Length is a measure of how long an object is from one end to the other end.

There are many ways of measuring length. Many years ago, people used parts of the human body to measure things, such as digits, palms, palm span, foot, cubit (the length from elbow to the tip of the middle finger), yard (length from the chin to the tip of the middle finger), fathom (length between the two arms spread out), foot, and pace (distance between the two feet when spread out for walking), etc.

They even formed some measurement norms like 4 digits = 1 palm, 3 palms = 1 span, 2 spans = 1 cubit, 2 cubits = 1 yard, and 2 yards = 1 fathom.

These units were called natural units, but these measurements were not reliable as the measurement varied from person to person.

Over time, the measurement systems became more standardised to two types or systems:

## Imperial measurements

These measure the length in inches, feet, yards, and miles. They are considered slightly older type of measurement, although some countries use this quite extensively (USA, etc.). The disadvantage with this system is the relative difficulty in converting the numbers from one measurement to the other, especially doing it mentally. For example, 1 mile = 1760 yards, how much would 5 miles be in yards?

## SI units of measurements

Now the standard type of measurements for length is metres using the SI unit system. The main advantage of the SI units of measurement is the conversion factor in in power of 10.

Now we measure with standard measurement using measuring instruments in metres. There are different measuring instruments to find the length of an object. They are a ruler, 1-metre ruler, builder’s tape, tape measure, trundle wheel and an odometer.

A **ruler** is used for shorter straight lengths, and it is generally marked in millimetres (mm) or centimetres (cm).

A **builder’s tape** is about 10-12 metres long and is used for measuring longer straight lengths. It can be marked in mm, cm or in metres (m).

A dressmaker’s **tape measure** is used for measuring material in straight lines as well as body parts that are not in straight lines. It is marked in mm, cm or occasionally in metres.

A **trundle wheel** is used to measure long distances (lengths) like a sporting field or playground. The wheel when rotated once covers one metre and makes a ‘click’ noise. It counts the distance in metres, and can also be used to get a finer measurement in centimetres, by finding the number of centimetres covered before the wheel ‘clicks’ to indicate a metre.

An **odometer** is used in vehicles, and measures very long distances. Typically you would use an odometer to find the distance between two cities, and the unit of measurement is in kilometres.

Reading various instruments is an important part of our everyday lives.

Learn here about rounding off the measurements.