About the Length Converter

The length converter translates between metric and imperial units of distance, including millimetres, centimetres, metres, kilometres, inches, feet, yards, miles, and nautical miles. Whether you are reading a European recipe in millimetres, an American construction plan in feet, or a maritime chart in nautical miles, this tool gives you an instant conversion in either direction.

The metric system, used by nearly every country in the world, is built on the metre as its base unit and uses powers of ten for its prefixes. The imperial system, still used in the United States and a few other countries for everyday measurements, uses the inch, foot, yard, and mile, which are related by factors of 12, 3, and 1760 rather than ten. The nautical mile, used in aviation and maritime navigation, is defined as exactly 1852 metres and corresponds to one minute of latitude.

Every conversion is computed by first converting the input to metres (the SI base unit) and then converting from metres to the target unit. This two-step approach guarantees consistency and avoids the accumulation of rounding errors that can occur when chaining direct conversions.

How It Works

Each unit has a defined relationship to the metre, the SI base unit of length. The converter uses these exact factors to translate between any two units:

1 mm   = 0.001 m
1 cm   = 0.01 m
1 m    = 1 m
1 km   = 1000 m
1 in   = 0.0254 m       (exact, by international agreement)
1 ft   = 0.3048 m       (12 inches)
1 yd   = 0.9144 m       (3 feet)
1 mi   = 1609.344 m     (1760 yards)
1 nmi  = 1852 m         (one minute of latitude)

The conversion proceeds in two steps: first the input value is multiplied by the source unit's factor to give a length in metres, then that length is divided by the target unit's factor to give the result in the target unit. This is mathematically equivalent to multiplying by the ratio of the two factors, but the two-step form is easier to verify and debug.

The imperial-to-metric conversions are exact by international agreement. The inch was redefined in 1959 as exactly 25.4 millimetres, which makes every other imperial unit exactly defined in metric terms. The nautical mile was defined as exactly 1852 metres by international agreement in 1929.

Worked Examples

To convert 1 mile to kilometres, set the input to 1 mile and the target to kilometres. The result is 1609.344 / 1000 = 1.609344 km. This is the conversion factor used on road signs in countries that display both units.

To convert 6 feet to metres, the calculation is 6 * 0.3048 = 1.8288 m. This is the height of a person who is six feet tall, expressed in metric units.

To convert 100 metres to yards, the calculation is 100 / 0.9144 = 109.36 yards. This is why a 100-metre sprint is slightly longer than a 100-yard sprint, which is why the two events are not directly comparable in track and field.

When to Use This Tool

Use the length converter whenever you need to translate between metric and imperial units:

Reading international recipes that use millimetres or centimetres instead of inches.
Interpreting construction or engineering drawings from another country.
Converting running, cycling, or driving distances for travel planning.
Working with maritime or aviation charts that use nautical miles.
Comparing the heights of people or objects measured in different systems.
Converting fabric, rope, or cable lengths for craft or industrial projects.
Teaching or learning the relationships between metric and imperial units.

Limitations & Disclaimer

This converter handles linear distance only and does not convert astronomical units, light-years, parsecs, or specialised surveying units. Conversions are exact by international agreement, but very large or very small results may be displayed in scientific notation due to floating-point representation. For surveying or legal work, verify with the appropriate authority. See our disclaimer for full details.

Frequently Asked Questions

Why is the inch exactly 25.4 millimetres?

By international agreement in 1959, the inch was redefined as exactly 25.4 millimetres to unify the American and British inch definitions. Before that, the two differed by a few parts per million, which caused measurable discrepancies in precision engineering. The 1959 agreement eliminated this source of error.

What is a nautical mile and how does it differ from a statute mile?

A nautical mile is exactly 1852 metres and corresponds to one minute of latitude on the Earth's surface. A statute mile is 1609.344 metres. Nautical miles are used in aviation and maritime navigation because they relate directly to the angular coordinate system of the Earth.

Why does the converter go through metres?

To avoid accumulating rounding errors. Every conversion is computed as input times source factor, divided by target factor, all in exact arithmetic. Going through a common base unit is also easier to verify and debug than maintaining a separate factor for every possible pair of units.

Are the imperial units still legally used?

Yes, in the United States, the United Kingdom (for some purposes), and a few other countries. Most of the world has officially adopted the metric system, but imperial units persist in everyday use for height, distance, and cooking in certain regions.

How accurate are the conversions?

The conversion factors are exact by international agreement. The only source of error is floating-point rounding, which is negligible for any practical purpose. The converter displays up to eight decimal places, which is more than enough for engineering, navigation, or everyday use.

What is the difference between a metre and a meter?

Nothing. Metre is the spelling used in most of the English-speaking world and in international standards; meter is the American spelling. The unit is the same, defined as the distance light travels in a vacuum in 1/299792458 of a second.

Last updated: July 21, 2026 · Author: HT99 Tools Editorial Team · Reviewed by: HT99 Tools Editorial Team