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Miles per Hour to Knots

Subtract about an eighth for a mental estimate.

Inputs
mph

The number of miles per hour to convert.

Knots
52.14kn
In km/h
96.56km/h
Nautical miles in an hour
52.14NM
10 mph
8.69kn
30 mph
26.07kn
60 mph
52.14kn
100 mph
86.9kn
knots = mph ÷ 1.15078

1 mph = 0.868976 kn

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Divide miles per hour by 1.15078. Sixty mph is 52.1 knots. Taking off roughly an eighth gets you close enough in your head for a forecast or a chart.

How to use MPH to Knots Converter

  1. Enter the speed in mph. From a forecast, a speedometer or a weather report.
  2. Read the knots. With km/h and the nautical distance covered in an hour.

About converting miles per hour to knots

Converting miles per hour into knots comes up more often than the reverse, because forecasts, speedometers and consumer specifications are quoted in mph while anything navigational is in knots. The exact factor is a division by 1.15078, and the mental shortcut is to take off about an eighth — accurate to under one percent, which is well inside the precision of the forecast you are reading. The two units differ because the miles underneath them were defined for unrelated reasons. The statute mile descends from Roman marching paces by way of English statute and has no geographic meaning at all. The nautical mile is one minute of arc of latitude, standardised at exactly 1,852 metres in 1929, and that definition is why it survives: distance on a chart and position in degrees and minutes are the same measurement, so navigation needs no conversion. The result is a fifteen percent gap that has no deeper explanation than the historical accident of two systems being built for different jobs. This is also why metrication stopped at the shoreline. Countries that abandoned miles on land decades ago still use nautical miles and knots at sea and in the air, and aircraft airspeed indicators read in knots almost everywhere. It is one of the few cases where a non-metric unit persists on genuine technical merit rather than habit.

Frequently asked questions

What is the quick mental conversion?
Take off about an eighth. Sixty mph less seven and a half is 52.5, against an exact 52.1 — accurate to under one percent, which is more than enough for reading a forecast.
Why is a nautical mile longer than a statute mile?
Because they were defined for different purposes. The statute mile descends from Roman paces and English statute; the nautical mile is one minute of latitude, fixed at 1,852 metres. There is no reason they would agree, and they differ by about 15%.
When would I need this direction?
Reading a land-based weather forecast in mph while sailing, flying or working from a marine chart, all of which use knots. It also comes up converting a boat manufacturer's advertised speed, which is sometimes quoted in mph for a consumer audience.
Do all countries use knots at sea?
Effectively yes. Knots and nautical miles are near-universal in shipping and aviation regardless of whether a country is otherwise metric, because the link to latitude makes navigation arithmetic work. Metric countries that abandoned miles on land kept them at sea.

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