Unit Converter

What a unit converter does

A unit converter takes one measurement written in one unit and rewrites it in another unit without changing the quantity. That sounds trivial until the units come from two different systems. The United States runs on US customary units for roads, ovens, body weight and lumber, while most of the world, and most of science and medicine, works in metric. NIST runs the federal metric program and states the policy in plain terms: agencies are to use the SI, with customary units kept where they serve the public (NIST metric policy). Daily life sits in the gap between the two.

This page covers ten sets: length, mass, volume, temperature, area, speed, pressure, energy, data storage and time. Each set has a base unit, and each unit inside the set carries a factor that converts it to that base. Convert 26.2 miles and the tool multiplies by 1,609.344 to reach meters, then divides by 1,000 to print kilometers.

The table under the result is the part people skip. It shows your value in every unit of the set at once, so 10 meters reads as 32.80839895 feet, 393.7007874 inches and 0.005399568035 nautical miles without retyping anything.

Two design choices are worth knowing before you trust a number. Every factor in the tool is exact by definition, so the only rounding happens in the display. Temperature is the exception: Fahrenheit and Celsius differ by a scale and an offset, so the tool converts through Celsius and keeps the direction straight.

What the tool does not do is judge the measurement. A bathroom scale that reads 165 pounds carries an uncertainty of a pound or two, and no converter repairs that. The arithmetic below is exact; the input is not.

The formula, and where every factor comes from

Conversion is multiplication by a ratio. Put the value into the base unit, then divide by the target unit's factor: result = value x factor of the unit you have / factor of the unit you want. A 165 pound person becomes 165 x 0.45359237 = 74.84274105 kilograms, because the pound factor is 0.45359237 and the kilogram factor is 1.

The origin of those factors matters more than the arithmetic. In 1959 the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa settled on one international yard and one international pound, and NIST lists the results as exact: 1 yard = 0.9144 meter, 1 foot = 0.3048 meter, 1 inch = 0.0254 meter, 1 pound = 0.45359237 kilogram (NIST SP 811, Guide for the Use of the SI). Every customary length and mass on this page is built from those four numbers.

The chain shows up in the small units. An inch is 0.9144 / 36 = 0.0254 meter. An ounce is 0.45359237 / 16 = 0.028349523125 kilogram. A stone is 0.45359237 x 14 = 6.35029318 kilograms. A short ton is 0.45359237 x 2,000 = 907.18474 kilograms. The troy ounce is the odd one: 480 grains, with the grain fixed at 64.79891 milligrams, which gives 31.1034768 grams, and that is why a troy ounce of gold weighs more than an ounce of sugar.

Volume follows from the inch. The US gallon is 231 cubic inches by law, and 231 x 0.016387064 = 3.785411784 liters, the figure printed in NIST Handbook 44 Appendix C, the weights and measures code most US states adopt (NIST Handbook 44, Appendix C). A fluid ounce is 1/128 of that gallon at 0.0295735295625 liters, a cup is 8 fluid ounces at 0.2365882365 liters, and a teaspoon is 1/6 of a fluid ounce at 0.00492892159375 liters. The imperial gallon is a separate definition, 4.54609 liters, fixed by UK law in 1985.

Area, speed and pressure reuse the same building blocks. An acre is 43,560 square feet, and 4,046.8564224 / 0.09290304 = 43,560 confirms the square meter figure. A mile per hour is 1,609.344 / 3,600 = 0.44704 meters per second, and a knot is 1,852 / 3,600 = 0.5144444444 meters per second. One psi is one pound-force on one square inch, 6,894.757293168361 pascals, and the standard atmosphere is 101,325 pascals by definition.

Energy and data carry their own conventions. The thermochemical calorie is 4.184 joules, the International Table BTU is 1,055.05585262 joules, and a kilowatt-hour is 3,600,000 joules because 1 kW for 3,600 s is 3,600,000 J. On the data side, SI prefixes step by 1,000 and binary prefixes step by 1,024, so a 2 TB drive holds 2,000,000,000,000 bytes, which is 1,862.645149 GiB once you divide by 1,073,741,824.

How to use this converter, step by step

Step 1. Pick the set. Length covers distance and height, weight covers mass, volume covers liquids and recipe amounts, and temperature has its own three units. Choosing the wrong set is the most common mistake, and the tool cannot catch it for you: 165 pounds converted as if it were miles gives a number that looks like a number.

