Mach number and the speed of sound
M = V / a, with a = √(γ R T) and γ = 1.4 for air. At sea-level ISA, a = 340.29 m/s (661 kt). Mach number is a ratio of speeds; it does not by itself tell you dynamic pressure.
Dynamic pressure
Dynamic pressure — the “q” or “q-bar” in every lift and drag formula — is
q = ½ ρ V² = ½ γ P M²
At sea level, 100 kt true (and essentially 100 kt equivalent) gives q ≈ 1,622 Pa, or about 33.9 psf. Equivalent airspeed is defined from that same q by q = ½ ρ0 VEAS² with ρ0 = 1.225 kg/m³, so EAS equals TAS only at sea-level density.
For indicated / calibrated airspeed from a pitot-static system, see the true airspeed calculator, which uses the subsonic compressible pitot formula. The impact pressure shown here, qc = P[(1+0.2M²)3.5 − 1], is that isentropic increment; it is not valid as a pitot reading above Mach 1.
Sources
NACA Report 1135; NASA Glenn Beginner’s Guide (dynamic pressure, Mach number); U.S. Standard Atmosphere 1976; Anderson, Introduction to Flight.
Frequently asked questions
Is dynamic pressure the same as pitot pressure?
No. Dynamic pressure q = ½ρV² is a defined quantity used in force coefficients. A pitot tube measures stagnation (total) pressure. The difference from static pressure is the impact pressure qc, which equals q only in the incompressible limit. At higher subsonic Mach they diverge, and above Mach 1 a shock stands in front of the tube.
Why is q also written ½γPM²?
Because a² = γRT and P = ρRT, so ½ρV² = ½ρ(aM)² = ½γPM². For air, ½γ = 0.7, which is a convenient check: q = 0.7 P M².
How is equivalent airspeed related to q?
EAS is the speed at sea-level density that produces the same dynamic pressure: q = ½ ρ0 VEAS². It is the natural speed to use when you are thinking in terms of aerodynamic force at altitude.