Unit Converter
Convert between common units of length, mass, temperature, time and digital storage.
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Frequently asked questions
Is any data sent to a server when I use this tool?
What is the difference between SI (metric) and imperial units?
What famous disaster was caused by a unit conversion error?
What are common pitfalls when converting between metric and imperial volumes?
How accurate are the conversion results?
Why did the world adopt the metric system but the US did not?
What is the difference between mass and weight?
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Is it true the metric system was originally based on the size of the Earth?
About Unit Converter
Units of measurement are the shared language through which humans quantify and communicate physical reality. Two major systems have competed for global dominance: the metric system, formally known as the International System of Units (SI), and the imperial system still used in the United States and a handful of other countries. The metric system was born from the French Revolution — in 1795, the French National Convention standardized the metre as one ten-millionth of the distance from the North Pole to the equator, creating a rational, decimal-based framework. The imperial system, by contrast, evolved organically over centuries from practical measures like feet (the length of a human foot) and pounds (a traditional counterweight), and was codified by the British Weights and Measures Acts of the 19th century. The world largely converged on SI through the late 20th century, with the US being the most notable holdout.
Unit conversion errors have real and sometimes catastrophic consequences. The most famous is the Mars Climate Orbiter disaster of 1999: NASA lost a $327.6 million spacecraft because one engineering team submitted thruster data in imperial pound-force seconds while the receiving software expected metric newton-seconds. The probe entered the Martian atmosphere at the wrong angle and was destroyed. Engineers, scientists, pilots, and medical professionals all depend on accurate unit conversion every day — an incorrect drug dosage calculation, a wrong runway length, or a misread weather forecast can have serious consequences. For everyday use, unit conversion is also essential for international shopping, cooking with foreign recipes, understanding weather reports while traveling, and interpreting technical specifications.
This tool handles the most commonly needed conversions: length (metres, feet, inches, miles, kilometres), mass (kilograms, pounds, ounces, tonnes), temperature (Celsius, Fahrenheit, Kelvin), time intervals (seconds, minutes, hours, days), and digital storage (bytes through terabytes). All conversion factors are hardcoded from authoritative SI definitions and run entirely client-side in your browser with no server calls and no data upload. Results use IEEE-754 double-precision arithmetic, the same standard used by virtually all scientific calculators.
A few tips for accurate conversions: always verify the exact definition of units that have multiple variants — for example, a US gallon (3.785 L) and a UK imperial gallon (4.546 L) differ by about 20%. Fluid ounces similarly differ between US and UK standards. When converting very large or very small numbers, watch for floating-point rounding at extreme ends of the scale. For weight, be aware that mass (kilograms) and weight (newtons) are related but distinct concepts — on the Moon, your mass stays the same but your weight drops by about 83%.
The $327 Million Mistake: When Unit Confusion Cost NASA a Spacecraft
On September 23, 1999, NASA's Mars Climate Orbiter completed a nine-month journey across 669 million kilometres of space, only to be destroyed at the final moment. The cause was devastatingly mundane: a software module developed by Lockheed Martin expressed angular momentum in pound-force seconds, while NASA's ground navigation system expected the data in newton-seconds. Nobody caught the mismatch during nine months of transit corrections. The spacecraft dipped too deep into the Martian atmosphere — about 57 km instead of the intended 150 km — and was either destroyed by atmospheric heating or skipped out of orbit entirely. The Mars Climate Orbiter was gone, along with its $327.6 million price tag and years of scientific planning.
The metre itself has a complicated history as a unit of measurement. Originally defined in 1793 by the French Academy of Sciences as one ten-millionth of the Earth's meridional quadrant, it was physically embodied in a platinum bar called the Mètre des Archives. The problem was that anyone who needed to measure something had to travel to Paris to compare against the original, or use a copy that might have slight dimensional differences. In 1960 the metre was redefined in terms of the wavelength of krypton-86 light, and then in 1983 it was finally anchored to a fundamental physical constant: the speed of light. One metre is now exactly the distance light travels in a vacuum in 1/299,792,458 of a second — a definition that can be replicated in any laboratory anywhere in the universe.
The United States has been officially metric since 1893, at least on paper. The Metric Conversion Act of 1975 made metrication a national goal, and American industry slowly adopted SI for trade, science, and manufacturing. Yet everyday American life remains stubbornly imperial. Temperatures are in Fahrenheit, roads are in miles, beer is sold in fluid ounces, and babies are weighed in pounds at birth. Three countries — the US, Liberia, and Myanmar — have not formally adopted the metric system as their primary system of everyday measurement, making the United States the most prominent holdout from a global standardization effort that began over two centuries ago.