Your Foot, My Foot

Try this: walk across a room heel to toe, one foot right in front of the other, and count your feet. Now ask an adult to do the same. You will get two different numbers for the same room. Which one is right?

Measuring with your body

In the Why We Measure lesson we saw that measuring means comparing a thing with something both people know. The first things people compared with were their own bodies, because a body is always with you:

a foot - the length of a foot, heel to toe;
a hand span - from the tip of the thumb to the tip of the little finger, with the hand stretched wide;
a cubit - from the elbow to the tip of the middle finger;
a pace - one step.

Four body measures: a foot from heel to toe, a hand span from thumb tip to little finger tip, a cubit from elbow to fingertip, and a pace, one step

Some of these are still used today. People who keep horses still give a horse's height in "hands".

The trouble with bodies

Everybody's body is different. A child's foot is smaller than an adult's, so the same room is more of the child's feet and fewer of the adult's.

The same room measured twice: it is 12 of a child's feet long, but only 9 of an adult's feet

Both counts are honest, but they don't agree. Now imagine buying cloth "by the cubit". A seller with short arms gives you less cloth for the same money than a seller with long arms. For hundreds of years, towns and countries each had their own "foot", all a little different, and traders from different places argued about whose was right.

A busy market in an old European town. A cloth seller stretches a length of striped cloth between his hands while a woman with a basket frowns and holds up her own arm, as if to check his measure. Rolls of colourful cloth are piled on his wooden stall

Picture: an illustration of buying cloth in a medieval market.

Everyone uses the same one

The fix was to stop using each person's own body and agree on one length that everybody uses. In ancient Egypt, builders used the royal cubit: one official length, copied onto rods of wood and stone, so every builder on the same job measured with exactly the same cubit.

A royal cubit rod from ancient Egypt: a straight rod of gilded wood, about 3,400 years old, made in the time of the pharaoh Amenhotep II

Picture: a royal cubit rod in the Museo Egizio, Turin (photo: Museo Egizio, CC0).

An amount that everyone agrees to measure with is called a unit. The official copy that everyone checks their own against is called a standard. Once there is a standard, your 10 cubits and my 10 cubits are the same length.

Try it at home

Measure a table in your hand spans and write down the number.
Ask someone with bigger (or smaller) hands to do the same.
Now cut a piece of string as long as your span, and both of you measure the table with the string. Do you agree now? The string is your standard.

Words to know

cubit - an old unit: the length from the elbow to the tip of the middle finger
unit - an amount everyone agrees to measure with
standard - the official copy of a unit that everyone checks against

Big idea

People first measured with their feet, hands and arms, but bodies differ, so their numbers didn't agree. Agreeing on one unit, with a standard to check against, makes everyone's measurements match.