# What is Engine Pressure Ratio?

I kind of know what it is but I don't fully understand it. Wikipedia says,

the engine pressure ratio (EPR) is the total pressure ratio across a jet engine, measured as the ratio of the total pressure at the exit of the propelling nozzle divided by the total pressure at the entry to the compressor.

I know where the entry of the compressor and exit of the propelling nozzle but I don't get what they mean by pressure. Do they mean air pressure?

• Related: Points of measure in a turbofan. EPR is more dependent on P2/P1 than P3/P0. Source. – mins Oct 16 '17 at 0:17

The wiki quote refers to total pressure at intake and outlet. Total pressure is defined as static pressure $p_s$ and dynamic pressure $p_d$ which for incompressible flow = $\frac{1}{2} \cdot \rho \cdot V^2$.
So the Engine Pressure Ratio is $$\frac {(p_s + \frac{1}{2} \cdot \rho \cdot V^2)_{exhaust}} {(p_s + \frac{1}{2} \cdot \rho \cdot V^2)_{freeStream}}$$