Length and Distance Converter: Metric, Imperial, Survey Chains and the US Survey Foot

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How to use this tool

Type a value, pick the unit you have and the unit you want. The big number is your answer, and the table underneath shows the same value in all 22 units at once, which is usually faster than converting twice. The Swap button flips the two selectors, and the chips under the result jump to the pairs people ask for most.

The one choice that catches people out is foot. There are two of them in the list, and they are not the same length. Pick “foot, international” unless you are working with older US state plane coordinates or land records, in which case you probably want “foot, US survey”. More on why that matters below.

What the conversion is actually doing

Every unit here is stored as a number of metres, and a conversion is just multiply by the source factor and divide by the target one. That sounds trivial, and for the arithmetic it is. The part that goes wrong is the factors themselves, because some are exact by definition and some are measured, and mixing the two quietly loses precision.

These are exact, not rounded: the inch has been exactly 25.4 mm since the international yard and pound agreement of 1959, the nautical mile is exactly 1852 m, and the astronomical unit was fixed at exactly 149597870700 m in 2012. The light year follows from the exact speed of light and a Julian year. The parsec is the one genuinely irrational value in the list, since it is defined as 648000 divided by pi astronomical units.

The surveyor chain, and why it refuses to die

Gunter invented the 66 foot chain in 1620, and it looks arbitrary until you see what it was designed to do. Ten square chains make exactly one acre. That is 43560 square feet, a number that would be miserable to work with in any other unit, and it falls straight out of the chain by construction.

The rest of the ladder hangs off it. One chain is 100 links, so a link is 7.92 inches. Four rods make a chain, ten chains make a furlong, and eighty chains make a mile. If you have ever wondered why a mile is 5280 feet rather than a round number, that is why: it is 80 chains, and the chain came first for land measurement purposes.

This is not trivia if you read deeds or old plans. Public Land Survey System descriptions in the United States are written in chains and links, and plenty of British and Commonwealth title plans use them too. Converting them by hand is where transcription errors creep in.

A worked check you can repeat on paper

Take a boundary recorded as 12 chains 34 links. That is 12.34 chains. Multiply by 66 to get 814.44 international feet, or multiply by 20.1168 to get 248.24 metres. Put 12.34 into the tool with chain selected and you get both, plus the value in rods and furlongs if the next call in the description uses those.

Now the check that proves the chain was designed around the acre. One chain is 66 feet, so a square chain is 4356 square feet. Ten of those is 43560, which is one acre exactly. No rounding anywhere.

The US survey foot, and the 2 ppm that bites

Before 1959 the United States defined the foot as exactly 1200 divided by 3937 metres. The 1959 agreement redefined it as 0.3048 m, but surveying kept the old value to avoid rewriting every existing coordinate, and called it the US survey foot. The difference is two parts per million, which sounds like nothing.

Two parts per million is nothing over a room. Over a 100 km traverse it is 0.66 feet, about 200 mm. State plane coordinate values run to hundreds of thousands of feet, so choosing the wrong foot shifts a point by a foot or more. That is well outside any survey tolerance, and it is a silent error, because both numbers look completely reasonable.

The US survey foot was officially retired for new applications at the end of 2022. It still appears everywhere in existing data, so the safe habit is to check which foot a dataset was published in before you convert anything, not after.

Mistakes that catch people out

Assuming one foot. The commonest error in coordinate work. Fix: find out which foot the source data uses before converting, and record it alongside the numbers.

Confusing nautical and statute miles. A nautical mile is 1852 m, a statute mile is 1609.344 m, a 15 percent difference. Fix: nautical for marine and aviation, statute for road distance.

Reading links as decimal chains. “12 chains 34 links” is 12.34 chains only because a link is exactly one hundredth of a chain. Rods and furlongs do not behave that way. Fix: convert each part separately unless you are certain of the ratio.

Rounding the factor rather than the answer. Using 0.305 for a foot introduces 65 ppm of error, which is thirty times worse than the survey foot problem everyone worries about. Fix: keep full precision through the calculation and round only at the end.

Treating thou and mil as different units. They are the same thing, one thousandth of an inch, and neither is a millimetre. Fix: watch for “mil” in drawings from mixed sources.

How this was checked

Every factor was tested against its legal definition before this page went live. The exact ones return zero error, not a small one: the inch at 25.4 mm, the foot at 0.3048 m, the nautical mile at 1852 m, and the astronomical unit at 149597870700 m. The survey identities were checked as whole numbers, so 1 chain returns exactly 66 feet and exactly 100 links, 1 mile returns exactly 80 chains, and 10 square chains return exactly 43560 square feet.

All 484 unit pairs were then round tripped, converting a value out and back again. The worst drift across the whole set was 1.8 times ten to the minus sixteen, which is floating point noise rather than a factor error. The parsec was checked against its published value of 206264.806 astronomical units.

Related Calculators and Reference

For everyday lengths without the survey units, the Length Converter handles inches, feet, millimetres, centimetres and miles with a tape measure check.

For speed rather than distance or mass, the Kmh to Mph Converter uses the same 1959 agreement, since the mile it is built on was fixed at exactly 1609.344 metres by that treaty.

For a single fixed pair, the Feet to Centimetres Converter is quicker. Slopes and grades convert differently, so use the Grade to Degrees Converter for those. If you are moving between coordinate systems rather than units, the Coordinate Converter handles latitude, longitude and UTM. For sediment and soil work the Grain Size Converter covers millimetres, phi and Wentworth classes, and the Pressure Unit Converter does the same job for psi, bar and kPa. The definitions used here come from the NIST Office of Weights and Measures.