How do you convert knots to km/h?
Type a speed in knots and the answer appears in kilometres per hour as you type. One knot is exactly 1.852 km/h, because a nautical mile is defined as exactly 1852 metres.
Round numbers are easy. 10 knots is 18.52 km/h, 20 knots is 37, and 100 knots is 185.2. Doubling and halving covers most of what you need.
Why does a separate unit exist?
Not tradition, and not stubbornness. A nautical mile is one minute of latitude, one sixtieth of a degree measured along a meridian.
That makes chart navigation almost arithmetic-free. A degree of latitude is sixty nautical miles, so you read distance straight off the latitude scale at the side of any chart with a pair of dividers. No conversion, no scale factor. And since a knot is one nautical mile per hour, a boat making 6 knots covers exactly 6 minutes of latitude every hour.
No other unit does that. Kilometres are tied to the earth too, but through the quarter-meridian rather than the angle, which isn't what a chart is ruled in.
Speeds in knots
| Vessel or aircraft | Knots | km/h |
|---|---|---|
| Rowing dinghy | 2 to 3 | 4 to 6 |
| Cruising yacht | 5 to 7 | 9 to 13 |
| Racing yacht | 8 to 12 | 15 to 22 |
| Container ship | 18 to 24 | 33 to 44 |
| Cross-channel ferry | 20 to 25 | 37 to 46 |
| RIB or speedboat | 30 to 45 | 56 to 83 |
| America's Cup foiler | 50+ | 93+ |
| Cessna 172 | 120 | 222 |
| Airbus A320 cruise | 450 | 833 |
| Boeing 787 cruise | 490 | 907 |
Wind and the Beaufort scale
Marine forecasts give wind in knots and often in Beaufort, a scale defined by what the sea and the land are doing rather than by an instrument.
| Force | Knots | km/h | At sea |
|---|---|---|---|
| 0 Calm | under 1 | under 2 | Sea like a mirror |
| 3 Gentle breeze | 7 to 10 | 13 to 19 | Large wavelets, scattered whitecaps |
| 4 Moderate | 11 to 16 | 20 to 29 | Small waves, frequent whitecaps |
| 5 Fresh | 17 to 21 | 30 to 39 | Moderate waves, many whitecaps |
| 6 Strong | 22 to 27 | 40 to 50 | Large waves, spray |
| 7 Near gale | 28 to 33 | 51 to 62 | Foam blown in streaks |
| 8 Gale | 34 to 40 | 63 to 74 | Moderately high waves |
| 10 Storm | 48 to 55 | 89 to 102 | Very high waves, sea white |
| 12 Hurricane | 64+ | 118+ | Air filled with foam |
Knots in aviation
Aircraft use knots for the same reason ships do. Air charts are ruled in nautical miles and latitude is the reference. Altitude is almost always in feet though, and vertical speed in feet per minute. It's a mixed system that survives because changing it worldwide would be more dangerous than living with it.
Measuring it yourself
If you want your actual speed in knots rather than a conversion, the boat speedometer opens in knots and measures distance in nautical miles, and the flight ground speed page does the same in the air. Both read speed over ground, which isn't the same as speed through the water or the air. The difference is the current or the wind.
Frequently asked questions
How do I convert knots to km/h?
Multiply by 1.852. So 20 knots is 37.04 km/h. The factor is exact, because a nautical mile is defined as exactly 1852 metres.
What is 10 knots in km/h?
18.52 km/h, or 11.51 mph.
Why do ships and planes use knots?
A nautical mile is one minute of latitude, so you can read distance straight off a chart with no conversion. A knot is one nautical mile per hour, which makes dead reckoning almost arithmetic-free.
Is "knots per hour" correct?
No. A knot already means one nautical mile per hour, so "knots per hour" would be an acceleration. Just say ten knots.
How fast is a knot compared with a mile per hour?
Slightly faster. One knot is 1.151 mph, because a nautical mile is about 15% longer than a statute mile.