Unit Converter

Whether you are reading a European recipe that lists ingredients in grams, checking the weather forecast in Celsius while traveling abroad, or converting kilometers to miles for a road trip, unit conversions are one of those tasks that come up constantly in daily life.

This unit converter handles all of them in one place. Select your category, choose your starting and target units, enter a value, and get an instant result with full decimal precision.

The tool covers every major measurement system used worldwide, including metric (SI), imperial (US customary), and several specialized units still used in specific industries. Categories include length, weight and mass, volume, temperature, speed, area, pressure, energy, data storage, and time. Each conversion uses exact conversion factors defined by international standards bodies, so the results match what you would get from any authoritative reference.

Unit conversion errors are more consequential than most people realize. NASA famously lost the $125 million Mars Climate Orbiter in 1999 because one engineering team used imperial units while another used metric. You probably are not sending anything to Mars, but getting the right number still matters when you are dosing medication, sizing building materials, or calculating fuel for a long drive. This tool eliminates the guesswork and gives you numbers you can trust.

Understanding Measurement Systems

The world primarily uses two measurement systems, and understanding the difference between them is the first step toward confident unit conversion.

The Metric System (SI) is the International System of Units adopted by virtually every country on Earth. It is built on base-10 relationships, which makes conversions within the system straightforward. One kilometer equals 1,000 meters. One kilogram equals 1,000 grams. One liter equals 1,000 milliliters. The prefixes are consistent across all measurement types: kilo always means 1,000, centi always means one-hundredth, and milli always means one-thousandth. This predictability is why the scientific community uses metric exclusively.

The Imperial System (and its close relative, US customary units) is used primarily in the United States, along with limited use in the United Kingdom, Liberia, and Myanmar. Imperial measurements evolved from older English units and lack the clean base-10 relationships of metric. There are 12 inches in a foot, 3 feet in a yard, 5,280 feet in a mile, 16 ounces in a pound, and 8 pints in a gallon. These seemingly arbitrary ratios are the reason conversion between imperial units often requires a calculator.

Converting between the two systems is where most people run into trouble. The conversion factors are not round numbers. One inch equals exactly 25.4 millimeters. One pound equals approximately 0.4536 kilograms. One gallon equals approximately 3.785 liters. Memorizing these factors is impractical for most people, which is exactly why a reliable conversion tool is worth bookmarking.

Common Length Conversions

Length is the most frequently converted measurement category, particularly conversions between metric and imperial units.

Inches to Centimeters. One inch equals exactly 2.54 centimeters. This is a defined relationship, not an approximation. It was internationally agreed upon in 1959. To convert inches to centimeters, multiply by 2.54. To go the other direction, divide by 2.54 (or multiply by 0.3937). A person who is 5 feet 10 inches tall is 177.8 centimeters tall.

Miles to Kilometers. One mile equals exactly 1.609344 kilometers. For quick mental math, multiply miles by 1.6 to get a close approximation. A 5K race is 3.1 miles. A marathon (26.2 miles) is 42.2 kilometers. Speed limits work the same way: 60 mph equals roughly 97 km/h, and 100 km/h equals roughly 62 mph.

Feet to Meters. One foot equals exactly 0.3048 meters. This conversion comes up frequently in construction, real estate, and aviation. A 2,000-square-foot home is approximately 186 square meters. A ceiling height of 9 feet equals 2.74 meters.

For more detailed length conversions with support for less common units like nautical miles, leagues, and fathoms, see the Length Converter.

Common Weight and Mass Conversions

Weight conversions are essential for cooking, shipping, fitness tracking, and international trade.

Pounds to Kilograms. One pound equals approximately 0.4536 kilograms. To do a rough mental conversion, divide pounds by 2.2. A 150-pound person weighs about 68 kilograms. A 50-pound suitcase is about 23 kilograms, which is relevant since most international airlines set baggage limits in kilograms.

Ounces to Grams. One ounce equals approximately 28.35 grams. This conversion is critical for cooking and baking, where precision matters. A recipe calling for 4 ounces of chocolate needs 113 grams. Note that fluid ounces (a volume measurement) are different from ounces of weight, a distinction that confuses many people.

Stones to Pounds and Kilograms. The stone is a unit still commonly used in the United Kingdom and Ireland for body weight. One stone equals 14 pounds or approximately 6.35 kilograms. A person weighing 12 stone is 168 pounds or 76.2 kilograms.

The dedicated Weight Converter offers additional units including metric tons, short tons, long tons, and troy ounces used for precious metals.

Volume and Liquid Conversions

Volume conversions are particularly tricky because the United States and the United Kingdom use different definitions for some of the same unit names.

US Gallons to Liters. One US gallon equals approximately 3.785 liters. One UK (imperial) gallon equals approximately 4.546 liters. This difference means that fuel economy figures, liquid recipes, and tank capacities can be significantly off if you use the wrong gallon. Always confirm whether a source is using US or imperial gallons.

Cups to Milliliters. One US cup equals approximately 236.6 milliliters. One metric cup (used in Australia and some other countries) equals exactly 250 milliliters. One UK cup is approximately 284 milliliters. When following recipes from different countries, using the wrong cup size can throw off your results noticeably, especially in baking. The Cooking Measurement Converter handles ingredient-specific conversions where density matters.

Tablespoons and Teaspoons. One US tablespoon equals approximately 14.79 milliliters. One US teaspoon equals approximately 4.93 milliliters. In metric countries, tablespoons are typically standardized at 15 milliliters and teaspoons at 5 milliliters.

