Aviation runs on a mixed bag of units that no physics textbook would choose: speed in knots, distance in nautical miles, height in feet, pressure in hectopascals or inches of mercury. None of it is arbitrary — each unit survives because it was convenient for someone navigating or flying — but a candidate who has only ever worked in metres and kilometres per hour will lose time converting under pressure, and the tests are timed. This lesson gives you the factors, the reasoning behind them, and the mental shortcuts that make them quick.
The nautical mile, and why it exists
A nautical mile is not a rounded-off version of anything. It is defined as one minute of arc of latitude — one sixtieth of a degree — measured along the Earth's surface. That makes it enormously convenient for navigation: a degree of latitude is sixty nautical miles anywhere on Earth, so a chart's latitude scale doubles as a distance scale, and a position given in degrees and minutes converts to distance without a calculation. Its modern definition is fixed at exactly 1,852 metres, which is close to but not identical with the historical measurement. In round terms it is about 1.85 kilometres, or roughly fifteen per cent longer than a statute mile.
The unit that follows from it is the knot: one nautical mile per hour. The name comes from the knotted line once paid out behind a ship to measure its speed, and the important thing to notice is that a knot already contains "per hour" — so "knots per hour" is a mistake, in the same way that "metres per second per second" means something different from "metres per second". If you are given a speed in knots and a time in hours, the distance in nautical miles falls straight out.
Distance in nautical miles
- in knots (kt), which already means nautical miles per hour
- in HOURS — convert minutes first, or the answer is out by a factor of 60
This is the same speed–distance–time relationship from the previous lesson wearing different clothes, and it rearranges the same way: divide distance by speed for the time, divide distance by time for the speed. The only new discipline is the time unit. Thirty minutes is half an hour, twenty minutes is a third, fifteen is a quarter — get into the habit of converting to a fraction of an hour before anything else, because leaving minutes in the sum is the single most common error in this whole family of questions.
If you think in km/h or mph
Most people arrive with an instinct calibrated in kilometres per hour or miles per hour, and the quickest way to make knots usable is to tie them to whichever of those you already feel. Since a nautical mile is 1.852 kilometres and a knot is one nautical mile per hour, the conversion is that same factor applied to speed rather than distance.
Knots to kilometres per hour
- kilometres in one nautical mile — the same factor, since a knot is one NM per hour
Before using it, settle the direction, because getting it backwards is the error that actually happens. A nautical mile is longer than a kilometre, so covering a given distance takes fewer nautical miles than kilometres — which means that for one physical speed, the number in knots is smaller than the number in km/h. An aircraft at 100 knots is doing 185 km/h, not 54. If a conversion ever hands you a knot figure larger than the km/h figure, it went the wrong way.
For mental work, treat "knots to km/h" as double it, then take off about eight per cent. Doubling 100 gives 200, and eight per cent of that is 16, leaving 184 — against a true 185.2. Going the other way, halve the km/h figure and add about eight per cent: half of 185 is 92.5, plus 8 per cent is 100. Neither is exact and neither needs to be; they are there to tell you instantly whether an answer is plausible.
Miles per hour sits between the two. One knot is about 1.15 mph, so a knot figure and an mph figure are within fifteen per cent of each other — close enough that they look interchangeable and far enough apart to make a convincing wrong option. Going from knots to mph, add roughly fifteen per cent; going from mph to knots, take off roughly thirteen. Three anchors will cover most sanity checks: 100 knots is 185 km/h or 115 mph, a light-aircraft cruise of 120 knots is around 220 km/h, and a jet at 500 knots is about 926 km/h, which is why airliner speeds are quoted as roughly 900 km/h in the press.
Feet, and where metres survive
Altitude is quoted in feet almost everywhere in the world, and one foot is exactly 0.3048 metres. Rather than carry that number, it is easier to remember that a metre is about 3.3 feet, so converting feet to metres means dividing by roughly three and a bit, and converting metres to feet means multiplying by it. A thousand feet is about 300 metres; ten thousand feet is about three kilometres; a cruising altitude of 36,000 feet is about eleven kilometres. Those three anchors will let you sanity-check almost any altitude conversion you meet.
Metres have not disappeared entirely. A handful of countries — China and, historically, Russia and some neighbouring states — use metres for flight levels, and runway dimensions are given in metres in many places even where altitude is in feet. Visibility is usually in metres for short distances and kilometres beyond that, while some countries use statute miles. The practical lesson is not to memorise which country does what, but to read the unit on the page rather than assuming it, because the same number means very different things in the two systems.
Pressure: hectopascals and inches of mercury
The SI unit of pressure is the pascal, one newton per square metre, and it is far too small to be convenient for atmospheric pressure — sea-level pressure is about 101,325 of them. So aviation uses the hectopascal, where "hecto" means a hundred, making one hectopascal a hundred pascals. Standard sea-level pressure is 1013.25 hPa, universally rounded to 1013. You may also meet the millibar, which is numerically identical to the hectopascal: 1013 mb and 1013 hPa are the same pressure, and the change of name was a tidying-up exercise rather than a change of unit.
North America instead quotes pressure in inches of mercury, written inHg, which measures how far the pressure will push mercury up a tube. Standard sea-level pressure is 29.92 inHg. The conversion factor is that one inch of mercury is about 33.9 hectopascals, so dividing hectopascals by roughly 34 gives inches. Because the two scales differ by more than a factor of thirty, mixing them up produces an answer so far out that it should be obvious — a pressure of 29.92 hPa would be near-vacuum, and 1013 inHg is not physically meaningful.
Temperature
Aviation uses degrees Celsius, which is one fewer thing to convert than most fields require. Fahrenheit still appears in North American weather reporting, and the conversion is worth having written down rather than described.
Celsius to Fahrenheit
- temperature in degrees Celsius (°C)
- the same temperature in degrees Fahrenheit (°F)
- 1.8 — a Fahrenheit degree is smaller, so it takes more of them per Celsius degree
Rearranged the other way, subtract 32 first and then multiply by five ninths — the order matters, and doing the multiplication before the subtraction is the usual mistake. Two anchors make the whole scale checkable without any arithmetic: 0 °C is 32 °F, and −40 is the same number on both scales, which is the one temperature where the two agree. A separate scale, the kelvin, matters for the gas laws and is covered in the heat module; for now it is enough to know it exists and is not used for reporting weather.
Fahrenheit to Celsius
- subtract 32 FIRST — the brackets are not decoration
- about 0.56 — the reciprocal of the factor above
Three worked conversions
Shortcuts worth carrying
A few rough equivalences will save you more time than any exact factor. Alongside the km/h and mph conversions above, a knot is about half a metre per second, so halving a speed in knots gives a usable figure in metres per second — 100 knots is close to 51 m/s, which is the form you need whenever a knots figure has to go into a physics formula. A nautical mile is about 1.85 kilometres, so doubling and taking a little off works in the other direction. A metre is about three and a third feet. Near sea level, one hectopascal of pressure change corresponds to roughly 27 feet of altitude, which is why a mis-set altimeter matters. None of these are exact, and none need to be: their job is to tell you instantly whether an answer is the right size.
What the test asks
Two errors recur, and both are unit errors rather than arithmetic ones. The first is leaving minutes in a calculation that wanted hours, which throws the answer out by a factor of sixty — and the wrong value will usually be sitting among the options. The second is treating a nautical mile as interchangeable with a kilometre or a statute mile; they differ by fifteen per cent or more, which is easily enough to make a plausible-looking distractor. Expect at least one item that gives you a speed in knots and a time in minutes precisely to see whether you convert before multiplying.