Miles per Hour to Knots Converter | mph to knot

What this converter does

Speeds on land come in miles per hour, but the moment you step onto a boat or into a cockpit, the unit changes to knots. This converter bridges the two: enter a speed in mph and it gives you knots. It is the tool you want when a figure you understand in everyday road terms needs to be read in the language of the water and the air.

What a knot actually is

A knot is one nautical mile per hour. The nautical mile is not the same as the mile on a road sign; it is tied to the size of the Earth, set so that one nautical mile equals one minute of latitude. That link to the globe is exactly why sailors and pilots use it, because it makes distance on a chart line up neatly with the latitude and longitude grid they navigate by. So a knot is a speed built for navigation, which is why it sits a little apart from the mph you are used to.

The conversion factor

One mile per hour is 0.868976 knots, so:

knots = mph × 0.868976

Because a nautical mile is a bit longer than a land mile, it takes fewer of them to cover the same ground in an hour, so the knot figure always comes out a little smaller than the mph you started with.

A quick estimate

The factor is close to 0.87, so for a rough figure, knock about an eighth off the mph value. Take 40 mph, an eighth of that is 5, so roughly 35 knots, and the exact answer is 34.8 knots. That is more than close enough to read a wind speed or a boat speed at a glance, and it is easy to remember that knots run a touch lower than the mph they came from.

A worked example

Say a weather report gives a wind gust of 45 mph and you want it in knots to match a marine forecast.

• 45 × 0.868976 = 39.1 knots

So a 45 mph gust is about 39 knots. On the water that is a meaningful number, the sort of wind that turns a pleasant sail into a serious one, and seeing it in knots lets you line it up directly against the forecasts and limits that boats work to.

Common mph speeds in knots

As a feel for it, 10 mph is a little under 9 knots, and a strong 60 mph wind is about 52 knots.

Who converts mph to knots

Boaters and pilots are the main ones, especially anyone newer to the water who still thinks in road speeds and wants to translate them into the knots on their instruments and charts. It is common with weather too: land forecasts often give wind in mph, while marine and aviation forecasts use knots, so converting lets you carry a single number between them. Anyone planning a trip on the water or in the air, sizing up a wind, a current, or a cruising speed, ends up making this jump from the familiar mph to the knots the activity runs on.

Questions people ask

How do I convert mph to knots?

Multiply the speed in mph by 0.868976. For a quick estimate, take about an eighth off the mph figure, since a knot is a little larger than a mile per hour.

Why are knots different from mph?

A knot is one nautical mile per hour, and a nautical mile is based on the Earth's latitude rather than a land measurement. That makes it slightly longer than a statute mile, so the knot figure is a bit lower.

Why is the knots number smaller than the mph?

Because a nautical mile is longer than a land mile, fewer of them fit into the same distance per hour, so the same speed reads as a smaller number in knots.

References

1. National Institute of Standards and Technology (NIST), Special Publication 811, Guide for the Use of the International System of Units (unit conversion factors). https://www.nist.gov/pml/special-publication-811
2. National Oceanic and Atmospheric Administration (NOAA), National Ocean Service, What is the difference between a nautical mile and a knot? https://oceanservice.noaa.gov/facts/nautical-mile-knot.html