Length Converter

Why Length Conversion Is Still a Daily Problem

Most countries have used the metric system since the 19th century, yet length conversion remains one of the most frequently searched unit topics online. The reason is simple: the United States, which drives a significant chunk of the world's media, construction standards, and consumer products, still publishes measurements in feet, inches, and miles. A timber merchant in Germany reading an American lumber spec needs to convert inches to millimetres. A runner in the UK comparing race times with a friend in Canada converts miles to kilometres. An architect submitting drawings to a Japanese client converts feet and inches to metres. This converter handles all of it — 13 units from nanometres to light years — instantly and without any login.

The 13 Supported Length Units

Each unit below covers a distinct practical context — this is not an arbitrary list:

• Kilometre (km): Road distances, geography, GPS coordinates
• Metre (m): The SI base unit — construction, fabric, athletics
• Centimetre (cm): Body measurements, furniture, everyday sizing
• Millimetre (mm): Engineering tolerances, printing, machined parts
• Micrometre (µm): Thin films, semiconductor layers, biological cells
• Nanometre (nm): Chip transistor nodes, optical wavelengths, DNA strands
• Mile (mi): US road distances, speed limits, marathon courses
• Yard (yd): American football, fabric retail in the US/UK
• Foot (ft): US construction, altitude readings, ceiling heights
• Inch (in): Screen sizes, pipe diameters, screw dimensions
• Nautical Mile (nmi): Maritime and aviation navigation, international airspace
• Light Year (ly): Interstellar distances in astronomy and astrophysics
• Astronomical Unit (AU): Solar system distances — Earth–Sun = exactly 1 AU

How the Converter Handles Precision

Precision matters more for length than for most other unit types, because the range spans 26 orders of magnitude — from a nanometre (10⁻⁹ m) to a light year (≈9.46 × 10¹⁵ m). The converter uses JavaScript's 64-bit double-precision floating point throughout, which gives 15–17 significant decimal digits. Results are formatted to 10 significant figures for ordinary values and switch to scientific notation for values larger than 10¹² or smaller than 10⁻⁹. This makes readability practical across the entire range — from semiconductor process nodes to interstellar distances — without sacrificing accuracy.

The underlying algorithm converts through a common base (metre): to go from feet to nautical miles, the tool first converts feet → metres, then metres → nautical miles. This two-step chain means only 13 conversion functions are needed to power all 169 possible unit pairings — not 169 separate formulas.

Exact Definitions You Can Rely On

This tool uses the official, exact definitions — not approximations. Key values:

• 1 inch = exactly 2.54 cm (international inch, defined 1959)
• 1 foot = exactly 0.3048 m (12 international inches)
• 1 mile = exactly 1,609.344 m (5,280 international feet)
• 1 nautical mile = exactly 1,852 m (international definition, 1929)
• 1 AU = exactly 149,597,870,700 m (IAU 2012 definition)
• 1 light year ≈ 9.4607 × 10¹⁵ m (Julian year × speed of light)

Common Conversion Scenarios

Here are four conversion situations people reach for a length converter most often: (1) Construction cross-border projects — converting US blueprint dimensions (feet/inches) to metric for European contractors or vice versa. (2) Athletic training — 5K and 10K runners comparing distances with friends using miles, or swimmers converting pool lengths. (3) Buying imported goods online — checking that a TV described as "65 inches" fits in a space measured in centimetres, or verifying that a table listed in metres matches a room in feet. (4) Science and education — converting between metric prefixes (mm, µm, nm) for physics coursework or checking whether a given transit exoplanet orbit makes sense in AU terms.

For problems that involve two dimensions, pair this tool with the Area Converter. If your project involves velocity, the Speed Converter handles km/h, mph, knots, and Mach numbers. For weight alongside dimensional measurements, the Weight Converter covers kg, lbs, grams, and more.

Privacy: Everything Runs Locally

No measurement values you enter are transmitted to any server. All arithmetic happens inside your browser tab via JavaScript. This means confidential inputs — engineering specs, proprietary tolerance drawings, medical height data — stay entirely on your device. There are no cookies related to your conversions, no analytics events triggered on input changes, and no session storage of the values you type.