How to Convert Units Online: Length, Weight, Temperature, and More

Unit conversion is one of those things that sounds straightforward right up until it is not. Most people can convert miles to kilometers if they remember to multiply by 1.60934. But asking someone to convert cubic feet to liters, or pounds per square inch to pascals, or kilowatt-hours to BTUs, without looking up the conversion factor first is asking them to memorize numbers they will use a handful of times in their life. Nobody does that. And so every unit conversion that is not completely routine involves looking something up.

The stakes of getting it wrong are higher than they might seem. In 1999, NASA lost the Mars Climate Orbiter because one engineering team used metric units and another used imperial units in the same calculation - a 125 million dollar spacecraft destroyed by a unit mismatch. In 1983, Air Canada Flight 143 ran out of fuel mid-flight because ground crew calculated fuel load in pounds instead of kilograms. These are extreme cases, but the underlying error - assuming units without verifying them - happens constantly in everyday contexts: a construction measurement taken in inches interpreted as centimeters, a recipe quantity in ounces mistaken for grams, a medication dose in milligrams given in micrograms.

This guide covers how measurement systems work, where conversion factors come from, the formulas for the most common conversions, and how to convert quickly and correctly for any combination of units.

Metric vs. Imperial: Why Two Systems Still Coexist

Almost every country in the world uses the metric system (officially called the International System of Units, or SI) as its primary system of measurement. The United States is the notable exception, using the US customary system for most everyday purposes, which overlaps significantly with but is not identical to the British imperial system.

The metric system is designed around powers of ten. One kilometer is exactly 1,000 meters. One meter is exactly 100 centimeters. One centimeter is exactly 10 millimeters. Converting within the metric system is always a matter of multiplying or dividing by a power of ten - moving a decimal point. This is why scientists and engineers everywhere use metric: calculations are simpler and unit mismatches are easier to spot.

The US customary system has no such internal logic. One mile is 5,280 feet (not a round number). One foot is 12 inches. One yard is 3 feet. One acre is 43,560 square feet. These numbers come from historical conventions that were standardized over centuries and then never changed. The reason the US did not adopt metric when the rest of the world did in the 19th and 20th centuries is largely path dependence - the infrastructure, the tooling, the education, and the public familiarity were all built around the existing system, and the cost of switching was judged to be too high each time it was considered.

Length Conversion: Metric and Imperial

The key conversion factor between metric and imperial length is the definition of the inch: exactly 25.4 millimeters. All other length conversions between the two systems derive from this one number.

From that, one foot is 12 inches, so one foot is 304.8 millimeters (12 x 25.4) or 0.3048 meters. One yard is 3 feet, so one yard is 0.9144 meters. One mile is 5,280 feet, so one mile is 1,609.344 meters or 1.609344 kilometers. To convert miles to kilometers, multiply by 1.60934 (rounded). To convert kilometers to miles, multiply by 0.62137.

Common length conversions people look up most often: 5 feet = 152.4 cm (useful for height); 6 feet = 182.88 cm; 1 inch = 2.54 cm; 1 meter = 39.37 inches; 100 meters = 328 feet; 1 km = 0.621 miles; 26.2 miles (marathon) = 42.195 km.

Weight Conversion: Pounds, Kilograms, and Stone

The key conversion for weight is that one kilogram is 2.20462 pounds, or equivalently, one pound is 0.453592 kilograms. To convert kilograms to pounds, multiply by 2.20462. To convert pounds to kilograms, multiply by 0.453592.

The stone is a unit of weight used primarily in the UK and Ireland for body weight. One stone is exactly 14 pounds, or about 6.35 kilograms. Converting a person's weight from kilograms to stone-and-pounds requires first converting to total pounds (multiply kg by 2.20462), then dividing by 14 to get the stone component and taking the remainder as pounds.

One common source of confusion is the distinction between mass and weight in scientific contexts. In physics, mass (measured in kilograms) is the amount of matter in an object, while weight is the force that gravity exerts on that mass (measured in newtons). In everyday use, weight and mass are used interchangeably - saying you weigh 70 kg is understood to mean your mass is 70 kg. This only becomes a real issue in aerospace engineering and physics problems.

Temperature Conversion: Celsius, Fahrenheit, and Kelvin

Temperature is the unit conversion that trips people up most often because it uses an offset formula rather than a simple multiplication. You cannot just multiply by a single number - you have to shift the scale first.

Celsius to Fahrenheit: multiply by 9/5 (or 1.8) and add 32. So 100°C = (100 x 1.8) + 32 = 212°F. Freezing point: 0°C = 32°F. Normal body temperature: 37°C = 98.6°F. Room temperature: 20°C = 68°F.

Fahrenheit to Celsius: subtract 32, then multiply by 5/9 (or 0.5556). So 98.6°F = (98.6 - 32) x (5/9) = 37°C.

Kelvin is used in science and engineering. It uses the same scale size as Celsius (a difference of 1 K equals a difference of 1°C) but starts from absolute zero (-273.15°C). To convert Celsius to Kelvin, add 273.15. Water freezes at 273.15 K and boils at 373.15 K. Absolute zero is 0 K or -273.15°C or -459.67°F.

