Exhaust Fan Static Pressure Calculator
Estimate the total static pressure an exhaust fan must overcome from CFM, round duct size, duct length, elbows, filter media, louver or shutter loss, silencer loss, velocity pressure, and safety margin.
Use the presets as starting points for grow rooms, barns, greenhouses, pack sheds, and drying rooms, then tune each restriction before checking the fan manufacturer's curve at the final inches water column value.
Static pressure result
Enter CFM, duct, fittings, and component losses to estimate the total fan static pressure requirement.
Calculate to compare friction against fittings and filters.
Calculate to see bend pressure from velocity pressure.
Calculate to show media resistance from the lookup.
Calculate to add shutter, silencer, and other losses.
| Item | Approximation Used | Units | Planning Note |
|---|---|---|---|
| Duct area | Area = pi x (diameter / 12)^2 / 4 | ft² | Round duct area drives velocity and pressure loss. |
| Duct velocity | Velocity = CFM / duct area | FPM | High velocity increases noise and static pressure quickly. |
| Velocity pressure | VP = (FPM / 4005)^2 | in w.c. | Elbow and fitting losses are estimated from VP. |
| Straight duct friction | 0.109136 x CFM^1.9 / diameter^5.02 | in w.c. / 100 ft | Approximation for round duct, adjusted by surface factor. |
| Design static | Total static x (1 + safety margin) | in w.c. | Use this pressure on the fan curve at target CFM. |
| Filter Type | Typical Loss | When It Rises | Calculator Use |
|---|---|---|---|
| No filter | 0.00 in w.c. | Only duct and accessory resistance remains. | Use for direct wall or roof exhaust. |
| Insect screen or mesh | 0.03 in w.c. | Dust, pollen, or small mesh opening. | Useful for greenhouse intake protection. |
| Clean prefilter pad | 0.07 in w.c. | Moisture, dust, or crop residue loading. | Use for washable pad or coarse filter. |
| Dust filter or MERV panel | 0.14 in w.c. | Higher MERV rating or undersized face area. | Use for seed room or packing area dust control. |
| Clean carbon filter | 0.25 in w.c. | Small filter, high CFM, dense carbon bed. | Common grow tent and odor-control input. |
| Loaded carbon filter | 0.38 in w.c. | Age, humidity, dust, or clogged prefilter. | Use when pressure has climbed in service. |
| Dense scrubber or HEPA-style | 0.55 in w.c. | Fine media or compact housing. | Check the actual manufacturer pressure curve. |
| Part | Loss Factor | Applied To | Best Use |
|---|---|---|---|
| Smooth long-radius elbow | K 0.22 each | Velocity pressure | Lowest bend loss for long runs and quiet rooms. |
| Standard adjustable elbow | K 0.45 each | Velocity pressure | General farm and grow-room ducting. |
| Tight stamped bend | K 0.75 each | Velocity pressure | Compact spaces where pressure budget is higher. |
| Flex duct bend | K 0.95 each | Velocity pressure | Short connections only; avoid kinks and sag. |
| Smooth metal duct | 1.00x | Friction rate | Baseline for permanent exhaust duct. |
| Long or sagging flex | 1.70x | Friction rate | Use larger duct or shorter flex to reduce loss. |
| Velocity Band | Static Pressure Trend | Noise Trend | Fan Selection Note |
|---|---|---|---|
| Under 500 FPM | Very low | Quiet | Duct may be large; good for occupied crop rooms. |
| 500 to 800 FPM | Low | Comfortable | Balanced target for grow tents and drying rooms. |
| 800 to 1200 FPM | Moderate | Noticeable | Common for greenhouse and utility exhaust. |
| 1200 to 1600 FPM | High | Louder | Use only when duct is short and fan curve has room. |
| Over 1600 FPM | Very high | Likely loud | Upsize duct or reduce CFM before final fan selection. |
Tip: Match the final design static pressure to the fan curve at your target CFM. A fan rated only at free air can lose a large share of airflow once filters, shutters, and duct are installed.
Tip: When the result is too high, the first fixes are usually a larger duct, fewer tight elbows, a bigger filter face area, or a cleaner prefilter.
