CFM to Static Pressure Calculator
Estimate the static pressure a fan must overcome from airflow, duct size or area, velocity, duct length, fitting K, filter loss, air density, and a fan curve point.
Pick a starting point, then adjust the duct size, length, fittings, filter, and fan curve point to match your run.
Loss breakdown
This grid keeps your airflow, length, fittings, filter, density, and fan point fixed while changing round duct diameter.
| Duct size | Velocity | Duct loss / 100 ft | Total required | Fan margin |
|---|
| Preset | Typical CFM | Duct | Common concern |
|---|---|---|---|
| Bath fan | 50-110 | 4-6 in | Long flex duct |
| Range hood | 300-900 | 6-10 in | Roof cap loss |
| HVAC return | 700-1600 | 12-20 in | Filter loading |
| Dust collector branch | 350-900 | 4-6 in | Keeping chips moving |
| Dust collector main | 1000-2500 | 7-12 in | Elbow and separator loss |
| Fume or laser exhaust | 250-700 | 4-8 in | Hood entry loss |
| Component | Typical K | Use in calculator | Note |
|---|---|---|---|
| Smooth radius elbow | 0.25-0.55 | Add each elbow | Large radius is lower loss |
| Tight 90 elbow | 0.75-1.50 | Add each elbow | Common in compact runs |
| Square entry | 0.50 | Add once at inlet | Bellmouth can be much lower |
| Blast gate or damper | 0.20-1.00 | Add if partly closed | Open hardware still adds loss |
| Branch wye | 0.30-0.90 | Add at active branch | Angle and balance matter |
| Hood or pickup | 0.60-2.50 | Add capture device | Shop hoods vary widely |
| Item | Clean loss | Loaded loss | Practical target |
|---|---|---|---|
| HVAC pleated filter | 0.10-0.30 in.wg | 0.30-0.80 in.wg | Lower face velocity |
| Carbon can filter | 0.25-0.60 in.wg | 0.60-1.20 in.wg | Match fan curve |
| Dust collector cartridge | 1.0-2.5 in.wg | 2.5-6.0 in.wg | Clean on schedule |
| HVAC supply duct | 600-1000 FPM | Noise rises fast | Comfort airflow |
| Dust branch duct | 3500-4500 FPM | Use enough CFM | Chip transport |
| Fume exhaust duct | 2000-3500 FPM | Balance noise | Capture stability |
When planning a line or range hood run, the measurements can seem good for the initial plan. However, when the system is actualy running, there may be issues. For example, there may be drop in airflow, there may be issues with the wood chips getting into the elbow in the system, the motor may overheat due to the fan being unable to overcome the resistance in the system.
The difference between the CFM that is rated for the fan and the CFM that the system demand is the concept of static pressure. The system’s component: the duct, the fittings, the filter, and the air itself create static pressure. Airflow are expressed in CFM.
Simple Guide to Static Pressure and Airflow
This number is a measurement of the volume of air that passes through the system. Static pressure is the force that the fan must exert to overcome the resistance created by the system components to move the air. These two elements are linked; however, they are not the same.
For example, a fan may have 1,000 CFM at 1000 sq. Ft. Of open bench area. However, once you add the duct and the filter to the system, there may be almost no airflow at all.
The calculator that was built allows a person to determine the gap between the rated CFM for a fan and the actual CFM that the system may demand. The length of straight duct runs is important. The longer the duct run, the more friction is created.
A short duct run will allow a fan to push air through the system. However, if a person add more length to that same size duct, the static pressure will drop. The material in which the duct is made will also play a role in friction and static pressure.
For instance, a system that uses flexible ducting will have more friction and static pressure loss then a system that uses metal ducting of the same size. The user can account for this factor in the calculator by choosing the correct roughness value for the ducting material. Fittings will also add to the static pressure loss of a system.
Each fitting will have a K factor that the calculator can account for. The more numerous or tight the elbow in the system, the more static pressure will be lost. The same velocity with a 90-degree elbow will produce more static pressure loss than the same velocity through a smooth radius elbow.
This static pressure loss will increase with the velocity of the air through the system. A high-velocity duct with many fittings will have higher static pressure loss than a low-velocity return that incorporate the same number of fittings. This factor can also be accounted for in the calculator.
The static pressure drop of a filter will increase with the airflow through the system. A filter may allow for 1 inch of static pressure loss at 500 CFM. However, that same filter could develop to 3 inches of static pressure loss at the same 500 CFM of airflow.
Therefore, the user needs to enter the rated airflow for the filter into the calculator to determine the correct static pressure loss for the system. This prevents the mistake of using the static pressure of a clean filter for a system that will load with much dust. Air density can also play a role in the static pressure of a system.
The density of air decrease with increased altitude and increased temperature. Therefore, fans will move more CFM through the system in thin air (high altitudes) or through heated areas of a building. However, the fan will produce less static pressure loss in thin air.
This factor can be accounted for in the calculator. If the user doesnt account for this factor, the static pressure calculation will be incorrect. The available static pressure of a fan is not a constant value.
Fan curve can be used to determine the available static pressure a fan will create at any given airflow. If a fan point is selected that is above the static pressure loss of the system, there will be a margin for error in the system for things like filter loading or variations in the system. If the selected fan point has less available static pressure than the system requires, the system will never reach the required airflow.
By comparing the fan point to the total static pressure loss in the system, a person can determine if there is a margin for error before purchasing fan for the system. The reference table included with this project provide context for the calculations. The tables do not contain the answers to the system design questions.
For instance, the tables show velocity recommendations for different sizes of ducting. However, the velocities will vary based off the function of the system. For instance, a dust exhaust may require high velocities to keep the wood chips from settling in the ducting.
A system that provides heated air to a building may have a much lower recommended velocity. The numbers in the tables are a starting point for system designer. The numbers in the tables are not rules.
To use the calculator, a person must first enter the required airflow in the system. The second field ask for the size of the ducting. A person also must add the static pressure loss of the filter and the systems fittings.
Finally, a person can compare the required static pressure to a fan curve to see if the available static pressure for the fans exceed the static pressure loss of the system. Based on this comparison, a person can determine if the system is likely to work with the selected fans. If the static pressure loss of the system exceeds the available static pressure of the fans, a person can adjust the system to fix the problem before purchasing the fans.
They should of checked this first.
