Duct Traverse Airflow Calculator
Enter the velocity pressure readings from a pitot-tube traverse to get the average velocity and the airflow in the duct, corrected for air density.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Airflow
- 1,402CFM
- Average velocity
- 1,402fpm
About 1,402 CFM at an average 1,402 fpm.
- Averaging the pressures before taking the square root would give 1,472 fpm (5 % high). Velocities must be averaged, as done here.
- Take readings on an equal-area grid at least several duct diameters downstream of elbows and fittings, as your instrument’s instructions describe.
- Duct area
- 1ft²
- Air density
- 0.075lb/ft³
- Velocity constant used
- 4,005
Velocity at each point
- Reading 1
- 801fpm
- Reading 2
- 1,202fpm
- Reading 3
- 1,602fpm
- Reading 4
- 2,003fpm
Show the working
- Air density by the ideal-gas law from standard air (0.075 lb/ft³)0.075 lb/ft³
- Velocity at each point V = 4,005 × √(0.075 / ρ) × √VPconstant 4,005; velocities 801, 1,202, 1,602, 2,003 fpm
- Average of the velocities (not of the pressures)1,402 fpm
- Airflow = average velocity × area1,402 × 1 ft² = 1,402 CFM
From velocity pressure to CFM
A pitot tube measures velocity pressure — the difference between total and static pressure. For standard air (0.075 lb/ft³), velocity in feet per minute is 4,005 × the square root of the velocity pressure in inches of water. For warmer, colder or higher-altitude air the calculator scales the constant by the square root of the density ratio.
Because velocity depends on the square root of pressure, each reading is converted to a velocity first and the velocities are averaged. Averaging the pressures first gives a result that is too high; the calculator shows by how much.
A good traverse
Take readings across the whole cross-section on an equal-area grid, in a straight run of duct away from elbows, transitions and dampers, with the fan running steadily. Your instrument’s instructions describe the grid; more points give a better average.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Duct shape
- Rectangular
- Inside width
- 12 in
- Inside height
- 12 in
- Air temperature in the duct
- 70 °F
- Site elevation
- 0 ft
- Number of readings
- 4 readings
- Reading 1: velocity pressure
- 0.04 in. w.c.
- Reading 2: velocity pressure
- 0.09 in. w.c.
- Reading 3: velocity pressure
- 0.16 in. w.c.
- Reading 4: velocity pressure
- 0.25 in. w.c.
Result
- Airflow
- 1,402 CFM
- Average velocity
- 1,402 fpm
- Duct area
- 1 ft²
- Air density
- 0.075 lb/ft³
- Velocity constant used
- 4,005
About 1,402 CFM at an average 1,402 fpm.
Assumptions
- V = 4,005 × √VP for standard air (0.075 lb/ft³), from V = √(2 g ΔP / ρ) with NIST unit factors; scaled by √(0.075 / ρ) for other densities.
- Density from the ideal-gas law using the standard-atmosphere pressure for your elevation and the duct air temperature; humidity is ignored.
Limitations
- Accuracy depends on the traverse: enough points on an equal-area grid, a straight run of duct and a steady fan.
Frequently asked questions
Why average velocities instead of pressures?
Velocity is proportional to the square root of velocity pressure. The average of square roots is lower than the square root of the average, so averaging pressures overstates airflow.
Do I need to correct for temperature?
For accurate work, yes. Hot or high-altitude air is less dense, so the same velocity pressure means a higher velocity. Enter the duct temperature and elevation.
What if some readings are zero?
Check the probe orientation and tubing first. True dead spots happen near obstructions, but they also suggest the measuring location is not ideal.
Sources
- NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.
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