When installing an exhaust fan, it is necessary to consider the static pressures that will be placed upon the fan. The static pressure of an exhaust fan will determine the amount of air that the fan can move. Static pressure is the measurement of the resistance that the exhaust fan must overcome in order to move the air through various obstacle in the exhaust system.
If the static pressure isnt correctly calculate for the exhaust fan that is to be installed in a structure, the exhaust fan can either work harder than necessary to move the required amount of air, or the exhaust fan may fail altogether to provide the amount of required exhaust air. Many people often begin to consider the free-air CFM rating of exhaust fans. The free-air CFM ratings is the CFM value that are printed on the exhaust fans when no components are attached to the fan.
How Static Pressure Affects Exhaust Fans
However, if some of the components of the exhaust system (such as a duct and filter) is attached to the fan, the amount of air that the fan delivers will decrease. The reason for this decrease in CFM ratings is due to the static pressure that is created within the exhaust system. One of the main contributor of static pressure is the friction that exists within the duct system of an exhaust fan.
Duct friction is created when the air that is moving within the duct system loses energy. The more friction that is created within the duct system, the more static pressure will be placed upon the exhaust fan. Friction can be created if the duct system is small in diameter or if the length of the duct system is large.
For instance, a four-inch diameter duct will experience more friction then an eight-inch duct. Additionally, if the diameter and length of the duct system is known, friction can be calculated. Another of the contributors to static pressure within the exhaust system are the elbows within the duct system.
Exhaust system elbows create resistance in the system due to the loss of momentum of the airstream as it changes direction. The more elbows within the system, the more static pressure will be placed upon the exhaust fan. Each elbow will create static pressure, but the amount of static pressure is related to the radius of the elbow; the longer the radius of the elbow, the less static pressure that it will place upon the system.
Thus, long radius elbows will add less static pressure to the system than a series of tight bend or a sagging section of flex duct. Another component of the exhaust system that add to the static pressure is the presence of filters. Filters will add to the static pressure of the system due to the way in which they tend to collect dust and moisture from the air that passes through the filter.
For instance, a clean carbon filter can add as much as a quarter inch of water column to the static pressure of the system; the static pressure will increase if the filter becomes loaded with debris. Thus, exhaust fans should of been designed with the loaded filter rather than the clean filter in mind. In addition to the ducts, elbows, and filters, additional component of the exhaust system will create additional static pressure losses.
For instance, louvers, shutters, silencers and hoods will all create a certain amount of static pressure loss within the system. For instance, a gravity shutter may work well on a short wall of exhaust air, but can become a problem when paired with a long duct run and a loaded filter. Silencers are used for noise control within the system, but will also create static pressure losses within the system.
Each of these component should be entered into the calculation of the static pressure of the system. Another component of the static pressure of an exhaust system is the velocity pressure of the exhaust system. Velocity pressure is created due to the speed at which the air is traveling through the duct system; the faster the velocity of the air, the higher the static pressure loss of the system.
Thus, the velocity pressure is one of the components of the total static pressure of the system. Additionally, if higher velocities are utilized within the system, the resistance of the system will increase at each elbow and junction within the duct system. Thus, high velocities of air within the system will create more resistance than slower velocity.
Due to this increased resistance of high velocities, increasing the size of the duct will reduce the total static pressure of the exhaust system. In addition to calculating the total static pressure of the system, a safety margin should be applied to that calculated total static pressure. The safety margin accounts for various factor within the exhaust system that may not be initially calculated for the static pressure of the system; these factors include dirty filters, sagging flex duct, and the tightness of the elbows within the system.
A fifteen-percent safety margin is often applied to the total static pressure calculation; however, the appropriate safety margin will depend upon the frequency with which the exhaust system can be cleaned. Another of the variables within the exhaust system that will contribute to the static pressure of the system are the fan curves of the exhaust fan that will be installed. Fan curves are a graphical representation of the relationship between the static pressure that can be emitted by the exhaust fan and the CFM of air that the fan moves.
The static pressure that can be emitted by an exhaust fan will usually decrease at higher CFM ratings for that
