Pitot Tube Airflow Calculator — Duct Traverse, Velocity and CFM
A pitot tube measures velocity pressure, not airflow. Enter the readings from a duct traverse together with the duct size and air temperature; the calculator converts each reading to velocity, averages them correctly and returns the average velocity, actual and standard CFM and total pressure.
No signup · Works on mobile · Formulas shown below · Updated 2026-11-02
Velocity Pressure → Velocity → Airflow
V = C × √(2 × Pv / ρ) for each point; average the velocities; Q = Vavg × A. Standard air: V (fpm) ≈ 4,005 × √Pv (in. w.g.).
Results
Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.
From velocity pressure to velocity
The total-pressure port of a pitot tube faces into the airflow; the static ports on its side read static pressure. The difference is the velocity pressure Pv, and Bernoulli's equation turns it into velocity:
Velocity (SI) V = C × √(2 × Pv / ρ) // m/s, Pv in Pa, ρ in kg/m³ Velocity (IP) V = 1,096.2 × C × √(Pv / ρ) // fpm, Pv in in. w.g., ρ in lb/ft³ Standard air V ≈ 4,005 × √Pv// fpm, 0.075 lb/ft³ Average velocity Vavg = (V₁ + V₂ + … + Vn) / n Airflow Q = Vavg × A // CFM = fpm × ft² Total pressure Pt = Ps + Pv // Air density changes with temperature and altitude — always correct hot air and high-altitude readings
The most common mistake is averaging the velocity-pressure readings and then taking the square root. Because velocity follows the square root of velocity pressure, that always overstates the average velocity, and the error grows with an uneven profile.
Velocity pressure to velocity and CFM chart (70 °F, sea level)
Velocity pressure in. w.g.
Pa
Velocity fpm
m/s
CFM in a 12 in. round duct
CFM in a 20 in. round duct
0.01
2.5
401
2.04
315
875
0.02
5.0
567
2.88
446
1,238
0.05
12.5
897
4.56
705
1,957
0.10
24.9
1,269
6.44
996
2,768
0.15
37.4
1,554
7.89
1,220
3,390
0.20
49.8
1,794
9.11
1,409
3,914
0.30
74.7
2,197
11.16
1,726
4,794
0.50
124.5
2,837
14.41
2,228
6,189
0.75
186.8
3,474
17.65
2,729
7,579
1.00
249.1
4,012
20.38
3,151
8,752
1.50
373.6
4,913
24.96
3,859
10,719
Air density 0.0749 lb/ft³ (1.200 kg/m³) at 70 °F and sea level, pitot coefficient 1.0; airflow assumes the cross-section average velocity equals the tabulated velocity.
How to run a pitot-tube traverse
Choose the plane: a straight duct section well away from elbows, tees, dampers and fan inlets or outlets. The longer the straight run upstream, the more even the velocity profile.
Lay out the points: in a round duct, take equal-area points along two perpendicular diameters; in a rectangular duct, divide the section into roughly equal rectangles and read at the center of each.
Read carefully: point the pitot tip straight into the airflow (yaw makes it read low) and average fluctuating readings over a few seconds.
Compare with the fan curve: convert the measured airflow and the static pressure to the fan inlet and outlet and find the point on the fan curve. That tells you whether the fan is short of capacity or the system resistance is higher than designed — use the fan operating point calculator.
Frequently Asked Questions
How do you convert velocity pressure to velocity?
V = √(2 × Pv / ρ) in SI units. For standard air (0.075 lb/ft³) this simplifies to V (fpm) ≈ 4,005 × √Pv with Pv in in. w.g. For example, 0.20 in. w.g. gives about 1,790 fpm.
How do I average several pitot-tube readings?
Convert each velocity-pressure reading to velocity first, then take the arithmetic mean of the velocities. Do not average the pressures and take the square root — that overstates the velocity. This calculator shows the difference between the two methods.
What is the pitot tube coefficient?
A standard (Prandtl or L-type) pitot tube has a coefficient close to 1.0. S-type pitot tubes, used for dusty or hot stack gas, are typically around 0.84 — but always use the value on the instrument's calibration certificate.
Why is the measured airflow lower than the fan catalog airflow?
Common reasons: system resistance is higher than designed (dirty filters, long ducts, many elbows), which moves the operating point left on the fan curve; poor inlet or outlet connections (system effect); air density different from the catalog's standard air; or a poor traverse location or too few points.
Can I use a pitot tube below about 600 fpm?
600 fpm corresponds to a velocity pressure of only about 0.022 in. w.g. (5–6 Pa), where an ordinary manometer's error becomes large. At low velocity, use a thermal (hot-wire) anemometer or a flow hood, or measure in a smaller duct section with higher velocity.
Link to or Cite This Tool
This calculator is free to reference in articles, forums, specifications and course material. Please credit it with a link:
<a href="https://www.longwellfans.com/resources/pitot-tube-airflow-calculator/">Pitot Tube Airflow Calculator (Velocity Pressure to FPM & CFM)</a> by LONGWELL Fans
