Length Converter

Metric units (mm, cm, m, km) are built on powers of 10 around the meter, the SI base unit.

Imperial / US units (inch, foot, yard, mile) use legacy ratios:

• 12 in = 1 ft
• 3 ft = 1 yd
• 1,760 yd = 1 mi

Converting between systems uses fixed factors:

• 1 in = 2.54 cm
• 1 ft = 0.3048 m
• 1 mi = 1,609.344 m

Our length converter handles both systems bidirectionally.

Beyond everyday units, this tool supports:

• micrometers (microns, 10⁻⁶ m) and nanometers (10⁻⁹ m) for science
• furlongs (1/8 mile) and fathoms (6 ft) for historic/marine use
• nautical miles (1,852 m) for aviation and navigation
• light-years (~9.461 × 10¹⁵ m) for astronomy

All converted through the meter as the common base.

To convert any length, multiply the value by the FROM unit's factor in meters, then divide by the TO unit's factor. Meters serve as the pivot because the metre is the SI base unit of length, defined by BIPM through the fixed speed of light.

For example, converting 5 miles to kilometers: 5 × 1,609.344 m = 8,046.72 m, then ÷ 1,000 = 8.04672 km.

The core factors are fixed constants:

• 1 in = 0.0254 m
• 1 ft = 0.3048 m
• 1 yd = 0.9144 m
• 1 mi = 1,609.344 m
• 1 nautical mile = 1,852 m

This meter-pivot method, standardized by NIST, keeps every conversion consistent and fully bidirectional.

There are exactly 2.54 centimeters in one inch. This is a defined value, not a measured approximation: since the 1959 international yard-and-pound agreement, 1 inch = 25.4 mm = 2.54 cm exactly, a relationship maintained by NIST and BIPM.

To convert inches to centimeters, multiply by 2.54; to go from centimeters to inches, divide by 2.54. For example, a 15-inch laptop screen measures 15 × 2.54 = 38.1 cm, and 100 cm ≈ 39.3701 inches.

Because the factor is exact, inch-to-centimeter conversions carry no rounding error at the definition level — any imprecision comes only from how many decimals you keep in your final answer.

To convert meters to feet, multiply by 3.28084, or equivalently divide by 0.3048, since 1 foot equals exactly 0.3048 meters by international definition (NIST, BIPM). For example, 10 m × 3.28084 = 32.8084 ft, and a 100-meter running track equals about 328.08 feet.

To reverse the conversion, multiply feet by 0.3048 to get meters. Because a foot is defined as exactly 0.3048 m, this factor is a fixed constant rather than a rounded estimate.

If you need feet-and-inches, take the whole-number feet, then multiply the remaining decimal by 12 to get inches — useful for height, ceiling clearances, and construction measurements.

One statute mile equals exactly 1.609344 kilometers, since 1 mile is defined as 1,609.344 meters (NIST, based on 1 mi = 5,280 ft × 0.3048 m). To convert miles to kilometers, multiply by 1.609344; to convert kilometers to miles, multiply by 0.621371.

For quick mental math, a mile is roughly 1.6 km, so a 26.2-mile marathon is about 42.2 km. This is the most common conversion for travelers comparing road signs and speedometers, since the United States posts distances in miles while most of the world uses kilometers.

Do not confuse the statute mile with the nautical mile (1,852 m), which is longer and used in aviation and marine navigation.

Length conversion shows up across daily life and technical work:

• Travelers convert miles to kilometers to read foreign road signs and interpret car speedometers.
• Engineers and machinists convert inches to millimeters when working with international CAD drawings and ISO-standard tolerances.
• Builders and interior designers translate meters to feet for room dimensions, flooring, and furniture.
• Runners and cyclists convert miles to kilometers for race distances.
• Scientists move between nanometers and micrometers for semiconductor and microscopy work, while sailors and pilots rely on nautical miles.
• Astronomers scale up to light-years.

In each case, using the exact SI factors published by NIST and BIPM prevents the small errors that compound into costly mistakes in manufacturing, navigation, and construction.

A meter contains about 3.28084 feet, a yard contains exactly 3 feet, and a mile contains exactly 5,280 feet. These follow directly from the definitions maintained by NIST: 1 yd = 0.9144 m = 3 ft, and 1 mi = 1,760 yd = 5,280 ft.

So to convert yards to feet, multiply by 3; to convert miles to feet, multiply by 5,280. For example, an American football field is 100 yards = 300 feet = 91.44 meters.

Because the yard and foot are both defined exactly in terms of the meter (1 ft = 0.3048 m exactly since 1959), all of these imperial-to-imperial ratios are exact whole numbers, making them easy to remember and error-free.

SI prefixes let the meter span from the atomic scale to astronomical distances, all in powers of ten as defined by BIPM:

• A nanometer (nm) is 10⁻⁹ m, used to describe semiconductor process nodes and light wavelengths; visible light ranges roughly 380–750 nm.
• A micrometer (µm), or micron, is 10⁻⁶ m, common for machining tolerances, cell biology, and particle sizes.
• A millimeter is 10⁻³ m, a centimeter 10⁻² m, and a kilometer 10³ m.

To move between adjacent metric units you simply shift the decimal: 1 mm = 1,000 µm = 1,000,000 nm.

This decimal structure, standardized in the SI, is why the metric system is so convenient for scientific and engineering length conversions.

One light-year is approximately 9,460,730,472,580.8 kilometers (about 9.461 × 10¹² km), or roughly 5.879 trillion miles. A light-year is a unit of distance, not time: it is the distance light travels in a vacuum during one Julian year of 365.25 days, using the fixed speed of light (299,792,458 m/s) defined by BIPM.

Astronomers use light-years for interstellar distances — the nearest star system, Alpha Centauri, is about 4.37 light-years away. For even larger scales, they use the parsec (about 3.26 light-years).

Because the light-year depends on exact SI constants, this converter computes it precisely from the meter rather than treating it as a loose approximation.

The most frequent error is confusing statute miles (1,609.344 m) with nautical miles (1,852 m); they differ by about 15%, which matters greatly in navigation.

• Another is mixing up the exact factor 1 in = 2.54 cm with rounded values like 2.5 cm, introducing avoidable error.
• People also invert factors — multiplying instead of dividing — so always check whether your result should be larger or smaller (converting a big unit to a small unit yields a bigger number).
• Treating a light-year as a measure of time rather than distance is a classic mistake.
• Finally, watch decimal-shift errors in metric conversions: 1 km = 1,000 m, not 100.

Using the exact NIST and BIPM factors and sanity-checking the direction prevents nearly all of these.