Altitude
Set it once here, in feet, meters, or flight level. It drives the Mach conversion below and the local speed of sound and ISA temperature shown here.
Speed ↔ Mach
Mach is a ratio, not a fixed speed: it's how fast you're going relative to the speed of sound around you. Climb higher and the air gets colder, sound travels slower through cold air, so the same true airspeed reads as a higher Mach number up high than it does at sea level.
Worked Examples
Five common speed and altitude pairs, run through the converter above. Pick any row to load it into the tool.
| True Airspeed | Altitude | Speed of Sound | Mach |
|---|---|---|---|
| 250 kt | 10,000 ft | 638 kt328.4 m/s | 0.392 |
| 450 kt | FL350 | 576 kt296.5 m/s | 0.781 |
| 520 kt | 20,000 ft | 614 kt316.0 m/s | 0.846 |
| 598 kt | 15,000 ft | 626 kt322.3 m/s | 0.955 |
| 1,000 km/h | 11,000 m | 574 kt295.1 m/s | 0.941 |
Mach Speed Chart
Mach 1 is about 761 mph (1,225 km/h, 661 knots) at sea level and about 663 mph at 35,000 ft, where the air is colder.
Sea Level (ISA, 15 °C)
| Mach | mph | km/h | knots | m/s |
|---|---|---|---|---|
| 0.5 | 381 | 613 | 331 | 170 |
| 0.8 | 609 | 980 | 529 | 272 |
| 0.85 | 647 | 1,041 | 562 | 289 |
| 1 | 761 | 1,225 | 661 | 340 |
| 2 | 1,522 | 2,450 | 1,323 | 681 |
| 3 | 2,284 | 3,675 | 1,984 | 1,021 |
| 4 | 3,045 | 4,900 | 2,646 | 1,361 |
| 5 | 3,806 | 6,125 | 3,307 | 1,701 |
| 6 | 4,567 | 7,350 | 3,969 | 2,042 |
| 8 | 6,090 | 9,800 | 5,292 | 2,722 |
| 10 | 7,612 | 12,251 | 6,615 | 3,403 |
| 20 | 15,224 | 24,501 | 13,230 | 6,806 |
| 25 | 19,030 | 30,626 | 16,537 | 8,507 |
35,000 ft (ISA, -54.3 °C, Typical Airliner Cruise)
| Mach | mph | km/h | knots | m/s |
|---|---|---|---|---|
| 0.5 | 332 | 534 | 288 | 148 |
| 0.8 | 531 | 854 | 461 | 237 |
| 0.85 | 564 | 907 | 490 | 252 |
| 1 | 663 | 1,068 | 576 | 297 |
| 2 | 1,327 | 2,135 | 1,153 | 593 |
| 3 | 1,990 | 3,203 | 1,729 | 890 |
| 4 | 2,653 | 4,270 | 2,306 | 1,186 |
| 5 | 3,317 | 5,338 | 2,882 | 1,483 |
| 6 | 3,980 | 6,405 | 3,459 | 1,779 |
| 8 | 5,307 | 8,540 | 4,611 | 2,372 |
| 10 | 6,633 | 10,675 | 5,764 | 2,965 |
| 20 | 13,267 | 21,351 | 11,528 | 5,931 |
| 25 | 16,583 | 26,688 | 14,410 | 7,413 |
Each value is the Mach number times the local speed of sound in the International Standard Atmosphere, so real speeds shift with the actual air temperature. Above about Mach 5 the figures are a straight conversion only.
Speed of Sound by Altitude
| Altitude (ft) | Altitude (m) | Temperature (°C) | knots | mph | km/h | m/s |
|---|---|---|---|---|---|---|
| 0 | 0 | 15.0 | 661 | 761 | 1,225 | 340.3 |
| 5,000 | 1,524 | 5.1 | 650 | 748 | 1,204 | 334.4 |
| 10,000 | 3,048 | -4.8 | 638 | 735 | 1,182 | 328.4 |
| 20,000 | 6,096 | -24.6 | 614 | 707 | 1,138 | 316.0 |
| 30,000 | 9,144 | -44.4 | 589 | 678 | 1,091 | 303.2 |
| 35,000 | 10,668 | -54.3 | 576 | 663 | 1,068 | 296.5 |
| 36,089 (tropopause) | 11,000 | -56.5 | 574 | 660 | 1,062 | 295.1 |
| 40,000 | 12,192 | -56.5 | 574 | 660 | 1,062 | 295.1 |
| 45,000 | 13,716 | -56.5 | 574 | 660 | 1,062 | 295.1 |
| 50,000 | 15,240 | -56.5 | 574 | 660 | 1,062 | 295.1 |
| 60,000 | 18,288 | -56.5 | 574 | 660 | 1,062 | 295.1 |
The speed of sound depends on temperature, so it falls with height until the tropopause (about 36,089 ft in ISA), then stays constant in the lower stratosphere.
How this is calculated
This uses the International Standard Atmosphere: a fixed model where sea level starts at 15°C and temperature drops in a straight line, about 1.98°C per 1,000 ft, up to the tropopause at 36,089 ft (11,000 m). Above that, in the lower stratosphere, it holds steady at -56.5°C. The speed of sound is derived from that temperature:a = 38.97 × √T(K), from a = √(γRT) with γ = 1.4 and R = 287.05 J/(kg·K).
It's the model every performance chart and flight manual is built on, and it's a standard-day approximation, not the actual weather outside the window. Real air runs warmer or colder than ISA depending on the day and the latitude, and Mach depends on the actual local temperature, not the standard-day one. So the same true airspeed can read a slightly different Mach number on a hot day over the equator versus a cold one over Siberia, even at the identical altitude. Treat these numbers as the standard-day baseline, not a live reading from row 34.
Curious why airliners settle into the 35,000 to 38,000 ft band in the first place? We wrote about how high planes actually fly.
Only have the indicated airspeed, the number on the cockpit dial? The airspeed converter turns it into true airspeed and Mach.
Questions People Ask
What Is Mach 1 at 35,000 Feet?
In the standard atmosphere, Mach 1 at 35,000 ft is about 576 kt, which is 663 mph, 1,068 km/h, or 296.5 m/s. That is about 85 kt slower than at sea level, where Mach 1 is about 661 kt, because the air up there is much colder.
Why Does Mach 1 Change With Altitude?
The speed of sound depends on the temperature of the air, and sound travels slower through cold air. In the standard atmosphere the air cools by about 1.98°C for every 1,000 ft you climb, so Mach 1 drops as you go up. Above about 36,000 ft (the ISA tropopause, at 36,089 ft) the temperature stops falling and holds at -56.5°C, so Mach 1 holds steady too, at about 574 kt.
Is This True Airspeed or Indicated Airspeed?
This converter uses true airspeed (TAS), the speed of the plane through the air around it. Indicated airspeed reads lower than true airspeed at altitude, so enter TAS to get a correct Mach number.
How Fast Is Mach 1 in mph?
At sea level in the standard atmosphere, Mach 1 is about 761 mph, which is 1,225 km/h or 661 knots. There is no single answer for every altitude, because Mach 1 is the local speed of sound and that changes with air temperature.
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