Knots to mph: what a knot is and how to convert it

How many mph is a knot?

A knot is a speed: the speed of something that covers one nautical mile in one hour. A nautical mile is exactly 1,852 meters, about 6,076 feet, so one knot is about 1.151 mph and exactly 1.852 km/h.

These are the equivalents you’ll use most:

Unit	One knot equals	In knots
mph (statute miles per hour)	1.151 mph	1 mph = 0.869 knots
km/h	1.852 km/h (exact)	1 km/h = 0.540 knots
m/s	0.514 m/s	1 m/s = 1.944 knots

The formulas are simple:

• mph = knots × 1.151
• knots = mph × 0.869 (or mph ÷ 1.151)
• km/h = knots × 1.852

A few examples: someone walking at 3 mph is doing 2.6 knots; a sailboat at 6 knots is going 6.9 mph; a 25-knot wind is 28.8 mph; and a 2-knot tidal current moves the water at 2.3 mph, faster than most people can swim, as we explain in tidal currents. Going the other way, a 15-mph breeze on a land forecast is 13 knots, force 4 on the Beaufort scale.

Knots to mph chart, 1 to 60

The last column is the Beaufort scale force for an average wind of that speed.

Knots	mph	km/h	m/s	Beaufort
1	1.2	1.9	0.5	1
2	2.3	3.7	1.0	1
3	3.5	5.6	1.5	1
4	4.6	7.4	2.1	2
5	5.8	9.3	2.6	2
6	6.9	11.1	3.1	2
7	8.1	13.0	3.6	3
8	9.2	14.8	4.1	3
10	11.5	18.5	5.1	3
12	13.8	22.2	6.2	4
15	17.3	27.8	7.7	4
17	19.6	31.5	8.7	5
20	23.0	37.0	10.3	5
22	25.3	40.7	11.3	6
25	28.8	46.3	12.9	6
28	32.2	51.9	14.4	7
30	34.5	55.6	15.4	7
34	39.1	63.0	17.5	8
35	40.3	64.8	18.0	8
40	46.0	74.1	20.6	8
45	51.8	83.3	23.2	9
50	57.5	92.6	25.7	10
55	63.3	101.9	28.3	10
60	69.0	111.1	30.9	11

Mental math shortcuts

On a boat or a beach you don’t always have a calculator. These shortcuts get you very close to the exact answer:

• Knots to mph: add 15%. 20 knots: 20 plus 3 is 23 mph (exact: 23.0). 40 knots: 40 plus 6 is 46 mph (exact: 46.0).
• Mph to knots: subtract 13%. 30 mph: 30 minus 4 is 26 knots (exact: 26.1).
• Knots to m/s: halve it. 20 knots is about 10 m/s (exact: 10.3). The other way, double the m/s to get knots. That helps, because many weather models and stations report wind in m/s.
• Knots to km/h: double it, then take off 7%. 15 knots: 30 minus 2 is 28 km/h (exact: 27.8). Handy for European forecasts.

If you just want to know whether it’s blowing hard, treat knots and mph as roughly the same and add a bit: the mph figure is always the bigger one.

Common conversion mistakes

• Mixing up knots and mph. In the US, land forecasts give wind in mph and marine forecasts give it in knots. 20 mph is 17.4 knots, not 20.
• Reading m/s as if it were mph. 10 m/s is 22.4 mph, nearly 20 knots and force 5. Read it as 10 mph and you’re more than twice too low.
• Mixing up miles. On a nautical chart, miles are always nautical miles. A trip of 100 nautical miles is 115 statute miles, which matters when you plan time and fuel.

Knots and the Beaufort scale

The Beaufort scale, used in marine forecasts around the world, is defined in knots: each force is a range of average wind speeds. Here are the ones worth remembering:

• Force 4 (moderate breeze): from 11 knots, about 13 mph. Whitecaps everywhere.
• Force 6 (strong breeze): from 22 knots, about 25 mph. Large waves with foam crests.
• Force 8 (gale): from 34 knots, about 39 mph. This is where an NWS gale warning starts.
• Force 10 (storm): from 48 knots, about 55 mph. NWS storm warning territory.
• Force 12 (hurricane force): from 64 knots, about 74 mph.

That 64-knot line is also where a tropical storm becomes a hurricane, which is why hurricanes start at 74 mph. A hurricane with 100-knot winds is blowing at about 115 mph: add 15% and you’re there.

