In compressible fluid dynamics, **impact pressure** (dynamic pressure) is the difference between total pressure (also known as pitot pressure or stagnation pressure) and static pressure.^{[1]}^{[2]} In aerodynamics notation, this quantity is denoted as **$q_{c))$** or **$Q_{c))$**.

When input to an airspeed indicator, impact pressure is used to provide a calibrated airspeed reading. An air data computer with inputs of pitot and static pressures is able to provide a Mach number and, if static temperature is known, true airspeed.^{[citation needed]}

Some authors in the field of compressible flows use the term *dynamic pressure* or *compressible dynamic pressure* instead of *impact pressure*.^{[3]}^{[4]}

##
Isentropic flow

In isentropic flow the ratio of total pressure to static pressure is given by:^{[3]}

${\frac {P_{t)){P))=\left(1+{\frac {\gamma -1}{2))M^{2}\right)^{\tfrac {\gamma }{\gamma -1))$

where:

$P_{t))$ is total pressure

$P$ is static pressure

$\gamma \;$ is the ratio of specific heats

$M\;$ is the freestream Mach number

Taking $\gamma \;$ to be 1.4, and since $\;P_{t}=P+q_{c))$

$\;q_{c}=P\left[\left(1+0.2M^{2}\right)^{\tfrac {7}{2))-1\right]$

Expressing the incompressible dynamic pressure as $\;{\tfrac {1}{2))\gamma PM^{2))$ and expanding by the binomial series gives:

$\;q_{c}=q\left(1+{\frac {M^{2)){4))+{\frac {M^{4)){40))+{\frac {M^{6)){1600))...\right)\;$

where:

$\;q$ is dynamic pressure