Everyday Life · Unit Conversions
Unit Converter (Length)
Convert any length measurement between metric and imperial units including meters, feet, inches, kilometers, miles, and more.
Calculator
Formula
L_source is the input length value, f_source is the conversion factor of the source unit to meters, and f_target is the conversion factor of the target unit to meters. All conversions are performed via the SI base unit (meter).
Source: NIST Special Publication 330 (2019): The International System of Units (SI). National Institute of Standards and Technology.
How it works
Every length conversion in this calculator is grounded in the International System of Units (SI), where the meter is the base unit of length. The conversion process works in two steps: first, the input value is multiplied by the source unit's conversion factor to express it in meters; then, that meter value is divided by the target unit's conversion factor to produce the result. This two-step approach through a common base unit ensures that any unit can be converted to any other unit without compounding rounding errors from a chain of direct conversions.
The conversion factors used in this calculator are the exact definitions adopted by international standards bodies. For example, one inch is defined as exactly 0.0254 meters by the International Yard and Pound Agreement of 1959, and one mile is exactly 1,609.344 meters. The nautical mile — used in aviation and maritime navigation — is exactly 1,852 meters, as defined by the International Hydrographic Organization. These are not approximations; they are fixed, exact legal definitions that underpin engineering, trade, and science worldwide.
It is worth understanding the historical division between metric and imperial systems. The metric system, standardized as SI, is used by the vast majority of countries and all scientific disciplines globally. Imperial and US customary units persist primarily in the United States for everyday use, and in the United Kingdom for certain road and height measurements. Engineers, scientists, and anyone working across international contexts routinely need to convert between these systems, making a reliable, multi-unit converter an essential everyday tool.
Worked example
Suppose you want to convert 26.2 miles (a standard marathon distance) into kilometers and meters.
Step 1 — Express in meters: Multiply 26.2 miles by the conversion factor for miles (1,609.344 m/mile):
26.2 × 1,609.344 = 42,164.8128 meters
Step 2 — Convert to kilometers: Divide the meter value by 1,000:
42,164.8128 ÷ 1,000 = 42.1648 km
Step 3 — Convert to feet for a cross-check: Divide the meter value by 0.3048:
42,164.8128 ÷ 0.3048 = 138,336 feet
This example illustrates how all three outputs are derived from the same intermediate meter value, ensuring consistency. The official marathon distance is defined as 42.195 km, confirming our conversion is accurate to within rounding of the original 26.2-mile input.
Limitations & notes
This converter applies exact or high-precision conversion factors for all listed units and is suitable for engineering, science, and everyday use. However, extremely large values such as light years or astronomical units may be subject to JavaScript floating-point precision limits (approximately 15–17 significant digits), so results at the extremes of scale should be verified with arbitrary-precision tools if sub-part-per-million accuracy is required. The tool also does not handle units of angular length (degrees, arcminutes) or units that depend on context such as typographic points, which vary by system (PostScript vs. traditional).
Frequently asked questions
What is the exact conversion factor between feet and meters?
One foot is exactly 0.3048 meters, a definition fixed by the International Yard and Pound Agreement of 1959, signed by the United States, the United Kingdom, Canada, Australia, New Zealand, and South Africa. This means one meter equals approximately 3.28084 feet. This exact definition applies to both the international foot and, since 2023, the US survey foot, which was retired.
How many kilometers are in a mile?
There are exactly 1.609344 kilometers in one international mile, making the conversion factor precise and not an approximation. This means 1 km equals approximately 0.621371 miles. The shorthand approximation of '1 mile ≈ 1.6 km' is accurate to about 0.6% and is commonly used for quick mental math.
What is the difference between a nautical mile and a statute mile?
A nautical mile is exactly 1,852 meters, while a statute (land) mile is exactly 1,609.344 meters, making the nautical mile about 15.1% longer. Nautical miles are used in maritime and aviation navigation because one nautical mile corresponds closely to one arcminute of latitude, making chart reading and position calculations more intuitive. Speeds in aviation and shipping are expressed in knots, where one knot equals one nautical mile per hour.
Why is the meter the standard base unit for length?
The meter is the SI base unit of length and is defined by the 2019 SI redefinition as the distance light travels in a vacuum in exactly 1/299,792,458 of a second, tying it to the fixed numerical value of the speed of light. This physical definition makes the meter reproducible anywhere in the universe to extraordinary precision, which is why it underpins all modern scientific measurement. Before 1983, it was defined by the length of a physical platinum-iridium bar kept in France.
How do I convert inches to centimeters?
To convert inches to centimeters, multiply by 2.54, since one inch is defined as exactly 2.54 centimeters. For example, a standard sheet of US letter paper is 8.5 × 11 inches, which is 21.59 × 27.94 cm. This is one of the most commonly needed conversions for clothing sizes, screen dimensions, and product specifications when dealing between US and international markets.
Last updated: 2025-01-15 · Formula verified against primary sources.