Length & Distance Converter

The Metric System Explained

During the French Revolution in the 1790s, scientists created the metric system as a universal way to measure things. Local units varied wildly—sometimes even neighboring towns used different measurements. The meter was originally defined as one ten-millionth of the distance from the North Pole to the equator along a meridian passing through Paris, though this definition has since been replaced with one based on the speed of light.

The metric system works well because it uses base-10. Converting between metric units just means moving the decimal point. One kilometer contains 1,000 meters, one meter contains 100 centimeters, and one centimeter contains 10 millimeters. The pattern holds from kilometers down to nanometers—all linked by powers of ten. Try converting 3.7 miles to inches—you have to multiply by 5,280, then by 12. Metric is simpler.

Today, one meter equals the distance light travels in 1/299,792,458 of a second. This 1983 definition tied the meter to a physical constant instead of a metal bar that could change over time. Scientists can now measure with extreme precision—far beyond what the French scientists who invented the system could have imagined.

The Imperial System

Imperial units developed over centuries. Most came from body parts. An inch was roughly a thumb's width. A foot was the length of a man's foot. A yard stretched from nose to fingertip. People could measure without tools, but that also meant big differences between regions and eras. A "foot" in medieval Paris was not the same as a "foot" in London or Rome.

In 1824, Britain standardized imperial units, and the United States largely adopted these measures, though some differences remain. The gallon, for instance, differs between US and UK measurements, though length units stayed consistent. Imperial units follow tradition, not simple math: 12 inches make a foot, three feet make a yard, and 1,760 yards make a mile. These numbers look random, but 12 divides by 2, 3, 4, and 6—handy when doing math in your head.

The 1959 International Yard and Pound Agreement tied imperial to metric with exact definitions. One inch became 25.4 millimeters, one foot 0.3048 meters, and one yard 0.9144 meters. Every imperial measurement now has an exact metric match. The agreement ended small differences that had existed for centuries and made international trade and engineering much simpler.

Nautical Measurements

Sailors and pilots use nautical units because they match up with navigation charts. One nautical mile equals one minute of latitude on any chart—count the minutes between two points and you have the distance. Travel one degree of latitude (60 nautical miles) and you've covered one-360th of Earth's circumference.

By international standard, one nautical mile equals 1,852 meters, close to the original definition based on Earth's geometry. Pilots use nautical miles for the same reason sailors do—they line up with latitude and longitude, which makes planning routes simpler. Speed at sea and in the air uses knots, where one knot equals one nautical mile per hour. The name comes from the practice of measuring ship speed by counting knots on a rope as they passed through a sailor's hands.

A fathom is six feet (about 1.83 meters). The name comes from arm span—how far you can stretch both arms. It became the standard for measuring water depth; sailors would "sound" the bottom by lowering a weighted rope marked in fathoms. A cable length (about 185 meters or 1/10 of a nautical mile) measured shorter distances at sea. Modern boats have electronic depth finders that show meters or feet, but nautical charts still use fathoms.

Astronomical Distance Units

Regular length units don't work well for cosmic distances. Write the distance to the nearest star in kilometers and you get a number too big to mean anything. Astronomers use special units that make huge distances easier to grasp.

One Astronomical Unit (AU) is the average distance from Earth to the Sun—about 150 million kilometers. Inside our solar system, this gives a useful scale: Mars orbits at about 1.5 AU, Jupiter at 5.2 AU, and Neptune at 30 AU.

The parsec, used mainly by professional astronomers, equals about 3.26 light-years.

For interstellar distances, the light-year describes how far light travels in one year—about 9.46 trillion kilometers. Even though it has "year" in the name, this measures distance, not time. Proxima Centauri, our nearest stellar neighbor, lies 4.24 light-years away. The light you see from it tonight left that star more than four years ago. The Andromeda Galaxy, visible to the naked eye on clear nights, is 2.5 million light-years distant—we see it as it looked 2.5 million years ago.

Units From the Past

Before standard units existed, people used their bodies to measure things. Ancient Egyptians and Near Eastern cultures used the cubit—elbow to fingertip—about 18 inches or 45 centimeters. The span stretched from thumb tip to pinky tip of an outstretched hand, about nine inches. The hand, now standardized at four inches, originally measured the width of a palm and remains the official unit for measuring horse height today.

Farming and land surveying created other units. A furlong ("furrow long") was how far oxen could plow before resting—about 660 feet. An acre was how much land one man and one ox could plow in a day. Edmund Gunter invented the surveyor's chain in 1620, 66 feet long. Ten square chains equal an acre. Eighty chains equal a mile. These round numbers made property math much easier.

Sailors kept the fathom (six feet) for measuring water depth and cable length. The rod or perch, at 16.5 feet, served as a standard surveyor's measure throughout England and its colonies. Most of these units are gone from daily life, but some survive. Horse heights still use hands, water depths may be charted in fathoms, and agricultural land is still sold in acres. Knowing these old units helps when reading old documents and explains where many common words come from.

Quick Conversion Tips

For quick mental math: one inch is about 2.5 cm, one foot is about 30 cm, and one meter is roughly three feet three inches. Miles to kilometers? Multiply by 1.6. Kilometers to miles? Multiply by 0.6, or double and subtract 20%.

For exact work, the official factors are: 1 inch = 25.4 mm, 1 foot = 0.3048 m. Engineers need these because small errors add up. Match your precision to the task—round driving distances, but machine parts might need thousandths of an inch.

Real-World Size References

At the microscopic end, a nanometer is about the width of a DNA double helix (2 nanometers across). A human hair ranges from 50 to 100 micrometers in diameter, about the same as a fine grain of sand. Moving to everyday scales, a credit card is about one millimeter thick, a fingernail about one centimeter wide, and a standard door about two meters tall.

These comparisons help when you don't have a ruler handy.

How to Use This Converter

Single Conversion mode handles direct conversion between any two units—enter your value, pick your units, and click Convert. The swap button reverses the direction, and quick conversion buttons set common unit pairs with one click. Convert to All Units mode shows your value in every supported unit at once, organized by category with copy buttons for each result.

Height Converter mode handles the feet-and-inches to centimeters conversion that comes up when discussing human height. Pick your direction, enter the height, and get the result with a breakdown of the math. A reference table shows common height conversions for quick lookup.