Speed of Sound Guide: m/s, mph, Mach Number & Temperature Effects
Sound travels at 343 m/s in air at 20°C — but temperature, altitude, and medium all matter. This guide covers the speed of sound in every unit and explains the Mach number system.
Last updated: 2026-04-28
Speed of Sound in Air at Different Temperatures
Unlike the speed of light, the speed of sound is not constant — it depends on the medium and its temperature. In an ideal gas, sound speed scales with the square root of absolute temperature (Kelvin). For dry air, the working formula is:
v (m/s) = 331.3 + 0.606 × T (°C)
The table below shows speed of sound in air at five temperatures across the range encountered in weather and aviation, converted to all major speed units.
| Temperature | m/s | km/h | mph | knots | ft/s |
|---|---|---|---|---|---|
| −20°C (−4°F) | 319.1 | 1,148.8 | 713.8 | 620.0 | 1,047 |
| 0°C (32°F) | 331.3 | 1,192.6 | 741.0 | 643.9 | 1,087 |
| 15°C (59°F) — ISA | 340.3 | 1,225.0 | 761.1 | 661.5 | 1,116 |
| 20°C (68°F) | 343.2 | 1,235.6 | 767.7 | 667.2 | 1,126 |
| 40°C (104°F) | 355.4 | 1,279.4 | 794.9 | 690.8 | 1,166 |
Speed of Sound in Different Materials
Sound is a mechanical wave — it requires a medium and travels faster through denser, stiffer materials. Liquids and solids transmit sound much faster than gases because their molecules are more closely packed and the restoring forces (elastic modulus) are stronger.
| Medium | Speed (m/s) | Speed (km/h) | Speed (mph) | Relative to Air (20°C) |
|---|---|---|---|---|
| Dry air at 20°C | 343 | 1,235 | 768 | 1.0× |
| Carbon dioxide | 267 | 961 | 597 | 0.78× |
| Helium | 972 | 3,499 | 2,174 | 2.83× |
| Fresh water at 20°C | 1,481 | 5,332 | 3,313 | 4.32× |
| Seawater at 20°C | 1,522 | 5,479 | 3,404 | 4.44× |
| Concrete | 3,100 | 11,160 | 6,835 | 9.0× |
| Aluminum | 5,100 | 18,360 | 11,408 | 14.9× |
| Steel | 5,960 | 21,456 | 13,335 | 17.4× |
| Diamond | 12,000 | 43,200 | 26,843 | 35.0× |
The Mach Number System
The Mach number (M) is the ratio of an object's speed to the local speed of sound: M = v / vsound. It is named after Austrian physicist Ernst Mach. Because the speed of sound varies with altitude and temperature, a given Mach number corresponds to different speeds in m/s depending on conditions.
| Aircraft / Object | Mach Number | Speed at Sea Level (km/h) | Speed at Sea Level (mph) |
|---|---|---|---|
| Typical airliner (cruise) | Mach 0.82–0.85 | 1,005–1,041 | 624–647 |
| Speed of sound (Mach 1) | Mach 1.0 | 1,225 | 761 |
| F/A-18 Hornet (max) | Mach 1.8 | 2,205 | 1,370 |
| Concorde (cruise) | Mach 2.04 | 2,499 | 1,553 |
| SR-71 Blackbird (max) | Mach 3.3 | 4,042 | 2,512 |
| X-15 (max, unpowered glide) | Mach 6.7 | 8,207 | 5,100 |
| Space Shuttle re-entry | Mach 25 | 30,625 | 19,030 |
Subsonic, Transonic, Supersonic, Hypersonic
- Subsonic (M < 0.8): Most commercial aviation. No shock waves.
- Transonic (M 0.8–1.2): Mixed subsonic and supersonic airflow. High drag (wave drag) region.
- Supersonic (M 1.2–5.0): Shock waves attached to the craft; sonic boom heard on ground.
- Hypersonic (M > 5.0): Extreme aerodynamic heating; requires special materials.
Sonic Boom
When an aircraft exceeds Mach 1, it produces a continuous shock wave — a cone of compressed air called a Mach cone. The angle of the cone (θ) satisfies sin(θ) = 1/M. People on the ground hear this shock wave as a "sonic boom" as the cone sweeps past, not just at the moment of breaking the sound barrier.
Use our speed converter to convert m/s, km/h, mph, knots, and Mach numbers, and our temperature converter to find the exact speed of sound at any air temperature.