When a source does not say which system it uses, assume US for American sites and check any figure above a quart.

Fluid ounces against ounces. Coffee is sold by weight, milk by volume, and both get called ounces. If a label says fl oz, a scale cannot check it without knowing the density.