Why do we use imperial measurements when measuring chain?

By Oliver McCann

When working with customers to identify leaf chain, something which regularly comes up in conversation is why are chain sizes in imperial (feet and inches) and not nice, neat whole round numbers like metric millimetres?  Let’s take a closer look.

First of all, the contenders: The Metric system originated in France during the 1700’s. It’s based on the metre (length), gram (weight) and litre (volume). There are terms for making things bigger (kilo- , mega- , giga- , tera- ) or smaller (milli- , micro- , nano- ). Everything comes in multiples of 1000 – the metric system is always based on powers of 10. Quite simple really.

Imperial on the other hand takes a bit more understanding as it’s based on the inch / foot / yard / mile (length), the ounce / pound / stone / hundredweight (weight / mass) and the fluid ounce / pint / quart / gallon (volume). There are 12 inches in a foot, 3 feet in a yard, and 1760 yards in a mile – not to mention furlongs (think horse racing) poles and chains. Confused yet?

When in school I went through an education system seemingly oblivious to inches the UK having adopted a decimal outlook along with the monetary system in the early seventies. I would regularly think to myself, will I ever see a fraction again?  So, when I joined FB Chain, I was shocked when part of my initial training was measuring in 1/8th and 1/16th of an inch. So then, why is chain measured in inches? 

The reason is, as ever, historical. During the late 1800s and early 1900s, British companies dominated the chain industry. It was not until the second decade of the 20th century that challengers from elsewhere in Europe started to appear but many of these based their chains on the standards already established by British companies, so dimensions were mainly based on imperial inch.

The largest market for leaf chain was in The United States and their standards committee was one of the first to publish a standard for leaf chain, in fact, BL leaf chains which are now the most popular in the world come from the ANSI standard and the United States customary system of measurement is in inches.

As chain manufacturing spread out around the world chain companies used readily available materials and some dimensions moved while still staying inside the standard. For example: 1/8th (3.175mm) of inch plates become 3.2 or even 3.3 mm.

How to convert inches to millimeters

1 inch is equal to 25.4 millimeters:

1″ = 25.4mm

The distance d in millimeters (mm) is equal to the distance d in inches (″) times 25.4:

d(mm) = d(inch) × 25.4


Convert 20 inches to millimeters:

d(mm) = 20″ × 25.4 = 508mm

How many inches in a millimeter

One millimeter is equal to 0.03937 inches:

1mm = 1mm / 25.4mm/in = 0.03937in

How many millimeters in an inch

One inch is equal to 25.4 millimeters:

1in = 25.4mm/in × 1in = 25.4mm

How to convert 10 inches to millimeters

Multiply 10 inches by 25.4 to get millimeters:

10in = 25.4mm/in × 10in = 254mm

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