Temperature Conversion

Temperature conversion between Celsius and Fahrenheit is one of the most common conversion needs, especially for travelers.

Celsius to Fahrenheit: F = (C x 9/5) + 32. Water freezes at 0 degrees Celsius (32 degrees Fahrenheit) and boils at 100 degrees Celsius (212 degrees Fahrenheit). Normal human body temperature is approximately 37 degrees Celsius (98.6 degrees Fahrenheit).

Fahrenheit to Celsius: C = (F – 32) x 5/9. Some useful reference points to memorize: 72 degrees Fahrenheit equals about 22 degrees Celsius (comfortable room temperature), 32 degrees Fahrenheit is 0 degrees Celsius (freezing), and 100 degrees Fahrenheit is about 38 degrees Celsius (a hot day or a mild fever).

Kelvin is the SI unit for temperature used in scientific contexts. Zero Kelvin (absolute zero) is -273.15 degrees Celsius. To convert Celsius to Kelvin, simply add 273.15. Kelvin uses the same scale increments as Celsius, so a 1-degree change in Celsius equals a 1-degree change in Kelvin.

For detailed temperature conversion with support for Rankine and other scales, use the Temperature Converter.

Speed, Area, and Pressure Conversions

Speed. The most common speed conversion is miles per hour (mph) to kilometers per hour (km/h). Multiply mph by 1.609 to get km/h. Knots, used in maritime and aviation contexts, equal 1.852 km/h or 1.151 mph. Mach numbers represent multiples of the speed of sound (approximately 1,235 km/h at sea level).

Area. One acre equals 43,560 square feet, 4,047 square meters, or 0.4047 hectares. One square mile equals 640 acres or 2.59 square kilometers. Real estate listings in different countries use different units, making area conversion essential for international property comparison.

Pressure. Pressure has multiple commonly used units depending on the field. Atmospheric pressure at sea level is approximately 101,325 Pascals, 1,013.25 millibars, 14.696 psi (pounds per square inch), 760 mmHg (millimeters of mercury), or 29.92 inches of mercury. Tire pressure is typically measured in psi in the US and kPa or bar in metric countries.

Tips for Accurate Unit Conversion

Know Your Precision Needs. For everyday use, rounding to two or three decimal places is fine. For engineering, scientific, or medical applications, you may need full precision. This converter provides results to multiple decimal places so you can round as needed.

Watch for Naming Conflicts. A "ton" can mean 2,000 pounds (short ton, US), 2,240 pounds (long ton, UK), or 1,000 kilograms (metric ton). A "gallon" can be US or imperial. An "ounce" can be avoirdupois (common) or troy (precious metals). Always confirm which unit variant is intended.

Understand the Difference Between Mass and Weight. Technically, kilograms measure mass and pounds measure force (weight). On Earth’s surface, the distinction is negligible for everyday purposes, but in scientific contexts it matters. A kilogram of mass weighs approximately 9.81 Newtons on Earth but would weigh only about 1.62 Newtons on the Moon.

Use Standard Conversion Factors. This tool uses conversion factors defined by the National Institute of Standards and Technology (NIST) and the International Bureau of Weights and Measures (BIPM). These are the same factors used in engineering, science, and international trade.

Frequently Asked Questions

What is the difference between metric and imperial units?

The metric system (SI) is a decimal-based system used by most countries worldwide, where units scale by powers of 10 (1 km = 1,000 m, 1 kg = 1,000 g). The imperial system, used mainly in the United States, uses non-decimal relationships (12 inches = 1 foot, 16 ounces = 1 pound). Metric is standard in science and international commerce, while imperial remains dominant in everyday American life.

How do I convert miles to kilometers?

Multiply the number of miles by 1.609344. For quick mental math, multiply by 1.6. So 10 miles is approximately 16.1 kilometers. To convert kilometers to miles, divide by 1.609 or multiply by 0.6214.

What is the formula for Celsius to Fahrenheit?

The formula is F = (C x 9/5) + 32. For example, 25 degrees Celsius equals (25 x 9/5) + 32 = 77 degrees Fahrenheit. To convert Fahrenheit to Celsius, use C = (F – 32) x 5/9.

Why do some countries still use the imperial system?

The United States is the primary country that uses imperial units for everyday measurements. The UK uses a mix of both systems. Historical inertia, the cost of switching infrastructure (road signs, manufacturing standards), and cultural familiarity are the main reasons the transition has not occurred. US science and military already use metric extensively.

Is a fluid ounce the same as an ounce of weight?

No. A fluid ounce (fl oz) measures volume, while an ounce (oz) measures weight. One fluid ounce of water weighs approximately one ounce, but this relationship does not hold for other liquids. One fluid ounce of honey weighs about 1.5 ounces because honey is denser than water.

How accurate is this unit converter?

This converter uses exact conversion factors defined by international standards organizations (NIST, BIPM). The factors are the same ones used in engineering and scientific applications. Results are accurate to the full precision displayed. For everyday purposes, you can round to two or three decimal places.

What is the difference between a US gallon and an imperial gallon?

A US gallon equals approximately 3.785 liters, while an imperial (UK) gallon equals approximately 4.546 liters. An imperial gallon is about 20% larger than a US gallon. This distinction matters for fuel economy comparisons, recipes, and liquid volume calculations.

How many grams are in a pound?

One pound equals approximately 453.592 grams or 0.4536 kilograms. For a quick estimate, divide pounds by 2.2 to get kilograms or multiply by 454 to get grams.

Conversion factors based on NIST and BIPM international standards. Data accurate as of: March 2026