21 knots in miles per hour
Conversion
21 knots is equivalent to 24.1663684084944 miles per hour.[1]

Conversion in the opposite direction
The inverse of the conversion factor is that 1 mile per hour is equal to 0.041379821042888 times 21 knots.
It can also be expressed as: 21 knots is equal to miles per hour.
Approximation
An approximate numerical result would be: twenty-one knots is about twenty-four point one seven miles per hour, or alternatively, a mile per hour is about zero point zero four times twenty-one knots.
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.