All Length Units

How to Convert Length Units?

Length conversion is essential for everyday tasks like measuring rooms, calculating travel distances, or working on construction projects. This converter uses standardized conversion factors based on the International System of Units (SI). Simply enter a value, select your source unit, and the tool instantly calculates the equivalent in all other length units.

Units

Meter (m)

The meter is the base unit of length in the SI system. It was originally defined as one ten-millionth of the distance from the equator to the North Pole. Today, it's defined by the speed of light in vacuum. Meters are used worldwide for most length measurements in science, construction, and daily life.

Foot (ft)

The foot is a unit of length used primarily in the United States and United Kingdom. One foot equals exactly 0.3048 meters or 12 inches. It's commonly used for measuring height, room dimensions, and in construction. The plural form is 'feet'.

Inch (in)

The inch is a unit of length equal to 1/12 of a foot or 2.54 centimeters. It's widely used in the US and UK for small measurements, screen sizes, and in manufacturing. The inch is subdivided into fractions or decimal values for precision work.

Kilometer (km)

The kilometer equals 1,000 meters and is used worldwide for measuring road distances, geographic features, and long distances. It's the standard unit for vehicle odometers and speed limits in most countries except the US and UK.

Mile (mi)

The mile is a unit of length equal to 5,280 feet or approximately 1.609 kilometers. It's primarily used in the United States and United Kingdom for measuring road distances, running races, and geographic distances.

Common Length Conversions

Frequently Asked Questions

How do I convert miles to kilometers quickly in my head?

The exact conversion factor is 1 mile = 1.609344 kilometers, but for mental math the easiest approximation is to multiply miles by 1.6, or add 60% to the mile figure. So 10 miles is roughly 16 km, 50 miles is 80 km, 100 miles is 160 km. A more accurate trick uses the Fibonacci sequence: consecutive Fibonacci numbers (3, 5, 8, 13, 21, 34, 55, 89) are very close to the actual mile-to-km ratio. So 5 miles is about 8 km, 8 miles is about 13 km, 13 miles is about 21 km, and so on with errors under 1%. For high-precision work — surveying, aviation, marine navigation — always use the full factor 1.609344, which is exact by international agreement since 1959.

Why does the world use meters but the US still uses feet and inches?

The meter was defined during the French Revolution in 1791 as one ten-millionth of the distance from the equator to the North Pole through Paris. Most countries adopted the metric system in the 19th and 20th centuries because it scales by powers of ten, simplifying trade and science. The United States legally authorized metric in 1866 and signed the Metre Convention in 1875, but never mandated it for daily use — building codes, road signs, and consumer products kept imperial units because retrofitting infrastructure and retraining workers would cost billions. The US is now one of only three countries (with Liberia and Myanmar) that have not officially adopted metric, though science, medicine, and the military all use it internally.

What is the difference between a US mile and a UK mile?

For modern road and statute use, there is no difference: both the US and the UK define the statute mile as exactly 5,280 feet, which equals 1,609.344 meters since the international yard agreement of 1959. However, two other miles still appear in specialized contexts. The nautical mile equals 1,852 meters exactly and is used worldwide for marine and air navigation — it equals one minute of arc along a meridian. The US survey mile is 5,280 survey feet, where the survey foot is slightly longer than the international foot (by 2 parts per million), still used for some US land surveying records but officially retired in 2023. For everyday driving distances, US miles and UK miles are identical.

What is the exact formula for converting inches to centimeters?

The exact conversion is 1 inch = 2.54 centimeters, defined by international agreement in 1959 — it is not an approximation but a legal definition. So to convert inches to centimeters multiply by 2.54, and to convert centimeters to inches divide by 2.54 (or multiply by 0.3937007874...). Common reference points: 1 foot = 30.48 cm exactly, 1 yard = 91.44 cm exactly, a 32-inch TV is 81.28 cm diagonal, a 6-foot person is 182.88 cm tall. For inches to millimeters, multiply by 25.4 exactly. This international inch replaced several slightly different historical inches that had differed by parts per million across countries, eliminating tiny errors in precision manufacturing and aerospace work.

What do the SI prefixes kilo, centi, milli, micro, nano actually mean?

SI prefixes are powers of ten that scale a base unit up or down without changing its definition. Going up: deca = 10, hecto = 100, kilo = 1,000, mega = 1,000,000 (million), giga = 10^9 (billion), tera = 10^12, peta = 10^15. Going down: deci = 0.1, centi = 0.01, milli = 0.001, micro = 10^-6 (millionth), nano = 10^-9 (billionth), pico = 10^-12, femto = 10^-15. So 1 km = 1,000 m, 1 cm = 0.01 m, 1 mm = 0.001 m, 1 micrometer (often called a micron) = 0.000001 m. The prefixes apply uniformly to grams, liters, seconds, watts, and every other SI unit — learn them once and they work everywhere.

How were the meter and foot originally defined and how are they defined today?

The meter began as one ten-millionth of the Paris meridian quadrant, then became the length of a platinum-iridium bar kept in Sevres, France. Since 1983, the meter is defined as the distance light travels in vacuum during 1/299,792,458 of a second — a definition that ties length to the speed of light, an absolute physical constant. The foot's origin is anthropomorphic (literally the length of a human foot) and varied for centuries: the Roman foot, English foot, and Paris foot all differed. The modern international foot was fixed at exactly 0.3048 meters in 1959, so the foot is now derived from the meter, not the other way around. Every length measurement on Earth ultimately traces back to the speed of light.

How many significant figures should I keep when converting lengths?

Match the precision of your source measurement, do not invent extra digits. If a tape measure shows 5.0 ft, the converted value is 1.5 m (2 significant figures), not 1.524 m. If a CAD file says 5.000 ft (4 sig figs), then 1.524 m is appropriate. For engineering drawings, building codes, and machined parts, follow the tolerance class specified by the standard (e.g., ISO 2768). For scientific work, propagate uncertainties through the conversion factor — fortunately the inch-to-cm factor (2.54) is exact, so it does not add error. Avoid the common mistake of converting then rounding then converting back, which can drift by a few parts per thousand over multiple steps. Always store the most precise value internally and round only for display.

What does a yard equal in meters and where did the yard come from?

One international yard equals exactly 0.9144 meters since 1959, or equivalently 3 feet or 36 inches. Historically, the yard was supposedly the distance from the tip of King Henry I's nose to the end of his outstretched thumb, though this is folk etymology — the actual origin is Anglo-Saxon land measurement based on the gird, a stick or rod. The yard remained slightly variable across English-speaking countries until the 1959 international agreement unified US, UK, Canadian, Australian, New Zealand, and South African yards into one identical value. Today the yard is used mainly in the US and UK for fabric, sports field measurements (American football, cricket pitch length 22 yards, golf drive distances), and informal short distances. Most of the world uses meters exclusively.