Units of length

Length measurement

This article will review the units of length used by the UK imperial, US customary and metric system. Furthermore we will discuss the conversion between these length measurements.

The imperial and US customary system divides the length measurement into three classes, namely:

  • Linear
  • Nautical
  • Surveyor length

The conversion in these unit systems can become quit complicated, as we will find out later. The metric system in contrast uses only scales of 10 the convert between the units. This makes unit conversion less comprehensive.  However, the imperial system approaches the real length of the used goods better.

Units of linear length

The linear length units used in the United Kingdom include in increasing order of length: line, inch, feet (foot), yard, statute mile (land mile) and statute league (land league). The people in the United States, however,  only uses inch, feet (foot), yard and statute league (land league).

An inch consists of 12 lines (line).

Furthermore, a feet (ft) consists of 12 inch.

And yard (yd) is the equivalent of 3 feet.

The land mile (st. mi) consists of 1760 yard.

And finally the land league (st. lg) is the equivalent of 3 land miles.

So how can we convert to the metric system and back? The Weight and Measurement act of 1963 (WMA 1963) defined the feet as an equal length of 0.3048 meter of the metric system.  Therefore unit conversion becomes a possibility. See table 1 for internal en SI conversion factors.

A yard consist of 3 feet, as stated before. However, of how many feet does a yard consists? Also, a feet consists of 12 inches. So a yard consists of 12 x 3 = 36 inches.

Imperial UK/US linear units of length

Table 1 Imperial UK/US linear units of length

A line consists of 12 points (pt). This means that the point is the equivalent of 0.000176 m (0.176 mm). Quite pointless….

Nautical units of length

In the navigation on sea the fathom and nautical mile units of length are used by both the UK and US. Other nautical length measurements are not used anymore and will not be reviewed in this article.

A nautical mile (naut. mi) equals 1853.184 meters of the SI system.

Furthermore, a nautical mile consists of about 3040/3 (or about 1013)  fathom (fath) units.

Imperial UK US nautical units of length

Table 2 Imperial UK/US nautical units of length

Surveyors units of length

The surveyors units of length used by people in the United States include: link, chain, rod and furlong. People in the United Kingdom, however, only used the furlong (horse races).

We can define 1 Ramsden’s chain as 30.48 meter in the SI system.

Furthermore we know that a rod (rd) consists of 16.5 link (lk).

Also, the chain (ch) consists of 100 links.

And finally a furlong (fur) is the equivalent of  6.6 chain.

Imperial UK/US surveyors units of length

Table 3 Imperial UK/US surveyors units of length

Units of length: metric system

The metric system is based on the meter (m) and scales up and down with a factor of 10. The most used units are millimeter (mm), centimeter (cm), decimeter (dm), meter (m) and kilometer (km). Although some specific units are used in specialized fields.

Metric units of length

Table 4 Metric units of length

Share your experience

I hope you learned something out of this article. Let me know in the comment section below!


@ Engineerings fundamentals

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