Enter the total pressure and airflow (CFM), then choose the duct shape and dimensions (to determine area) to calculate static pressure. This calculator uses the standard HVAC relation for standard air: VP (inH₂O) = (V/4005)², where V is velocity in ft/min.
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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)
