When I was 11 years old, my Girl Guides leader taught us how to estimate length and distance. The knowledge I acquired that day has served me well my whole life.
We used a tape measure to find the average length of one pace so that we could estimate distances by pacing them off. Using a ruler, we discovered that our thumb was approximately one inch wide. The width of my hand was approximately three inches. The span of my outstretched fingers, from the tip of the thumb to the tip of the little finger, was approximately seven inches. These approximations of distance are collectively called “rule of thumb”.
Using the body as a measuring tool is nothing new, of course. The measurement we use today called “foot” is 12 inches long and was actually the length of the foot of King Henry 1 (1068-1135) of England. He established the inch as the length of three grains of barley end to end or the width of a man’s thumb. The length between a man’s outstretched arms was called a fathom. He set the yard as the distance from his nose to the thumb of his outstretched arm. Today, a yard is 36 inches.
“Rule of thumb” originates from the centuries-old practice of using the human thumb for rough measurements. The phrase is associated with tradesmen and first appeared in print in 1658 in a sermon by James Durham referring to “foolish builders” working by guess and “rule of thumb” and not by “square and rule house”.
Historically, the width of a thumb was used as a standard for an inch and the span for longer, less precise measurements.The thumb was used to measure cloth, wood, or other materials. Cooks and brews used fingers and thumbs to gauge quantities.
We still use the body to provide rough estimates. You’ve likely seen someone hold the edge of fabric to their nose and pull it as wide as their arm will stretch to the side to estimate one yard of fabric. It’s not an exact measurement, but it’s a good way to determine if there is enough fabric for the use intended.
In today’s digital world, we have many sophisticated methods of measuring distances, both long and short. However, even though the concept is centuries, old, you probably use “rule of thumb” daily for quick, practical estimations.
