If someone asks you to give your height, if you live in the United States, you most likely prepare yourself to give two separate but related numbers, such as 5 and 10, without even thinking about it. If you live outside the U.S., the odds are that you'll return precisely the same information by giving a single number, like 1.78 or 178.

This difference reflects the different ways height and other things are measured around the world. A combination of feet and inches has been the standard in the U.S. forever, whereas in Europe and elsewhere, the meter and its derivatives are the default units of height. It's fairly easy to work between these units, even if you have to do it in your head in a pinch.

A foot was originally selected as a standard unit of measure because the length of a man's foot seemed as convenient as anything. The inch, at first equated to the width of a man's thumb but later changed to the width of three adjoining pieces of barleycorn. The fact that a foot contains 12 inches makes converting between these inconvenient.

Other imperial measurement units for length, such as hands, rods, fathoms and furlongs, have fallen almost entirely out of use except in limited contexts.

The metric system, today also called the SI system or international system of units, was implemented across Europe in the late 1700s. The meter and the gram (or kilogram) were picked to be the fundamental units of length and mass, with the gram arising from the meter and being the mass of (0.01 m × 0.01 m × 0.01 m)³ = 1 cm³ of water at its maximum density.

The meter was defined at the outset as 1/10,000,000th the distance between the equator and the North Pole. Although modern land surveying equipment has resulted in a slight modification of the original definition, the length of a meter remains unchanged and is equal to 3.2808 feet or 37.37 inches.

You can immediately see the advantages of a system that uses powers of 10 instead of seemingly random equivalents such as "3 feet per yard" and "12 inches per foot." Thus means you have to be careful when reading and converting. 5.3 feet in meters is (5.3)/(3.28) = 1.615 m, but 5 ft 3 in, which is actually 5.25 feet, is 1.600 m.

Since height is normally given in both feet and inches when these units are in play rather than using a decimal, the easiest way to convert height from feet to meters is to convert height entirely to inches and then divide by 39.37 to get meters. For example, 5 ft 10 in is 70", and 70/39.37 = 1.778 m.

If you know your height in meters, you can give your height in any other metric length unit just by moving the decimal point in your answer. For example, because there are 100 centimeters (cm) in a meter, a height of 1.743 m is the same as. 174.3 cm. You can see that centimeters are more convenient to objects roughly as long as humans as meters are.

If you are in a hurry and need to convert a height from between feet to meters or from meters to feet, you can start with the knowledge that an inch is about 2.5 centimeters and that 10 cm is about the same 4 in. (The true conversion is 2.54 cm = 1 in.) This tells you that, for example, 44 inches, because it holds 11 units of 4 inches, must be about 11 times 10 cm = 110 cm.

See the Resources for an example of a height growth chart for children that allows you to see where your son or daughter stands in percentile terms for his or her age, as well as a handy foot to meters and meters to feet online tool.