Discharge of Air Through An Orifice Equation

Fluid Flow Table of Contents
Hydraulic and Pneumatic Knowledge

Discharge of Air Through An Orifice Equation

The table given below gives the discharge of air through an orifice in cu. ft. of free Air per minute at atmospheric Pressure of 14.7 psia, and 70 °F

Based on 100% coefficient of flow. For well-rounded entrance multiply values by 0.97. For sharp-edged orifices a multiplier of 0.65 may be used.

This table will give approximate results only. For accurate measurements see ASME Power Test Code, Velocity Volume flow Measurement.

Values for pressures from 1 to 15 psig, calculated by standard adiabatic formula.

Values for pressures above 15 psig, calculated by approximate formula proposed by S. A. Moss:

Moss Equation


W = discharge in lb. per sec.
P = upstream total pressure in psia
a = area of orifice in sq. in.
C = coefficient of flow
T = upstream temperature in deg. F. abs.

Values used in calculating above table were C = 1.0,
P = gauge pressure + 14.7 psi.,
T = 530 F. abs.

Weights (W) were converted to volumes using density factor of 0.07494 lb. per cu. ft. This is correct for dry air at 14.7 psia, and 70 F.

Formula cannot be used where P is less than two times the barometric pressure.

Click of Chart Image to enlarge

Pressure Drop vs Orifice Size