Hong Life CalculatorsUnderstanding the Pitot tube airspeed calculator — concept and principle
A Pitot tube measures the speed of a fluid by comparing the pressure where the flow is brought to rest (total or stagnation pressure) with the undisturbed static pressure. Their difference is the dynamic pressure. Enter it with the fluid density and the calculator returns the speed in m/s, km/h and knots. It is used for aircraft airspeed, wind tunnels, duct air balancing in HVAC and drone or race-car testing.
How it is calculated
- v = √(2Δp ÷ ρ), with Δp the dynamic pressure in Pa (total minus static) and ρ the density in kg/m³.
- km/h = m/s × 3.6; knots = m/s × 1.943844, since one knot is 1,852 m per hour.
- Sea-level air in the ISO standard atmosphere (15 °C, 101.325 kPa) has ρ = 1.225 kg/m³.
The principle
Apply Bernoulli's equation along a streamline that ends at the tip of the tube. At the tip the fluid stops, so all of its kinetic energy per volume, ½ρv², turns into extra pressure: p_total = p_static + ½ρv². Solving for v gives the formula. The method was introduced by Henri Pitot in 1732; the modern Pitot-static probe measures both pressures in one instrument. The formula treats air as incompressible, which is accurate at low speed: below about Mach 0.3 the speed error is roughly 1% or less. At higher speeds compressible-flow relations are needed.
Worked example
- Inputs: Δp = 500 Pa, ρ = 1.225 kg/m³.
- 2Δp ÷ ρ = 1000 ÷ 1.225 = 816.33 m²/s².
- v = √816.33 = 28.571 m/s (shown as 28.57142857).
- In km/h: 28.571 × 3.6 = 102.86 km/h.
- In knots: 28.571 × 1.943844 = 55.54 kn (shown as 55.5384).
| Density (kg/m³) | Speed (m/s) |
|---|---|
| 0.306 | 57.14285714 |
| 0.613 | 40.40610178 |
| 0.919 | 32.99144395 |
| 1.23 | 28.57142857 |
| 1.53 | 25.5550626 |
| 1.84 | 23.32847374 |
| 2.45 | 20.20305089 |
How to read the result
Speed rises with the square root of Δp: four times the pressure difference means only twice the speed. Small pressures therefore mean low resolution at low speeds; a 5 Pa reading in air corresponds to only about 2.9 m/s. Density matters too: at the same Δp, thinner air at altitude or in hot weather means a higher true speed. If you always use 1.225 kg/m³, the result is what pilots call equivalent airspeed, close to indicated airspeed, not true airspeed.
Common mistakes
- Entering the total pressure instead of the difference between total and static.
- Using 1.225 kg/m³ at altitude or for other gases; use the actual local density.
- Misaligning the probe with the flow; yaw of more than a few degrees reduces the reading.
- Using water manometer readings without converting (1 mmH₂O ≈ 9.81 Pa).
Frequently asked questions
How do I get density at another altitude or temperature?
For dry air use ρ = p ÷ (R·T), with p in Pa, T in kelvin and R = 287.05 J/(kg·K).
Can a Pitot tube be used in water?
Yes. Use ρ ≈ 998 kg/m³; the same Δp then corresponds to a much lower speed (500 Pa gives about 1 m/s).
Why do Pitot tubes on aircraft have heaters?
Ice can block the opening and produce false airspeed readings, so the probe is heated in flight.
Sources: ISO 2533, Standard Atmosphere · F. M. White, Fluid Mechanics, McGraw-Hill (Pitot-static tube) · J. D. Anderson, Introduction to Flight, McGraw-Hill