A useful mental shortcut for rough Celsius-to-Fahrenheit estimates: double the Celsius temperature and add 30. This gives approximately the right Fahrenheit temperature within a few degrees for the range from 0°C to 30°C (32°F to 86°F), which covers most weather-related questions.

Volume Conversion: Liters, Gallons, and Fluid Ounces

Volume is where US customary units become most confusing because there are two separate unit systems for liquid measure in different countries: US customary and imperial (UK). A US gallon is 3.785 liters. A UK (imperial) gallon is 4.546 liters. A US fluid ounce is 29.574 mL. A UK fluid ounce is 28.413 mL. These differences are small enough that they do not matter for everyday use but significant enough to cause errors in precise applications like fuel economy calculations or chemistry.

Common volume conversions: 1 liter = 33.814 US fluid ounces = 4.227 US cups; 1 US gallon = 128 fluid ounces = 8 pints = 4 quarts = 3.785 liters; 1 tablespoon = 3 teaspoons = 14.787 mL; 1 cup = 16 tablespoons = 236.588 mL; 1 liter = approximately 4.23 cups.

Cooking conversions are a particularly common need. A recipe that uses metric measurements (250 mL of cream) needs to be converted to cups (about 1 cup plus 1 tablespoon) for someone using US measuring cups. Going the other direction - a US recipe that calls for 2 cups of flour - becomes 473 mL, which a European cook would typically measure by weight instead (flour is typically measured in grams in metric countries because volume measurements for dry ingredients are less consistent).

Area and Speed Conversions

Area conversions involve squaring the linear conversion factor. One meter is 3.281 feet, so one square meter is 3.281 x 3.281 = 10.764 square feet. One kilometer is 0.621 miles, so one square kilometer is 0.386 square miles. Common area conversions: 1 acre = 43,560 square feet = 4,047 square meters = 0.405 hectares; 1 hectare = 10,000 square meters = 2.471 acres; 1 square mile = 640 acres = 258.999 hectares.

Speed conversions are straightforward multiplications of the length conversion. One mile per hour = 1.609 kilometers per hour. One kilometer per hour = 0.621 miles per hour. One knot (nautical mile per hour) = 1.852 km/h = 1.151 mph. The speed of light is 299,792,458 meters per second, 186,282 miles per second, or 670,616,629 miles per hour.

Data Storage and Transfer Rates

Digital storage units are a source of persistent confusion because the industry uses two different base systems. Hard drive manufacturers typically use decimal (base 10) prefixes: 1 kilobyte = 1,000 bytes, 1 megabyte = 1,000,000 bytes, 1 gigabyte = 1,000,000,000 bytes. Operating systems typically use binary (base 2) prefixes: 1 kibibyte = 1,024 bytes, 1 mebibyte = 1,048,576 bytes, 1 gibibyte = 1,073,741,824 bytes.

This is why a 1 TB hard drive shows as 931 GB in Windows: the drive manufacturer's 1,000,000,000,000 bytes becomes 931 gibibytes when divided by 1,073,741,824. Neither number is wrong - they are just using different definitions of "gigabyte."

Internet speed is measured in megabits per second (Mbps), not megabytes. Since one byte = 8 bits, a 100 Mbps internet connection transfers 12.5 megabytes per second (100 / 8 = 12.5). A 1 Gbps connection transfers 125 megabytes per second.

How Conversion Factors Are Derived

Most conversion factors are exact by definition. The inch is defined as exactly 25.4 mm. The pound is defined as exactly 0.45359237 kilograms. The international nautical mile is exactly 1,852 meters. These definitions mean there is no rounding error in the conversion factor itself - only in how many decimal places you choose to use.

Temperature is different: the Fahrenheit scale was defined by Daniel Gabriel Fahrenheit in 1724 using brine as a zero reference and body temperature as the upper reference, leading to the non-round numbers (32°F freezing, 212°F boiling) that make temperature conversion slightly more complex than multiplication alone. The formulas are exact - they just require two steps.

Some conversions involve approximations by nature. When converting between cooking volume measurements and weight, the conversion depends on the ingredient's density. One cup of water weighs 236 grams, but one cup of flour weighs only about 120-130 grams depending on how compactly it is packed. For these conversions, the ingredient matters as much as the unit.

A Note on Significant Figures

When you convert a measurement, the result cannot be more precise than your input. If you know a distance is "about 5 miles," converting that to kilometers gives you "about 8 kilometers" - not 8.04672 kilometers. Reporting seven significant figures from a two-significant-figure input is meaningless precision. In everyday use this is rarely a problem - you know whether you need an exact number or an approximation. In scientific or engineering work, tracking significant figures matters and can affect whether a calculation result is reported correctly.

For any unit conversion you need, the unit converters on FlipCaps cover over 100 combinations across length, weight, temperature, volume, area, speed, energy, pressure, data storage, and more. Each converter gives you the result instantly as you type and shows you the formula behind the calculation so you can verify the math yourself.