Length Converter (mm, cm, m, km, in, ft, yd, mi)

How the conversion works

Rather than storing a separate factor for every possible pair of units (which would mean 72 different numbers for the nine units on this page), each unit is defined by how many meters it represents. A conversion then takes two steps: the input is converted to meters, and those meters are converted into the target unit.

Converting 3 miles to kilometers, for example:

1. 3 mi × 1609.344 = 4828.032 m
2. 4828.032 m ÷ 1000 = 4.828032 km

The general formula is:

result = value × (meters per source unit) ÷ (meters per target unit)

This approach has a practical advantage beyond simplicity: rounding happens only once, at the end, instead of accumulating through a chain of intermediate conversions.

Why most of these factors are exact

A common assumption is that inches, feet, and miles are "roughly" defined and that converting them always introduces error. That has not been true since 1959. Under the International Yard and Pound Agreement, the United States, United Kingdom, Canada, Australia, New Zealand, and South Africa agreed to define the yard as exactly 0.9144 meters. Every other imperial length unit follows from that definition:

So "1 inch = 2.54 cm" is not an approximation that was rounded for convenience, it is the definition itself. Any imprecision you see in a result comes from rounding the display, not from the underlying factors.

Quick reference table

Values are rounded to five decimal places where they are not exact.

Everyday conversions worth knowing

Body height between systems

Height in the US and UK is usually given in feet and inches, which is not a single unit but a combination. Convert each part separately and add the results: 5 ft 10 in becomes (5 × 30.48) + (10 × 2.54) = 152.4 + 25.4 = 177.8 cm. Going the other way, divide the centimeter value by 30.48 to get feet, then multiply the remainder by 12 for inches: 180 cm ÷ 30.48 = 5.9055 ft, and 0.9055 × 12 ≈ 10.9 in, so roughly 5 ft 11 in.

Screen and display sizes

Monitor, laptop, and TV sizes are quoted as the diagonal in inches almost everywhere in the world, including in countries that otherwise use the metric system. A 15.6-inch laptop screen has a diagonal of 39.6 cm, a 55-inch TV measures 139.7 cm. Note that the diagonal alone does not determine width and height, those depend on the aspect ratio.

Road distances and speed limits

When driving between countries, the useful shortcut is that 1 km is roughly 0.62 miles and 1 mile is roughly 1.61 km. For quick mental math on road signs, multiplying miles by 1.6 gets you within about 1% of the correct kilometer figure, which is more than accurate enough for judging distances.

Nautical distances

Shipping and aviation use the nautical mile because it maps directly onto the Earth's geometry: one nautical mile equals one minute of arc of latitude. That makes chart work straightforward, since a degree of latitude is always 60 nautical miles regardless of where you are. A knot, the corresponding speed unit, is one nautical mile per hour.

Accuracy and rounding

Results are shown with up to 10 significant digits and trailing zeros removed, so 1 m to feet gives 3.280839895 rather than 3.2808398950. This avoids two opposite problems: showing 3.28 would hide precision that matters in engineering contexts, while showing the raw floating-point result would produce artifacts like 3.2808398950131235.

For most practical purposes, far fewer digits are appropriate. Carpentry and construction typically work to the nearest millimeter, land distances to the nearest meter, and road distances to one decimal place in kilometers. Carrying more digits than your original measurement justifies creates false precision: if you measured a room as "about 4 meters", converting to 13.12335958 feet does not make that measurement any more accurate.

Frequently asked questions

How many centimeters is one inch?

Exactly 2.54 cm. This is the internationally agreed definition rather than a rounded approximation, so the conversion introduces no error at all.

What is the difference between a mile and a nautical mile?

A statute mile (1609.344 m) is the everyday road-distance mile used in the US and UK. A nautical mile (1852 m) is about 15% longer and is based on the Earth's circumference, specifically one minute of latitude. Nautical miles are used in maritime and air navigation, statute miles on land.

How do I convert feet and inches to centimeters?

Multiply the feet by 30.48, multiply the inches by 2.54, and add the two results. For 6 ft 2 in: (6 × 30.48) + (2 × 2.54) = 182.88 + 5.08 = 187.96 cm.

Why does the US still use feet and inches?

The US formally adopted the metric system as its preferred system in 1975 but never mandated it for everyday use, so both systems coexist. Scientific, medical, and military contexts in the US are largely metric, while construction, retail, and daily life remain imperial. This is why converting between the two remains a routine task rather than a historical curiosity.

Is a meter exactly 100 centimeters?

Yes. All metric prefixes are exact powers of ten: 1 m = 100 cm = 1000 mm, and 1 km = 1000 m. This is the main practical advantage of the metric system, conversions within it require only moving a decimal point.

What is the most accurate way to convert large distances?

Use the exact factors rather than rounded shortcuts. Converting 500 miles with the rounded factor 1.6 gives 800 km, while the exact factor 1.609344 gives 804.672 km, a difference of nearly 5 km. The shortcut is fine for rough estimates but not for anything where the total matters.

Does this converter send my input anywhere?

No. The conversion runs entirely in your browser using JavaScript. No value you enter is transmitted to a server, and nothing is stored.

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