Measuring Wind Speed in Knots

In both meteorology and sea and air navigation, a knot is a unit 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 U.S, wind speeds over land are expressed in miles per hour, while those over water are expressed in knots. This is largely because knots were invented over a water surface, as explained below. Since meteorologists deal with winds over both surfaces, they adopted knots 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?

Sea winds are measured in knots simply because of maritime tradition. In centuries past, sailors didn't have GPS or even speedometers to know how fast they were traveling across the open sea. To estimate their vessel's speed, they crafted a tool made up of a rope several nautical miles long 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. As the knots slipped off of the ship out to sea, the number of them was counted over 30 seconds (timed using a glass timer). The number of knots that unspooled within that 30-second period indicated an estimation of the ship's speed.

This tells us not only 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.

Converting Knots to Miles Per Hour

Being able to convert knots to miles per hour (and vice versa) is an important skill in both meteorology and navigation. 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 miles per hour:
# kts * 1.15 = miles per hour

Formula to convert miles per hour to knots:
# mph * 0.87 = knots

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

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

Formula to convert miles per hour 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.