Pitot Traverse Airflow (Velocity Pressure to CFM)
Duct airflow from a Pitot traverse reading.
Example
You enter
- Average velocity pressure (in. w.c.) 0.15
- Duct width (in) 24
- Duct height (in) 12
You get
- Traverse velocity 1551
- Duct area 2 ft^2
- Airflow 3102 CFM
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeField airflow from a Pitot traverse: V = 4005 x sqrt(VP_avg) for standard air, then CFM = V x duct area. 0.15 in wc in a 24x12 duct -> 1551 fpm, 2.0 ft^2, 3102 CFM; a lighter 0.05 in wc reads 896 fpm (the square-root relation). This takes one pre-averaged VP; to average raw point readings the right way, use the velocity-averaged traverse. A field measurement, not a substitute for a calibrated flow station.
Formula or tableV = 4005 sqrt(VP_avg); A = (w x h)/144; CFM = V x A.
Source and editionThe Pitot-traverse airflow measurement (velocity-pressure to velocity, standard air) from ASHRAE Fundamentals / AABC-NEBB field practice, by name.
Free accessThe V = 4005 sqrt(VP) standard-air relation and the traverse-average method are standard published air-balancing procedures.
What governsEstimate. AHJ and licensed professional govern.
API field namesvp_avg_inwc, w_in, h_in, v_fpm, area_ft2, cfm
- Air density optional (added 2026-10-09): V = 4005 sqrt(VP x 0.075/rho), the standard-air constant scaled for the entered density (the exact form is 1096.2 sqrt(VP/rho)); 0.075 lb/ft^3 leaves it unchangedASHRAE Fundamentals; air-density-correction
- Velocity from VP V = 4005 sqrt(VP) for standard airASHRAE Fundamentals
- Traverse average takes one pre-averaged VP; averaging point velocities is more exact (pitot-traverse-average)AABC / NEBB field practice
- Standard air 0.075 lb/ft^3 unless an air density is entered; air-density-correction gives the density at altitude or high temperaturescope of this tile