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Airflow from velocity pressure
Turn a velocity pressure reading into feet per minute and CFM, and know when the standard-air shortcut stops being accurate.
The standard-air formula
Velocity pressure (VP) is total pressure minus static pressure, in inches of water. With standard air, velocity in feet per minute is:
FPM = 4005 × √VP
Then multiply by the duct's cross-section area in square feet:
CFM = FPM × arearectangular area = width × height ÷ 144 · round area = π × (diameter ÷ 2)² ÷ 144, with sizes in inches
Worked example
24 × 12 in duct, VP = 0.25 in. w.c.
4005 × √0.25 = 4005 × 0.5 = 2002.5 FPM
24 × 12 ÷ 144 = 2 ft² · 2002.5 × 2 = 4005 CFM
Airflow calculator
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When to correct for density
The 4005 only holds for standard air, which is 0.075 lb/ft³ (about 70 °F at sea level). Hot air and high altitude make air thinner, so the same velocity pressure means a higher real velocity. The full formula is:
FPM = 1096.7 × √(VP ÷ density)density in lb/ft³
You can estimate density from the air temperature and the barometric pressure:
density ≈ 1.325 × barometric pressure (in. Hg) ÷ (°F + 460)
Same reading at about 5,000 ft
At roughly 24.9 in. Hg and 70 °F, density is about 1.325 × 24.9 ÷ 530 = 0.0622 lb/ft³.
1096.7 × √(0.25 ÷ 0.0622) ≈ 2198 FPM
That is about 10% more air than the 2002.5 FPM the standard-air formula gives. At sea level with air near room temperature, the correction is small.
Field notes
- On a traverse, convert each reading to velocity first and then average the velocities. Averaging the pressures and taking one square root reads high.
- Use the inside dimensions of the duct. Internal liner makes the free area smaller than the sheet metal size.
- Flow rings on terminal units and airflow measuring stations amplify the signal, so 4005 does not apply to them. Use the manufacturer's flow constant (K factor) for that device and size.
- Take readings in the straightest run you can find. Readings just after an elbow, damper or fan are uneven and hard to trust.
- Very low velocity pressures are hard to measure. At 400 FPM the velocity pressure is only about 0.01 in. w.c.
Pass criteria. ASHRAE Guideline 13 lists airflow reporting accuracy as ±10% of full scale for terminal units and ±5% for measuring stations. See the tolerances page for the full list and source. The balancing report and the project specification decide what passes on your job.
Keep the math in your pocket
The free Panelside app includes field math like this and works with no signal. The checkout workbook records the airflow points you verify.
Coming soon. The app and the store are not open yet.