Speed Units Explained: mph, km/h, knots, m/s, and Mach
By Convert Calc Editorial Team · Published
A weather report gives wind in knots, a road sign gives a limit in km/h, a physics problem gives a velocity in m/s, and a documentary about the SR-71 gives a speed as “Mach 3.” All four describe the same physical quantity — distance covered per unit of time — and all four survive because the people using them are solving different problems.
Most of the conversions are trivial arithmetic. The errors that cost you something come from somewhere else: assuming two units measure the same thing when they measure against different references.
The five units and who uses which
Meters per second (m/s) is the coherent SI unit: the base unit of length divided by the base unit of time, defined in the BIPM SI Brochure. Physics and engineering work in m/s because it drops straight into equations for acceleration, momentum and kinetic energy without a stray conversion factor. You will not find it on a road sign anywhere, because hours and kilometers suit the scale of a journey better than seconds and meters do.
Kilometers per hour (km/h) is m/s scaled to the distances and durations of road travel, and it’s what nearly every country outside the US and UK posts on its speed limits. It’s a decimal step away from m/s: exactly 3.6 km/h per m/s.
Miles per hour (mph) is the road unit in the United States and the United Kingdom. UK national speed limits are set in mph — gov.uk lists them as 30 mph in built-up areas in England, Scotland and Northern Ireland, 20 mph in Wales — even though the UK is otherwise largely metric. One mile per hour is exactly 1.609344 km/h, because the international mile is defined as exactly 1,609.344 meters (NIST SP 811, Table B.8).
Knots (kn) are used at sea and in the air, and the reason is navigational rather than traditional. A knot is one nautical mile per hour, and the nautical mile is tied to the Earth’s coordinate grid: NOAA describes it as “one minute of latitude,” standardized at exactly 1.852 kilometers in 1929. That relationship is what makes the unit worth keeping. If you are moving at 10 knots on a northerly heading, you are crossing 10 minutes of latitude every hour, and you can read your progress off a chart without converting anything. The name itself is older than the standard — NOAA traces it to the common log, a knotted rope paid out behind a ship and counted against a timer.
Aviation inherited the convention along with marine charts, and it is now baked into the data
pilots read: US aviation weather reports code wind speed in knots, abbreviated KT, per the
National Weather Service ASOS/METAR documentation.
Mach is the odd one out, because it isn’t a speed at all. NASA defines the Mach number as “the ratio of the speed of the aircraft to the speed of sound in the gas” (NASA Glenn Research Center). It exists because the aerodynamics of a wing change according to that ratio. Below about Mach 1, air behaves as though it were nearly incompressible; near Mach 1, compressibility effects dominate; above Mach 1, shock waves form; and above Mach 5, the air’s own chemistry starts absorbing energy. An airframe designer needs to know which of those regimes the aircraft will fly in, and the answer in mph does not tell them.
Reference table
| km/h | mph | knots | m/s |
|---|---|---|---|
| 10 | 6.21 | 5.40 | 2.78 |
| 30 | 18.64 | 16.20 | 8.33 |
| 50 | 31.07 | 27.00 | 13.89 |
| 60 | 37.28 | 32.40 | 16.67 |
| 80 | 49.71 | 43.20 | 22.22 |
| 100 | 62.14 | 54.00 | 27.78 |
| 120 | 74.56 | 64.79 | 33.33 |
| 200 | 124.27 | 107.99 | 55.56 |
The underlying factors are exact by definition, not measured: 1 mph = 1.609344 km/h, 1 knot = 1.852 km/h, 1 m/s = 3.6 km/h. Only the decimals above are rounded.
Four pitfalls worth knowing about
Mach 1 is not a fixed speed
Of the four pitfalls here, this one produces the largest errors. The speed of sound depends on the temperature of the air, as NASA’s page states, and air temperature drops with altitude through the troposphere. Mach 0.85 at cruise altitude and Mach 0.85 near the ground are two different speeds, and the gap between them runs to tens of miles per hour.
Any converter that turns a Mach number into mph, including ours, has to pick a reference condition, and the conventional choice is the standard atmosphere at sea level (15 °C), where the speed of sound is about 340 m/s — roughly 1,225 km/h or 761 mph. Treat a Mach conversion as “Mach n at sea level in standard conditions.” For anything involving actual flight at altitude, the sea-level number will overstate the true speed.
A knot is already “per hour”
“Knots per hour” is a unit of acceleration, and almost never what the person writing it meant. The “per hour” is inside the definition of the knot, the same way it’s inside mph. If you see “20 knots per hour” in a forecast or a listing, read it as 20 knots.
