Decibel Shield
Speed of sound calculator
Sound travels at about 343 m/s (1,236 km/h, 768 mph) in dry air at 20 °C, and about 0.6 m/s faster for every degree warmer. Enter the air temperature for the exact speed in m/s, km/h, mph and ft/s, or the seconds between a flash and its sound to see how far away it was: about 1 km for every 3 seconds, or a mile for every 5.
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Speed of sound in dry air at 20 °C
343.4 m/s
- 1,236 km/h
- 768 mph
- 1,127 ft/s
- 667 knots
How far away was it?
The sound came from about 1.03 km (0.64 mi) away.
Dry air. Humidity makes sound slightly faster, by about 0.4% in saturated air at 20 °C; pressure on its own does not change it.
Speed of sound in air by temperature
| Temperature | m/s | km/h | mph | ft/s |
|---|---|---|---|---|
| −40 °C (-40 °F) | 306.2 | 1,102 | 685 | 1,005 |
| −30 °C (-22 °F) | 312.7 | 1,126 | 700 | 1,026 |
| −20 °C (-4 °F) | 319.1 | 1,149 | 714 | 1,047 |
| −10 °C (14 °F) | 325.3 | 1,171 | 728 | 1,067 |
| 0 °C (32 °F) | 331.5 | 1,193 | 741 | 1,087 |
| 10 °C (50 °F) | 337.5 | 1,215 | 755 | 1,107 |
| 15 °C (59 °F) | 340.4 | 1,226 | 762 | 1,117 |
| 20 °C (68 °F) | 343.4 | 1,236 | 768 | 1,127 |
| 25 °C (77 °F) | 346.3 | 1,247 | 775 | 1,136 |
| 30 °C (86 °F) | 349.2 | 1,257 | 781 | 1,146 |
| 40 °C (104 °F) | 354.9 | 1,278 | 794 | 1,164 |
The formula
In dry air, c = 331.46 × √(1 + T / 273.15) m/s, with T in °C: 331.46 m/s is the measured speed at 0 °C, and the formula gives 343.4 m/s at 20 °C. It follows from c = √(γRT / M) for a gas, so it depends on temperature but not on pressure. Humidity adds a little: saturated air at 20 °C carries sound about 0.4% faster than dry air. Altitude matters only through the temperature, which is why the standard atmosphere puts the speed of sound at about 295 m/s at an airliner's cruising height of 11 km, where it is −56.5 °C.
Sources: Kaye & Laby, Tables of Physical and Chemical Constants, 2.4.1 (NPL); Bohn, Environmental Effects on the Speed of Sound, J. Audio Eng. Soc. 1988; OpenStax University Physics, 17.3 Speed of Sound; HyperPhysics, Speed of Sound in Various Bulk Media; U.S. Standard Atmosphere 1976 tables (PDAS); US National Weather Service, Understanding Lightning: Thunder; Met Office, Thunder and lightning.
Speed of sound in other materials
| Material | m/s | Condition |
|---|---|---|
| Fresh water | 1,481–1,482 | 20 °C |
| Seawater | 1,522 | 20 °C |
| Ice | 3,840 | −20 °C |
| Steel | 5,960 | 20 °C |
| Aluminium | 6,374–6,420 | 20 °C |
| Glass (Pyrex) | 5,640 | 20 °C |
| Concrete | 3,700–4,700 | typical |
| Helium | 972 | 0 °C |
| Hydrogen | 1,286–1,290 | 0 °C |
| Carbon dioxide | 258–259 | 0 °C |
Solids carry more than one kind of sound wave. The table gives the longitudinal wave in the bulk material, the fastest kind; a thin rod carries sound more slowly (about 5,100 m/s in aluminium). Where the two tables differ, the range covers both.
Using the speed of sound
The same speed sets the wavelength of every tone (see frequency to wavelength) and the delay between a speaker and your ears: at 343 m/s, every metre adds about 2.9 ms. The sound distance calculator works out how much quieter a sound gets as it travels.
What is the speed of sound in mph and km/h?
In dry air at 20 °C (68 °F) sound travels at about 343 m/s, which is 1,236 km/h or 768 mph. At 0 °C (32 °F) it is 331.5 m/s, 1,193 km/h or 741 mph. The often quoted 761 mph (340.3 m/s) is the standard atmosphere's figure at sea level, where it sets the temperature at 15 °C.
Why is sound faster in warm air?
Sound is passed along by molecules bumping into each other, and in warmer air they move faster. The speed goes with the square root of the absolute temperature, which near room temperature works out to about 0.6 m/s more for every degree Celsius. Pressure on its own makes no difference, which is why sound is slower high up, where the air is cold, not because it is thin.
How do you work out how far away lightning is?
Count the seconds between the flash and the thunder. Sound covers about 1 km in 3 seconds, or a mile in 5, while the light arrives almost at once, so 6 seconds means about 2 km or 1.2 miles. The calculator does the same sum with the speed for the temperature you set.
Is sound faster in water or in steel?
Both are much faster than air. Sound travels about four times faster in water than in air, and well over ten times faster in steel, because these materials are far stiffer for their density. The table above gives figures for common materials.
