This table converts fractional 16ths of an inch into decimal inches and centimeters.
inches centimeters 1/16 0.0625 0.15875 1/8 0.125 0.3175 3/16 0.1875 0.47625 1/4 0.25 0.635 5/16 0.3125 0.79375 3/8 0.375 0.9525 7/16 0.4375 1.11125 1/2 0.5 1.27 9/16 0.5625 1.42875 5/8 0.625 1.5875 11/16 0.6875 1.74625 3/4 0.75 1.905 13/16 0.8125 2.06375 7/8 0.875 2.2225 15/16 0.9375 2.38125
Note that the equivalents in centimeters are exact, as one inch is 2.54 centimeters long. In the U.S., before 1964, it was instead the case that a meter was exactly 39.37 inches long, making the inch slightly larger than 2.54 centimeters; and in the British Commonwealth, the inch was slightly smaller than 2.54 centimeters, being defined on the basis of an independent physical standard.
If we're dealing with conversions between the English and metric units of length, a handy conversion chart such as that shown at right may be useful. It also embraces conversions from the height of horses, expressed in hands, and from a cubit of 18 inches in length.
In a cubit, four digits (each 3/4 inch long) make a span (three inches) and six spans make a cubit. In addition to the span being based on the width of a finger, and matching the spaces between the keys on a typewriter or computer keyboard, the cubit is based on the distance between the elbow and the tips of the fingers, as this diagram illustrates:
It also illustrates the existence, in ancient Egypt, of a royal cubit of seven spans in length, which figured in the construction of the Great Pyramid, accounting for the seeming appearance of the mathematical constant pi in its design, when what happened was that its approximation 3 1/7, closer to the one actually in use in Egypt in those days, simply appeared by a coincidence.
As typewriters typically print with six lines to the inch, another group of fractions of an inch is often used. Here, even the decimal inches usually cannot be exact
inches centimeters 1/6 0.1666667 0.4233333 1/3 0.3333333 0.8466667 1/2 0.5 1.27 2/3 0.6666667 1.6933333 5/6 0.8333333 2.1166667
Another interesting pair of units of length are the light-year and the parsec. The speed of light is now exactly 2.99792458 * (10^8) meters per second by definition. With astronomers, however, there are many kinds of year; the tropical, the sidereal, the anomalistic, and the Gregorian calendar year. As it happens, the light-year is, according to Wikipedia, the distance light travels in a Julian year of 365 1/4 days, which is 9.4607304725808 * (10^15) meters.
The astronomical unit is 149,597,870,691 metres, give or take 30 metres. Since the intent of the parsec is to permit one to calculate the distances to celestial bodies from their parallaxes, simply by taking the reciprocal of the parallax, and these bodies are all be at distances greater than 1 parsec, it might actually be more reasonable to use the reciprocal of the angle in radians rather than calculating the cotangent of one second of arc exactly. With the exact cotangent, a parsec is approximately 20264.806245480 astronomical units in length; if one takes the reciprocal of the angle, it would be 206264.806247096 astronomical units in length, for a difference of .000001616 astronomical units.
Based on the cotangent, a parsec is 3.08567758128 * (10^15) metres, give or take about 600,000 metres, and so a parsec is approximately 3.26156377694566 light years, a figure that one does not usually see quoted to this precision.
The topic of units of measurement go in many directions, and so the discussion continues on several other pages.