Calculate duct static pressure from CFM and total pressure, or find airflow from static pressure using round or rectangular duct size.
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Static Pressure From CFM Formula
The following formula is used to calculate the Static Pressure From CFM.
Pstatic (inH₂O) = Ptotal (inH₂O) - ((CFM/A)/4005)^2
- Where Pstatic is the static pressure (inH₂O)
- Ptotal is the total pressure (inH₂O)
- CFM is the airflow (ft³/min)
- A is the duct cross-sectional area (ft²)
To calculate the static pressure from CFM, compute the duct velocity V by dividing CFM by area (V = CFM/A, in ft/min). Then divide V by 4005, square the result to get velocity pressure in inches of water, and subtract that from the total pressure. Note: the 4005 constant is the standard HVAC approximation for standard air density; if you work in other pressure units, convert to/from inH₂O (for example, 1 inH₂O ≈ 5.2026 lbf/ft² ≈ 249.09 Pa).
| CFM (ft³/min) | Static Pressure (inH₂O) |
|---|---|
| 100 | 0.999 |
| 200 | 0.998 |
| 300 | 0.994 |
| 400 | 0.990 |
| 500 | 0.984 |
| 600 | 0.978 |
| 700 | 0.969 |
| 800 | 0.960 |
| 900 | 0.950 |
| 1000 | 0.938 |
| 1100 | 0.925 |
| 1200 | 0.910 |
| 1300 | 0.895 |
| 1400 | 0.878 |
| 1500 | 0.860 |
| 1600 | 0.840 |
| 1700 | 0.820 |
| 1800 | 0.798 |
| 1900 | 0.775 |
| 2000 | 0.751 |
| * Rounded to 3 decimals. Uses formula: Static = Ptotal − (Q/A/4005)², with Ptotal=1.00 inH₂O and A=1.00 ft², output shown in inH₂O. | |
How to Calculate Static Pressure From CFM?
The following example problems outline how to calculate the Static Pressure From CFM.
Example Problem #1
- First, determine the total pressure. In this example, the total pressure is 1.20 inH₂O.
- Next, determine the total airflow. For this problem, the airflow is 1000 ft³/min (CFM).
- Next, determine the duct area. In this case, the area is 1.00 ft².
- Finally, calculate the static pressure using the formula above.
Pstatic (inH₂O) = Ptotal (inH₂O) - ((CFM/A)/4005)^2
Inserting the values from above and solving yields:
Pstatic = 1.20 - ((1000/1.00)/4005)^2 = 1.1377 (inH₂O)
Example Problem #2
Using the same method as above, determine the variables required by the equation. For this example problem, these are provided as:
total pressure = 2.00 inH₂O
total CFM = 2500 ft³/min
area = 2.00 ft²
Entering these given values yields: Pstatic = 2.00 - ((2500/2.00)/4005)^2 = 1.9026 (inH₂O)
