Length Converter

Enter a distance, pick the unit it is in and the unit you want, and this converter handles the rest. It covers thirty-one units, from the angstrom and the fermi at the subatomic end through the everyday metric and imperial units to the astronomical unit, light year and parsec. Typesetting units — the em, the pica and the point — are in there too, alongside older measures like the fathom, furlong, rod and ell.

Most people arrive needing one of about six conversions. The rest of the list is there for the day you meet a fathom on a nautical chart or a parsec in an astronomy paper and would rather not go looking.

Which conversions actually matter day to day?

These are the ones worth committing to memory, because they are exact by definition rather than rounded:

• 1 inch = 2.54 cm exactly — every other imperial-to-metric length follows from this
• 1 foot = 30.48 cm (12 inches)
• 1 yard = 0.9144 m (3 feet)
• 1 mile = 1,609.344 m (1,760 yards)
• 1 nautical mile = 1,852 m exactly — a different thing from a statute mile

The inch was defined as exactly 2.54 centimetres by international agreement in 1959, which is why these are clean numbers rather than approximations that drift.

A worked example: height in feet and inches

Someone is 5 feet 9 inches tall and you need it in centimetres for a form.

1. Convert the whole height to inches: (5 × 12) + 9 = 69 inches.
2. Multiply by 2.54: 69 × 2.54 = 175.26 cm.

Round to 175 cm. Going back the other way, 180 cm ÷ 2.54 = 70.87 inches, which is 5 feet 10.9 inches — so just under 5′11″.

Statute miles and nautical miles

They are not the same and the difference is significant: a nautical mile is 1,852 metres against 1,609.344 for a statute mile, about 15% longer. The nautical mile exists because it corresponds to one minute of latitude, which makes chart work straightforward at sea and in the air. A knot is one nautical mile per hour.

The tool lists several: international nautical mile, UK nautical mile and US nautical mile. The first is the modern standard; the other two are historical definitions that survive in older documents and differ by a few metres.

The astronomical units

At the other end of the scale the tool handles the three distances used in astronomy, which are each anchored to something physical:

• Astronomical unit (AU) — the Earth-Sun distance, defined as exactly 149,597,870,700 metres. Convenient for talking about the solar system.
• Light year — how far light travels in a Julian year, about 9.461 × 10¹⁵ metres. It is a distance, not a time, despite the name.
• Parsec — about 3.2616 light years. It comes from parallax measurement and is what professional astronomers actually use.

How to use it

1. Type the number into the input field.
2. Choose the unit you have from the first dropdown.
3. Choose the unit you want from the second.
4. Copy the result. Very large and very small values are shown in scientific notation, which is the only sensible way to display a fermi next to a parsec.

The older units still worth recognising

A few of the entries in the list are no longer in daily use but appear often enough in documents to be worth knowing. A fathom is six feet, used for water depth on charts. A furlong is 220 yards, an eighth of a mile, and survives in horse racing. A rod is 5.5 yards and still turns up in older land deeds and property descriptions. An ell is 45 inches, a cloth measure. Meeting one of these in a survey or a historical text and having to guess is a good way to be badly wrong about a distance.

Frequently asked questions

How many centimetres are in an inch?

Exactly 2.54. This is a definition, not a measurement, so it does not round.

Why are there three different nautical miles?

The UK and US each had their own definition before the international nautical mile of exactly 1,852 metres was adopted. The differences are a few metres and only matter when reading historical charts or documents.

What is a mil?

One thousandth of an inch, used in engineering for material thickness and tolerances. It is easy to confuse with the millimetre in written notes, and they differ by a factor of about 39.

Is a light year a measure of time?

No. It is the distance light covers in a year — roughly 9.46 trillion kilometres. The confusion is common enough that astronomy writing often spells it out.

What are the em, pica and point doing in a length converter?

They are typographic units and they are genuinely lengths. A point is 1/72 of an inch in the modern desktop-publishing definition, and a pica is twelve points. They appear here because print and layout work needs to move between them and millimetres.

Related tools

• CM to Feet Converter — a dedicated tool if height is all you need.
• Miles to Kilometers Converter — for road and running distances.
• Area Converter — the same units squared.
• Volume Converter — and cubed.

For the reasoning behind the metric and imperial systems and a set of reference charts, see our guide to metric and imperial length and distance conversion.