A trick for the middle of the scale: from force 4 to force 8, force times 5, minus 5 gives a speed in knots inside that force’s range. Force 6: 30 minus 5 is 25 knots, which falls between 22 and 27. The full table, with the sea state and wave height for each force, is in our guide to the Beaufort scale.

In the nautical converter, type a speed in knots and you’ll see it in mph, km/h and m/s, with its Beaufort force and sea state. On each place’s wind page, like the wind in San Francisco or the wind at Key West, the wind is shown in mph, and the current wind is given in knots too. If you’d rather see everything in knots, tap the gear icon in the header and pick knots under Wind in the Units panel.

The nautical mile: one minute of latitude

The knot makes more sense once you know its mile. Nautical charts are drawn with latitude and longitude, and each degree of latitude has 60 minutes. The nautical mile was meant to be one minute of latitude, so you can measure distance right on the chart with the latitude scale along its side: one minute, one mile.

The catch is that the Earth isn’t a perfect sphere, so a minute of latitude isn’t the same length everywhere: it’s a little shorter at the equator than near the poles. To get a fixed value, what is now the International Hydrographic Organization set the international nautical mile at exactly 1,852 meters in 1929. The US adopted it in 1954 and the UK in 1970.

One warning: always measure distance on the latitude scale, on the sides of the chart, never on the longitude scale along the top and bottom. Meridians converge toward the poles, so a minute of longitude is only a nautical mile at the equator. At New York’s latitude, about 41 degrees north, it’s roughly 0.76 nautical miles.

Two fun facts that follow:

• A trip around the Earth through the poles is 360 degrees times 60 minutes: 21,600 nautical miles, about 24,900 statute miles.
• A nautical mile is not the statute mile you see on road signs, which is 5,280 feet. The nautical one is about 15% longer.

Why sailors and pilots use knots

If distances are measured in nautical miles, it’s natural to measure speed in nautical miles per hour. Navigation math gets easy: 30 miles at 6 knots takes 5 hours, and at 10 knots a boat covers 10 minutes of latitude every hour.

The National Weather Service glossary puts it plainly: the knot is the unit of speed used in navigation, one nautical mile per hour. That’s why it’s the speed unit of the sea worldwide:

• Boats: boat speed and current speed are both given in knots.
• Marine forecasts: NWS coastal forecasts give wind in knots and seas in feet, and other countries use knots or Beaufort forces, which are defined in knots.
• Aviation: distances are measured in nautical miles too, and US airport weather reports (METARs) always give wind in knots, though other countries may use km/h or m/s. “27020G35KT” means wind from the west (270 degrees) at 20 knots, gusting to 35.

On land, though, US forecasts use mph, so it pays to switch between the two easily.

Where the name comes from: the chip log

In the 17th century, sailors measured a ship’s speed with a chip log: a small wooden board, weighted so it floated upright, tied to a line wound on a reel. The line had knots tied in it at regular intervals.

One sailor tossed the board over the stern, where it stayed almost still in the water while the ship moved on and the line ran out. Another turned a sandglass, and when the sand ran out they counted the knots that had gone over the side. The knots were spaced so that the count was the ship’s speed in nautical miles per hour.

The math is simple. If the sandglass ran 30 seconds, one 120th of an hour, the knots had to be one 120th of a nautical mile apart, about 50.6 feet. With a 28-second glass, the spacing drops to about 47 feet. Measurements varied by era and navy, but the idea was always the same.

Today boats measure speed with electronic logs and GPS, but the unit kept its name. And since a knot already includes the hour, “knots per hour” is wrong: a boat is simply doing 12 knots.

Speed through the water, speed over ground and apparent wind

At sea there are two boat speeds, and both are given in knots. Speed through the water is what the log measures: how fast you move relative to the water around you. Speed over ground is what GPS shows: how fast you move relative to the bottom.

With no current, they match. With a current, they add or subtract. A sailboat making 5 knots through the water with a 2-knot tidal current behind it is doing 7 knots over ground; against the current, 3 knots. A 15-mile trip can take just over 2 hours or 5 hours depending on the tide, which is why it pays to plan around it, as we explain in fishing the tides.

Wind works in a similar way. On board you feel the apparent wind, which combines the true wind with the wind your own motion creates: motoring at 6 knots straight into a 15-knot wind, you feel about 21 knots on deck. Forecasts, weather stations and wind pages always give the true wind, the one you’d feel standing still.