Step 2. Type the value you measured. Decimals are fine, and the field takes a leading minus sign for temperatures below zero. Leave it empty and the tool treats the value as 0 rather than throwing an error.

Step 3. Set the unit you have in the From box, and the unit you want in the To box. The first two units of each set load by default: meters to feet, kilograms to pounds, liters to gallons, Fahrenheit to Celsius, square meters to square feet, kilometers per hour to miles per hour, bar to psi, kilowatt-hours to BTU, terabytes to gibibytes, hours to minutes.

Step 4. Read the headline, then the table. The headline answers the question you asked. The table answers the question you will ask next, which is usually a third unit: a recipe in cups, a shipping label in ounces, a torque spec in pound-feet.

Step 5. Press Swap if the direction is backwards. Swapping exchanges the two units and keeps your value, which is the fast way to check a conversion. If 26.2 miles gives 42.1648128 kilometers, swapping should hand back 26.2 miles from 42.1648128, and it does.

For temperature, pick the direction before you type. Going from Fahrenheit to Celsius subtracts 32 and divides by 1.8, so 98.6 degrees Fahrenheit is 37 degrees Celsius. Going the other way multiplies by 1.8 and adds 32, so 37 degrees Celsius returns 98.6 degrees Fahrenheit. There is no single factor that does both jobs, and any converter that offers you one is wrong.

Four worked examples

Example 1: a marathon in kilometers. A race is 26.2 miles. Set the category to length, From to mile, To to kilometer, and type 26.2. The tool multiplies: 26.2 x 1,609.344 = 42,164.8128 meters, and 42,164.8128 / 1,000 = 42.1648128 kilometers. Most race results round that to 42.2 km. Run the same value through the table and you also get 42,164.8128 meters, 46,112 yards and 22.76717754 nautical miles.

Example 2: body weight in kilograms. A US scale reads 165 pounds and a European chart wants kilograms. 165 x 0.45359237 = 74.84274105 kilograms, which a doctor would write as 74.8 kg. The table adds the rest: 2,640 ounces, 11.78571429 stone and 0.0825 short tons. Note the ounce line: 2,640 is exact, because a pound is 16 ounces by definition, and exactness is why the converter does not disagree with itself.

Example 3: gallons, and the one that is not the same. Five US gallons of paint is 5 x 3.785411784 = 18.92705892 liters. Five imperial gallons, the kind sold in the UK, is 5 x 4.54609 = 22.73045 liters. The gap is 22.73045 - 18.92705892 = 3.80339108 liters, and 3.80339108 / 18.92705892 = 20.1 percent more paint in the imperial can. Anyone importing finishes across the Atlantic meets this difference on the label.

Example 4: a body temperature. A thermometer reads 98.6 degrees Fahrenheit. The conversion is (98.6 - 32) / 1.8 = 37 degrees Celsius, which is the number most of the world calls normal. Turn it around and 37 x 1.8 + 32 = 98.6 degrees Fahrenheit. The offset is why the shortcut of doubling and adding 30 breaks: 20 degrees Celsius is 68 degrees Fahrenheit, while doubling and adding 30 gives 70, a two degree error that grows as the temperature climbs.

One more, because storage trips people up: a drive sold as 2 TB holds 2,000,000,000,000 bytes, and 2,000,000,000,000 / 1,073,741,824 = 1,862.645149 GiB. Windows reports that same drive as about 1.82 TiB, and nothing is missing. The prefix changed meaning between the box and the operating system.

Where conversions go wrong

Rounded factors. Treating an inch as 2.5 centimeters is a 1.6 percent error: 1 inch is 2.54 centimeters, so 100 inches is 254 centimeters rather than 250. On a 10 foot wall that is 1.9 inches of drift, enough to break a layout. Use 2.54, 0.3048 and 0.45359237, never 2.5, 0.3 or 0.45.

Two gallons, two fluid ounces. The US fluid ounce is 0.0295735295625 liters and the imperial fluid ounce is 0.0284130625 liters, so a 12 ounce US can holds 354.8823547 milliliters while a 12 ounce UK can holds 340.95675. The same word, 4 percent apart.

Troy against avoirdupois. A troy ounce is 31.1034768 grams and an ordinary ounce is 28.349523125 grams, which makes the troy ounce 1.0971 times heavier. Silver priced per troy ounce and a kitchen scale reading ounces are not measuring the same thing.

Three tons. A short ton is 907.18474 kilograms, a metric ton is 1,000 kilograms, and a long ton is 1,016.0469088 kilograms. A truck rated at 10 short tons carries 9,071.8474 kilograms, not 10,000. Shipping paperwork that mixes the three produces claims disputes, which is why freight contracts name the ton they mean.

