What Is a Chain?
A chain (abbreviated ch) is a surveying unit of length equal to 66 feet, 22 yards, 4 rods or 100 links, which is exactly 20.1168 meters. Eighty chains make a mile and ten make a furlong. It is often called Gunter’s chain, after the English mathematician and clergyman Edmund Gunter, who introduced it in 1620.
Gunter’s chain was a real object: 100 iron or steel links joined by rings, each link 7.92 inches long, with brass tags every ten links so surveyors could count quickly. Two people dragged it across the land, counting chain lengths. Its genius was arithmetic. Gunter matched the chain to the existing English acre, which made area calculations simple: 10 square chains equal one acre, and 640 acres make a square mile.
The chain and link are listed with their exact metric values in NIST Handbook 44. They should not be confused with the engineer’s chain of 100 feet, used in some American civil engineering work, which is about 1.5 times longer.
What Is a Mile?
A mile is 80 chains, a relationship that comes from the 1593 English statute defining the mile as 8 furlongs. Each furlong was already 10 chains of 66 feet, so the statute mile came out at 5,280 feet. The word mile is Latin in origin, from mille passus, a thousand Roman paces.
The mile is now defined in metric terms. Under the 1959 International Yard and Pound Agreement it is exactly 1,609.344 meters, and since a chain is 22 international yards, the chain is exactly 20.1168 m.
Old Surveying Units in Land Deeds
If you read an old property deed in the United States, Canada, Australia or New Zealand, you are likely to find distances in chains and links. A typical “metes and bounds” description reads something like “thence north 12 degrees east 12 chains 50 links to a stake”. To use it with a modern map, convert links to chains (100 links per chain), then chains to miles or feet.
| Survey measure | Chains (ch) | Miles (mi) |
|---|---|---|
| One link | 0.01 | 0.000125 |
| One rod (pole or perch) | 0.25 | 0.003125 |
| One chain | 1 | 0.0125 |
| One furlong | 10 | 0.125 |
| Quarter-section side (half mile) | 40 | 0.5 |
| Section side (one mile) | 80 | 1 |
| Township side (six miles) | 480 | 6 |
The chain was built into the United States itself. The Land Ordinance of 1785 created the Public Land Survey System, which divided most of the country west of the original colonies into townships 6 miles square and sections of one square mile. Surveyors laid out those lines with Gunter’s chain, so a section is 80 chains on each side and a township is 480 chains, or 6 miles. That grid is why so many Midwestern roads run in straight lines exactly a mile apart.
The chain also lives on in transport. Britain’s railways still mark their lines in miles and chains, so a signal might sit at “112 miles 40 chains” from London, 0.5 miles past milepost 112. In Australia and New Zealand, many rural roads were surveyed as “one-chain roads”, with a reserve 66 feet (20.12 m) wide between property lines.
Measuring with a chain was a two-person job with its own routine. The lead chainman carried a set of marking pins, often ten or eleven, and pushed one into the ground at the end of each chain length; the rear chainman collected them, so the pins in his hand counted the chains covered. On slopes the chain had to be held level and stepped down the hill, a technique called “breaking chain”, because a chain laid along the ground would overstate the horizontal distance.
Steel measuring tapes replaced the iron chain around the end of the nineteenth century, and electronic distance meters and GPS have since replaced the tapes. The unit outlived the tool: surveys, plats and title records still use chains, and converting them to feet or miles is a routine step in property research.
How Many Chains in a Mile?
There are exactly 80 chains in a mile, so you multiply miles by 80 to get chains. Half a mile is 40 chains, a quarter mile is 20 chains, and a kilometer is 49.71 chains.