Measuring Wind Speed in Knots

Yacht sailing in the Southern Ocean. Australia.
John White Photos / Getty Images

In meteorology (and in sea and air navigation as well), a knot is a unit of speed typically used to indicate wind speed. Mathematically, one knot is equal to about 1.15 statute miles. The abbreviation for a knot is "kt" or "kts" if plural.

Why "Knot" Miles per Hour? 

As a general rule in the US, wind speeds over land are expressed in miles per hour, while those over water are expressed in knots (largely because knots were invented over a water surface). Since meteorologists deal with winds over both surfaces, they adopted knots for the for the sake of consistency.

However, when passing along wind information to public forecasts, knots are typically converted into miles per hour for the public's ease of understanding.  

Why Is Speed at Sea Measured in Knots?

The reason why sea winds are measured in knots at all has to do with maritime tradition. In centuries past, sailors didn't have GPS or even speedometers to know how fast they were traveling across the open sea. So to estimate their vessel's speed, they crafted a tool made up of a rope several nautical miles in length with knots tied at intervals along it and a piece of wood tied at one end. As the ship sailed along, the wood end of the rope was dropped into the ocean and remained roughly in place as the ship sailed away. The number of knots was counted as they slipped off of the ship out to sea over 30 seconds (timed using a glass timer). By counting the number of knots that unspooled within that 30-second period, the ship's speed could be estimated. 

This not only tells us where the term "knot" comes from but also how the knot relates to a nautical mile: it turned out that the distance between each rope knot equaled one nautical mile. (This is why 1 knot is equal to 1 nautical mile per hour, today.) 

Unit of Measure
Surface winds mph
Tornadoes mph
Hurricanes kts (mph in public forecasts)
Station Plots (on weather maps) kts
Marine forecasts kts
Units of Wind for Various Weather Events & Forecast Products

Converting Knots to MPH

Because being able to convert knots to miles per hour (and vice versa) is a must. When converting between the two, keep in mind that a knot will look like a lower numerical wind speed than a mile per hour. (One trick to remembering this is to think of the letter "m" in miles per hour as standing for "more.")

Formula to convert knots to mph:
# kts * 1.15 = miles per hour

Formula to convert mph to knots:
# mph * 0.87 = knots

Since the SI unit of speed happens to be meters per second (m/s), it might also be helpful to know how to convert wind speeds to these units.

Formula to convert knots to m/s:
# kts * 0.51 = meters per second

Formula to convert mph to m/s:
# mph * 0.45 = meters per second

If you don't feel like completing the math for the conversion of knots to miles per hour (mph) or kilometers per hour (kph), you can always use a free online wind speed calculator to convert the results.