Temperature by factor. Multiplying a Celsius reading by 1.8 and forgetting the 32 turns 37 degrees Celsius into 66.6 instead of 98.6 degrees Fahrenheit. Celsius and Fahrenheit share a zero only at minus 40, where both scales read the same number, and that single coincidence is not a conversion method.

Storage prefixes. A 1 TB drive is 1,000,000,000,000 bytes, and 1,000,000,000,000 / 1,073,741,824 = 931.3225746 GiB, which is the 931 GB an operating system shows. Manufacturers are not shorting you; they use decimal prefixes and the OS uses binary ones. The IEC introduced KiB, MiB and GiB in 1998 to end the confusion, and the labels on most drives still ignore it.

The survey foot. Until 2022, US surveying used a foot of 1,200 / 3,937 = 0.304800609601 meters, about 2 parts per million longer than the international foot. NIST and NOAA retired it: the last day of use was December 31, 2022, and from January 1, 2023 the international foot at 0.3048 meter applies to all applications (NIST U.S. Survey Foot page). Old plats and deeds still carry the survey foot, so a conversion of a historical boundary needs the older factor.

The standards that fix these numbers

Four documents cover almost everything on this page. The BIPM SI Brochure, 9th edition (2019) defines the seven base units and the prefixes, and it is the document that made the kilogram, ampere, kelvin and mole depend on fixed constants rather than physical objects. NIST SP 811 (2008) is the US guide to using the SI and carries the exact conversion factors in its appendix. NIST Handbook 44 Appendix C holds the US customary definitions used in commerce, including the 231 cubic inch gallon. IEC 80000-13 supplies the binary prefixes.

The 2019 redefinition changed no conversion factor on this page. The meter was already fixed by the speed of light in 1983, and the inch has been 0.0254 meter since 1959, so the chain from inch to millimeter never moved. What changed is the kilogram: it stopped being a platinum cylinder in France and became a value derived from the Planck constant. A pound is still 0.45359237 kilograms, and the arithmetic you run in this tool is identical before and after.

Because the factors are definitions rather than measurements, they carry no uncertainty. The uncertainty enters when a physical quantity is measured: a scale, a tape measure, a thermometer. That distinction decides how many digits to keep, which is the next section.

How many digits to keep

A definition is exact, a measurement is not. If a tape measure reads 12 feet 4 inches, the inches digit is already uncertain, and printing the metric answer to eight decimals implies precision the tape never had. Four significant figures is generous for a construction measurement; three is honest.

Cooking is looser still. A cup is 236.5882365 milliliters, and no kitchen scale needs that. Round to 237 mL and the recipe works. Where the rounding hurts is in scaling: multiply a 237 mL cup by 40 batches and you are 3.5 mL off, which is why bakeries weigh in grams instead of scooping cups.

Engineering and medicine want more digits. A drug dose in milligrams converted to micrograms is a factor of 1,000, and losing a digit there is a dosing error, so carry the full value and round only at the end. The same rule applies to pressure and torque: 14.50377377 psi per bar rounds to 14.5 for a tire gauge and stays at 14.5038 for a calibration sheet.

Never round in the middle of a calculation. Converting miles to kilometers through a rounded meter figure, 1,609 instead of 1,609.344, costs 0.02 percent, which is 8 meters on a marathon and enough to miss a qualifying time. Convert once, round once, and state the precision your measurement supports.

Which unit tool to use for which job

This converter is the general entry point, and it covers ten sets in one screen. Some jobs deserve a narrower page. Recipes that list ingredients by volume but want grams need a density, and the density converter supplies the bridge between milliliters and grams for flour, sugar, oil and water. Spoon measures have their own page for the milligram to teaspoon direction, since a teaspoon of salt and a teaspoon of baking powder do not weigh the same.

House and land questions land on the area converter for square feet against square meters and acres, while the length converter handles inches, feet and meters without the rest of the sets in the way. Height in feet and inches against centimeters has a dedicated page, which is the number most people want for a passport form or a gym app.

Vehicles and travel split two ways: the fuel economy converter turns miles per gallon into liters per 100 kilometers, and this page's speed set covers miles per hour against kilometers per hour for a rental car. Volume in the kitchen has a cups and quarts page for the US recipe units that show up on boxed mixes. When the question is "how much does this weigh in the unit my label uses," the answer is here.