Putting Cruise Mach Numbers on the km/h Scale
Airlines and manufacturers quote cruise speed as a Mach number because that is what matters aerodynamically: Mach 0.85 behaves the same way over the North Atlantic in January as it does over the Sahara in July, even though the actual speed differs. For a passenger, a journalist or a student, the useful question is the other one — how many kilometers per hour is that?
Mach is a ratio, not a speed. It compares an aircraft's speed with the speed of sound in the air around it, and that speed of sound depends on air temperature alone. This converter answers with the standard sea-level figure so every result is on one consistent basis.
Who Needs the Metric Answer
Frequent Flyers
Aerospace Coursework
Route Planners and Analysts
Why Two Sources Can Both Be Right
The speed of sound falls as air gets colder, and the air at cruise altitude is far colder than the air at the runway. Mach 1 is about 1,225 km/h in standard sea-level air at 15 °C, but only around 1,062 km/h in the −56.5 °C air above 11,000 metres. Two publications quoting different km/h figures for the same Mach number are usually just assuming different altitudes.
Converting a Quoted Cruise Speed
Cruise Mach numbers are almost always decimals, and the field is happy with either decimal mark, so 0,82 and 0.82 give the same answer.
Type the Mach number
Enter it on the left, decimals included. The km/h figure builds as you type, so you can compare 0.78, 0.82 and 0.85 in a few keystrokes.
Note which basis you are quoting
The answer is a sea-level equivalent. If you are describing an aircraft in the cruise, say so, or scale it with the altitude table further down this page.
Work backwards from a km/h claim
Press swap to enter a published km/h speed and see what Mach number it corresponds to — useful for checking whether a press release is describing supersonic flight at all.
Change the target unit when needed
The right-hand dropdown is searchable, so the same Mach value can be shown in m/s for an aerodynamics calculation or in knots for a flight-planning note.
The Speed of Sound at Airline Altitudes
These rows use the International Standard Atmosphere, the temperature model every performance chart is built on. Read them as the km/h value of Mach 1 at each height — multiply by the Mach number you care about to get that aircraft's true airspeed.
| Altitude | ISA temperature | Speed of sound | Mach 1 in km/h |
|---|---|---|---|
| Sea level (ISA) | +15 °C | 340.3 m/s | 1,225 km/h |
| Sea level, warm day | +20 °C | 343 m/s | 1,234.8 km/h |
| 5,000 m | −17.5 °C | 320.5 m/s | 1,154 km/h |
| 10,000 m | −49.9 °C | 299.5 m/s | 1,078 km/h |
| 11,000 m and above | −56.5 °C | 295.1 m/s | 1,062 km/h |
Run an airliner through it and the numbers stop being abstract. A Boeing 787 at Mach 0.85 in the tropopause is doing about 903 km/h through the air, not the 1,050 km/h a sea-level conversion suggests. Concorde cruised at roughly Mach 2.04 and was published at about 2,180 km/h, while the same Mach number on the sea-level basis used here returns 2,519 km/h. Boom's Overture is designed for Mach 1.7 over water, which the company describes as around 2,100 km/h.
One Fixed Sea-Level Baseline
Every result uses the same 343 m/s reference, so two Mach numbers converted here are always directly comparable with each other.
Decimal Mach Numbers Welcome
Cruise settings such as 0.785 are entered exactly as written, with a comma or a dot, and no rounding is forced on the input.
Check a Claim in Reverse
Swapping the fields turns a headline km/h figure back into a Mach number, which is the quickest test of whether a claim is plausible.
Clean Copy for Slides
The copy button takes the bare number without a unit label, which is what you want when the slide already carries its own axis titles.
Supersonic Flight Questions in Metric Terms
Why does this page take Mach 1 as 343 m/s?
343 m/s is the widely used textbook value for dry air at 20 °C, and physics courses quote it constantly. The colder ISA sea-level standard of 15 °C gives 340.3 m/s, about 0.8 percent lower, so the two agree closely enough for everything except formal performance work.
Is an airliner at Mach 0.85 really doing 1,050 km/h?
Not through the air. At a typical cruising level the surrounding air is around −56 °C, so Mach 0.85 works out at roughly 903 km/h true airspeed. Ground speed can still exceed 1,000 km/h when a jet stream tailwind is added on top.
What will the new supersonic airliners cruise at in km/h?
Boom's Overture targets Mach 1.7 over water, described by the company as roughly 2,100 km/h, with a slower boomless profile of up to about Mach 1.3 over land. The demonstrator aircraft that preceded it went supersonic at Mach 1.12 and Mach 1.18 before being retired.
Where does hypersonic begin in km/h?
Convention puts it at Mach 5, which is 6,174 km/h on this page's sea-level basis and closer to 5,300 km/h in the cold upper atmosphere where such vehicles actually fly. Above that, heating rather than drag becomes the limiting design problem.
Why does a climbing jet hold its Mach number while its km/h falls?
Because the air keeps getting colder, so the speed of sound drops and the same Mach number represents a lower true airspeed. Crews climb on a fixed indicated airspeed, switch to a fixed Mach number higher up, and the transition between the two is a routine part of the climb profile.
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