Miles to nautical miles (mi to nmi)

One statute mile is 0.8689762419 nautical miles. Both ends are exact: the international nautical mile has been exactly 1,852 meters since the International Hydrographic Conference of 1929, and the mile exactly 1,609.344 meters since 1959. The ratio of two exact numbers is exact; it does not come out round.

Worked through: 26.2 mi x 0.868976242 = 22.7672 nmi.

Going the other way, one nautical mile is 1.150779448 miles.

For mental arithmetic: Multiply by 0.87: take off 13%.

The nautical mile is not an arbitrary longer mile. It was originally one minute of arc of latitude, a sixtieth of a degree along a meridian, which is why sea and air navigation use it: a distance on a chart can be read straight off the latitude scale at the side. The Earth is not a perfect sphere, so a minute of latitude varies slightly between the equator and the poles, and the 1,852 meter definition fixed one figure for everyone. Converting a land distance to nautical miles makes it about 13% smaller.

Precision is worth matching to the input. A measurement given as 1 mi carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.