Calculate duct static pressure from CFM, duct size and total pressure, or find CFM from static pressure, for round or rectangular ducts in in. w.g. or Pa.
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Static Pressure From CFM Formula
The following low-speed, constant-density relation uses airflow, effective duct area, actual density and known total pressure. CFM alone does not determine static pressure.
Duct static pressure = duct total pressure โ ยฝฯVยฒ, with all pressures in Pa, ฯ in kg/mยณ and V in m/s. V = (CFM รท A) ร 0.3048 รท 60, where A is in ftยฒ. Both pressures must refer to the same duct station and pressure reference.
- Where Pstatic is the static pressure (inHโO)
- Ptotal is the total pressure (inHโO)
- CFM is the airflow (ftยณ/min)
- A is the duct cross-sectional area (ftยฒ)
Compute average duct velocity as CFM divided by area, then multiply ft/min by 0.3048/60 to obtain m/s. Calculate velocity pressure q = ยฝฯVยฒ in Pa and subtract it from total pressure in Pa. Divide by 249.08891 for conventional inches of water. The historical 4005 coefficient is a rounded standard-air approximation; the calculator uses the entered density directly. Signed duct static pressure may be negative. This relation does not determine fan static-pressure rise or duct friction losses.
| CFM (ftยณ/min) | Static Pressure (inHโO) |
|---|---|
| 100 | 0.999 |
| 200 | 0.998 |
| 300 | 0.994 |
| 400 | 0.990 |
| 500 | 0.984 |
| 600 | 0.978 |
| 700 | 0.970 |
| 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 | 0.841 |
| 1700 | 0.820 |
| 1800 | 0.798 |
| 1900 | 0.775 |
| 2000 | 0.751 |
| Rounded to 3 decimals. Static = total โ ยฝฯVยฒ using Pa and m/s, then รท 249.08891 for in. w.g. This assumes the stated total pressure remains fixed as airflow changes. | |
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 inHโO.
- Next, determine the total airflow. For this problem, the airflow is 1000 ftยณ/min (CFM).
- Next, determine the duct area. In this case, the area is 1.00 ftยฒ.
- Finally, calculate the static pressure using the formula above.
Pstatic (in. w.g.) = Ptotal (in. w.g.) โ [ยฝฯ ร (CFM รท A ร 0.3048 รท 60)ยฒ] รท 249.08891.
Inserting the values from above and solving yields:
With density 1.2014 kg/mยณ, V = 5.08 m/s and q = 15.50190448 Pa. Pstatic = 1.20 โ 15.50190448/249.08891 โ 1.13777 in. w.g. (1.1378 to four decimals).
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 inHโO
total CFM = 2500 ftยณ/min
area = 2.00 ftยฒ
At density 1.2014 kg/mยณ, V = (2500/2) ร 0.3048/60 = 6.35 m/s. Pstatic = 2.00 โ (0.5 ร 1.2014 ร 6.35ยฒ)/249.08891 โ 1.90276 in. w.g. (1.9028 to four decimals).
