### Centimeters

**Abbreviation/Symbol:**

- cm

**Unit of:**

- Length / distance

**Worldwide use:**

- The centimetre is used worldwide as a measurement of length. There are a few exceptions, most notably the United States, which still primarily uses the U.S. Customary (similar to imperial) system.

**Definition:**

The centimetre is a unit of length in the metric system, equal to one-hundredth of a metre.

1cm is equivalent to 0.39370 inches.

**Origin:**

The metric, or decimal, system of weights and measures was defined and adopted in France in 1795. Using the metre as the basis for length measurements, the system is now used officially across the globe.

John Wilkins was the first person to publicly recognise the need for a universal length measurement, ie a measurement that could be carried out locally to avoid having to have a definition that refereed to a stored reference unit. He proposed using the length of a “seconds pendulum”. - a pendulum that made half an oscillation per second – as he believed this would have a fixed length. It was later found there is a slight difference in this length around the world. Because of this, the Academy of Sciences devised a new measure, 1/10000000 of the distance from the Equator to the North Pole (measured through Paris). This new value was used to create a metal bar stored at the BIPM’s headquarters known as the “international prototype meter”. This standard has since been superseded by more accurate measures –first by the 11th conference of weights and measures, who defined the meter as “1,650763.73 wavelengths of the orange-red emission line of a krypton-86 atom in a vacuum”. Most recently, in 1983, the 17th conference defined the meter as “the distance travelled by light in a vacuum in 1/299792458 of a second”. From this we derive the centimetre as the distance travelled by light in a vacuum in 1/2997924580 of a second.

Defined at the 17th Conférence Générale des Poids et Mesures (CGPM)

**Common references:**

- A United States nickel (5 cent) coin is approximately 2cm diameter.
- The cornea of a human eye is approximately 1.15cm (11.5mm) diameter.
- One imperial foot is equal to approximately 30.5cm.
- 1 inch, 2.54 cm
- 1 metre, 100 cm
- 30 cm is approximately equal to 1 foot (actually 11.81 inches)
- A standard number 2 pencil is 19cm long when new
- An average mug from Ikea is about 10cm tall
- Theaveragebody.com reports that the palm of an average male is 8.4cm wide and an average female hand is 7.4cm wide

**Usage context:**

The centimetre is used as a day to day unit of measurement in countries that have adopted the International (SI) System of Units, in situations and applications where fractions of a centimetre are generally deemed unimportant to state.

Applications that require greater accuracy, such as engineering and design, will generally state distances using either millimetres or decimal fractions of a metre.

**Component units:**

- 1 cm = 10mm (millimetres)

**Multiples:**