📏 Length Converter

What is the Length Converter?

The world runs on two families of length units. The metric system, formalised today as the SI (International System of Units), builds everything on the metre, with prefixes for powers of ten: a millimetre is one-thousandth of a metre, a centimetre is one-hundredth, and a kilometre is a thousand metres. The imperial and US customary systems use the inch, foot, yard and mile, where the steps are 12, 3 and 1,760 rather than tidy powers of ten. Most countries use metric for everyday measurement; the United States and, for some purposes, the United Kingdom still lean on imperial, which is exactly why a quick converter saves so much friction.

Under the hood every conversion in this tool goes through a single base unit: the metre. Whatever you type is first scaled into metres, then scaled back out to each target unit. That two-step path is why you can mix any units freely, centimetres in and miles out, without a separate formula for each pair. It also keeps the maths consistent: there is exactly one definition of the metre, and every other unit is expressed against it. The same logic powers the height conversions people ask for most, such as turning 175 cm into 5 feet 9 inches.

The factors are not approximations. Since the 1959 international yard and pound agreement the inch has been defined as exactly 2.54 centimetres, which fixes the foot at exactly 0.3048 m, the yard at exactly 0.9144 m and the statute mile at exactly 1,609.344 m. Because these definitions are exact, the only error in a conversion is the precision you choose to round to; the underlying numbers are correct to as many decimals as you need.

A few unit quirks are worth knowing before you trust a number. The mile comes in more than one flavour: the everyday statute (land) mile is 1.609344 km, but the nautical mile used in shipping and aviation is 1,852 m, roughly 15 percent longer, because it is tied to one minute of latitude. The US survey foot, used in older American land surveys, differs from the international foot by about two parts per million, which is invisible for a room measurement but matters across a county. For science and astronomy there are far larger units too, such as the light-year (about 9.46 trillion km), though for everyday work the metre-based chain covers everything you are likely to need.

If you prefer mental shortcuts, a handful are accurate enough for quick estimates: a centimetre is a little under four-tenths of an inch (multiply cm by 0.4), a metre is about 3.28 feet or just over a yard, and to turn kilometres into miles you can multiply by 0.62 (or, faster, take roughly five-eighths). These rules of thumb get you close; this tool gives you the exact figure when close is not good enough, for instance when cutting material to size or reading a technical spec.

When to use it

• Reading screen and TV sizes quoted in inches when you think in centimetres (or vice versa).
• Converting running and cycling distances between kilometres and miles for race planning.
• Working out room and furniture dimensions in metres and feet when reading plans or property listings.
• Translating a person's height between centimetres and feet-and-inches for forms, visas or dating profiles.
• Making sense of travel distances and speed-limit signs abroad in unfamiliar units.
• Converting fabric, lumber or pipe lengths between millimetres, inches and yards for DIY and trade work.

How to use the Length Converter

1. Type a number into any unit field (for example centimetres).
2. Every other unit, including inches, feet, metres and miles, updates instantly.
3. For height, read the feet-and-inches value, or copy the unit you need.
4. Clear the field to start a fresh conversion.

Formula & method

Worked examples

Exact length conversion factors

Common heights: centimetres to feet and inches

Common mistakes to avoid

• Confusing statute and nautical miles. A statute mile is 1.609344 km, but a nautical mile (used at sea and in aviation) is 1,852 m, about 15% longer. Make sure you are converting the right kind of mile.
• Rounding 1 inch to 2.5 cm. An inch is exactly 2.54 cm, not 2.5. The 2.5 shortcut drifts by about 1.6%, which adds up fast over longer measurements.
• Multiplying the wrong way. Going from a larger unit to a smaller one (km to miles) the number gets smaller, while metres to feet the number gets larger. Multiply by the factor in one direction; divide in the other.
• Mixing units in a single dimension. Entering one part in feet and another in metres before adding throws the total off. Convert everything to one unit first, then combine.
• Treating feet-and-inches as a decimal. A height of 5 ft 9 in is not 5.9 ft. The inches are out of 12, so 5 ft 9 in is 5.75 ft. Convert the inch part to a fraction of a foot before doing arithmetic.
• Forgetting that area and volume scale differently. Length factors do not carry straight over to area or volume. One square metre is about 10.76 square feet, not 3.28, because the factor is squared. For volume it is cubed.

Glossary

Metric system

A decimal system of measurement built on the metre, where units scale by powers of ten (cm, m, km).

Imperial / US customary

The system of inches, feet, yards and miles. UK imperial and US customary share length units but differ for some volumes.

SI base unit

The metre is the SI base unit of length; every other length unit in this tool is defined as an exact multiple of it.

Statute mile

The ordinary land mile of 1,760 yards (1.609344 km), as opposed to the longer nautical mile of 1,852 m.

Nautical mile

A unit of 1,852 metres used in sea and air navigation, based on one minute of latitude. It is about 15 percent longer than a statute mile.

Hand

A traditional unit of exactly 4 inches (10.16 cm) still used to measure the height of horses.

Light-year

An astronomical distance unit equal to how far light travels in one year, about 9.46 trillion kilometres.

Frequently asked questions