Ultrasonic Level Calculator (Time of Flight and Temperature)
By Santaji GadepatilinUpdated October 28, 2026
An ultrasonic level transmitter sends a sound pulse down to the liquid surface and times the echo. Because the speed of sound in air changes about 0.6 m/s per °C, temperature compensation is essential. This ultrasonic level calculator converts echo time into distance and level — see ultrasonic level transmitter working and how an ultrasonic level transmitter works.
How to use the ultrasonic level calculator
Enter the round-trip echo time, the air temperature in the tank and the empty distance from the sensor face to the bottom (0% level). You get the speed of sound, the distance to the surface and the liquid level.
Ultrasonic level formula
c = 331.3 × √(1 + T/273.15) m/s · d = c × t ÷ 2 · Level = E − d
Speed of sound in air
| Temperature | Speed of sound | Distance error if 20 °C assumed |
|---|---|---|
| -20 °C | 318.9 m/s | -7.1% |
| 0 °C | 331.3 m/s | -3.5% |
| 10 °C | 337.3 m/s | -1.7% |
| 20 °C | 343.2 m/s | +0.0% |
| 30 °C | 349.0 m/s | +1.7% |
| 40 °C | 354.7 m/s | +3.4% |
| 60 °C | 365.9 m/s | +6.6% |
| 80 °C | 376.7 m/s | +9.8% |
Example
Echo time 20 ms at 30 °C, empty distance 6 m:
- c = 349.0 m/s, distance = 349.0 × 0.020 ÷ 2 = 3.49 m
- Level = 6 − 3.49 = 2.51 m
Installation tips
- Keep the liquid surface out of the blanking distance at maximum level.
- Mount away from the tank wall, inlet stream and internal ladders to avoid false echoes.
- Use the sensor’s built-in temperature probe; vapour layers with a different gas composition still cause errors.
- For foam, vapour or pressure, consider radar level measurement.
Frequently asked questions
How does an ultrasonic level sensor calculate distance?
Distance = speed of sound × echo time ÷ 2, because the pulse travels down and back. Level = empty distance − measured distance.
How much does temperature affect the reading?
The speed of sound changes by about 0.17% per °C. Without compensation, a 20 °C error causes about 3.5% distance error.
What is the blanking distance?
A zone just below the sensor where echoes cannot be measured while the transducer is still ringing — typically 0.25–0.5 m.
When is ultrasonic a poor choice?
With heavy vapour, foam, dust, high pressure or vacuum, or strong turbulence. Radar is usually better in those cases.