21 feet per second in miles per hour

Conversion

21 feet per second is equivalent to 14.3181818181818 miles per hour.^[1]

Conversion in the opposite direction

The inverse of the conversion factor is that 1 mile per hour is equal to 0.0698412698412698 times 21 feet per second.

It can also be expressed as: 21 feet per second is equal to $\frac{1}{0.0698412698412698}$ miles per hour.

Approximation

An approximate numerical result would be: twenty-one feet per second is about fourteen point three two miles per hour, or alternatively, a mile per hour is about zero point zero seven times twenty-one feet per second.

Units involved

This is how the units in this conversion are defined:

Feet Per Second

'The foot per second (plural feet per second) is a unit of both speed (scalar) and velocity (vector quantity, which includes direction). It expresses the distance in feet (ft) traveled or displaced, divided by the time in seconds (s, or sec). The corresponding unit in the International System of Units (SI) is the metre per second.Abbreviations include ft/s, ft/sec and fps, and the rarely used scientific notation ft s−1.'

Wikipedia page of feet per second

Miles Per Hour

"Miles per hour (abbreviated mph, MPH or mi/h) is an imperial and United States customary unit of speed expressing the number of statute miles covered in one hour. Although kilometres per hour is now the most widely used measure of speed, miles per hour remains the standard unit for speed limits in the United States, the United Kingdom, Antigua & Barbuda and Puerto Rico, although the latter two use kilometres for long distances."

Wikipedia page of miles per hour

Footnotes

[1] The precision is 15 significant digits (fourteen digits to the right of the decimal point).

Results may contain small errors due to the use of floating point arithmetic.