The two miles aren’t the same mile
A nautical mile is 1,852 m; a statute mile is 1,609.344 m. A boat’s speed in knots and a car’s speed in mph are therefore numbers on different scales, and the gap is large enough to matter: 25 knots is about 28.8 mph, not 25 mph. The same trap catches distances. A “12-mile” leg on a marine chart is 12 nautical miles, roughly 13.8 statute miles, and planning fuel or arrival time on the wrong mile puts you off by about 15%.
In aviation, converting the unit doesn’t settle what was measured
Airspeed comes in several flavors: what the instrument indicates, what the aircraft is doing relative to the air mass around it, and what it’s doing relative to the ground. All of them get quoted in knots. Converting 250 knots to 288 mph is correct arithmetic that tells you nothing about which of those three you started with, so check what the figure represents before you set it against a number from another source.
Mental math that holds up
- km/h → mph: multiply by 0.6, or halve it and add 10%. 100 km/h → 50 + 10 = 60, versus 62.1 exact. Close enough for a road sign.
- mph → km/h: multiply by 1.6, or add 60%. 70 mph → 112, versus 112.65 exact.
- knots → mph: add 15%. 20 knots → 23 mph, versus 23.0 exact.
- knots → km/h: multiply by 1.85, or double and take off 8%. 20 knots → 37 km/h, which is the exact answer.
- m/s → km/h: multiply by 3.6. No approximation at all; that factor is exact.
The km/h ↔ mph estimates drift at high speeds. At 200 km/h the “halve and add 10%” trick gives 120 mph against a true 124.3, an error of 3.5%, so use the km/h to mph converter or the mph to km/h converter when the figure matters.
Frequently Asked Questions
The forecast says winds at 25 knots. Is it safe to take a small boat out?
Convert first, then judge: 25 knots is 28.8 mph or 46.3 km/h, and that’s sustained wind, not a gust. Whether it’s safe depends on the vessel, the water, and your experience — the conversion only tells you the number isn’t as mild as “25” sounds next to the mph figures you’re used to from driving. Check the full marine forecast, including gust and sea-state figures, rather than deciding on the wind number alone.
My European car’s speedometer shows km/h and I’m driving in the US. What’s the quick check?
US interstate limits are commonly 65–75 mph, which is 105–121 km/h. The practical shortcut is to memorize the three or four limits you’ll actually meet: 25 mph ≈ 40 km/h (residential), 55 mph ≈ 89 km/h, 65 mph ≈ 105 km/h, 70 mph ≈ 113 km/h. Most cars sold in either market show both scales on the dial, with the secondary scale in smaller numbers on the inner ring.
A drone spec sheet lists max speed in m/s. How fast is that really?
Multiply by 3.6 for km/h, or by roughly 2.2 for mph. A 20 m/s top speed is 72 km/h or 44.7 mph, which is faster than most people expect from the raw number — part of why m/s specs feel deceptively modest. Wind limits on the same spec sheet are usually in m/s too, and a stated “10 m/s wind resistance” is a 36 km/h wind, around 19 knots.
Why does my flight tracker show a ground speed much higher than the aircraft’s cruise speed?
Ground speed includes the movement of the air mass the aircraft is flying through. An eastbound transatlantic flight riding a jet stream can show a ground speed 100+ knots above its airspeed, while the same route westbound shows the opposite. Both figures are in knots; they’re measuring against different references, which is the point of the fourth pitfall above.
Is Mach 1 the same everywhere?
No. The Mach number is a ratio against the local speed of sound, and the local speed of sound depends on air temperature. At sea level in the standard atmosphere (15 °C), Mach 1 is about 340 m/s, near 761 mph. In the roughly −57 °C air at a typical airliner’s cruise altitude it falls to about 295 m/s, near 660 mph — a 100 mph difference for the same Mach number. That’s why engineers quote supersonic thresholds as Mach numbers instead of a fixed mph figure.
Practical takeaway
For everyday conversions — a rental car speedometer, a foreign speed limit, a drone spec — the mental approximations above are accurate to within a couple of percent and take a second. Reach for an exact conversion in three situations: when you’re crossing between nautical and statute miles, when a Mach figure is involved, and when you’re above about 150 km/h, where the rough multipliers start to drift.
And before you convert anything from a marine or aviation source, confirm what the figure was measured against: nautical or statute mile, airspeed or ground speed, sea level or altitude. A converter will give you a correct answer to whichever question you asked it, which is no help if you asked the wrong one.