Static Pressure Calculator
Estimate round-duct static pressure from airflow, duct size, straight length, roughness, fittings, filter loss, hood loss, elevation, and velocity pressure allowance.
1 Duct presets
Load a realistic dust, fume, exhaust, or HVAC run, then tune the fitting counts and accessory losses for your layout.
2 Airflow, duct, fittings, and losses
Pressure breakdown
3 Duct and fitting comparison grid
These cards recalculate the same airflow and fitting counts through nearby duct sizes so you can see whether pressure or velocity is the limiting issue.
4 Material roughness grid
5 Round duct velocity reference
| Diameter | 350 CFM | 800 CFM | 1500 CFM | 2300 CFM |
|---|---|---|---|---|
| 4 in round | 4,010 FPM | 9,170 FPM | 17,190 FPM | 26,360 FPM |
| 5 in round | 2,570 FPM | 5,870 FPM | 11,000 FPM | 16,870 FPM |
| 6 in round | 1,780 FPM | 4,070 FPM | 7,640 FPM | 11,710 FPM |
| 8 in round | 1,000 FPM | 2,290 FPM | 4,300 FPM | 6,590 FPM |
| 10 in round | 640 FPM | 1,470 FPM | 2,750 FPM | 4,220 FPM |
| 12 in round | 450 FPM | 1,020 FPM | 1,910 FPM | 2,930 FPM |
6 Fitting equivalent length table
| Fitting | Equivalent length used | Typical note | Pressure effect |
|---|---|---|---|
| Long-radius 90-degree elbow | 14 duct diameters | Smooth sweep elbow | Moderate, count each turn |
| 45-degree elbow | 7 duct diameters | Offset or gentle turn | Lower than a full 90 |
| Wye or branch entry | 20 duct diameters | Branch merge or lateral takeoff | High when airflow turns sharply |
| Blast gate or damper | 6 duct diameters | Open gate in branch | Small, but adds up |
| Reducer, boot, or transition | 10 duct diameters | Abrupt shape or size change | Depends on angle and throat |
7 Roughness and loss reference
| Material | Roughness used | Best use | Calculator behavior |
|---|---|---|---|
| Smooth PVC or smooth steel | 0.00018 in | Short smooth runs | Lowest friction factor |
| Galvanized spiral or snap-lock | 0.00050 in | Common HVAC and shop duct | Good baseline |
| PVC with joints and seams | 0.00150 in | Shop branches with fittings | Moderate friction bump |
| Vinyl flex hose, stretched | 0.01200 in | Short machine drops | Large pressure penalty |
| Wire helix flex hose | 0.02500 in | Portable or cleanup hose | Very high friction |
| Internally lined duct | 0.01500 in | Acoustic HVAC runs | High roughness allowance |
8 Static pressure bands
| Total path loss | Typical read | Dust collection note | HVAC or exhaust note |
|---|---|---|---|
| Under 1.5 in. wc | Low resistance | Often easy for small collectors | Usually blower friendly |
| 1.5 to 3.5 in. wc | Moderate resistance | Common for short shop branches | Check available external static |
| 3.5 to 6.0 in. wc | High resistance | Typical of tight, filtered dust paths | Needs a fan selected for pressure |
| Over 6.0 in. wc | Very high resistance | Look for flex, hoods, and small ducts | May need redesign or different fan |
9 Tips
Static pressure are an invisible force in a system that has both a dust collection system and an HVAC system. The static pressure of a system is the total amount of energy that the air loses as it moves through the ductwork system. The air in the duct system can contact the duct walls and turn through elbows in the duct system or pushes through the filters in the system.
Each of these action creates a loss in the energy of the air within the duct system. These energy losses is measured as static pressure. If the static pressure of a system is too high for the system’s fan to handle, then the system will move less air then it is designed to move.
What Is Static Pressure in Duct Systems
Many people has issues with their dust collection systems because they dont account for the static pressure that the system’s components create. For instance, a 90 degree elbow in a duct system can create more static pressure than a straight piece of duct of the same size. However, a 90 degree elbow in a duct system can create the same amount of static pressure as fourteen diameters of the duct system.
Flex hose can also create more static pressure than a duct of the same diameter because the roughness of the flex hose creates more static pressure than that of a duct system. Finally, elevation can impact the static pressure of a system because the air is thinner at higher elevations, meaning that there is less velocity pressure of the air that move through the system. You can calculate static pressure for a system by entering the size of the duct system, the length of the system, the material of the duct, and the number of fittings in the system.
The airflow in the system is used to calculate the volume of air that should move through the system, the diameter of the duct is used to calculate the velocity of and the surface area of the air that create friction within the duct system. Additionally, the calculation can account for the roughness of the duct by entering information about whether the duct system uses smooth galvanized spiral ducts or wire reinforced flex ducts. Finally, the number of fittings in the system is used to account for each elbow in the system and each section of flex hose in the system.
There are different types of static pressure loss then that of friction loss in the duct system. For instance, a used filter in the system will create more static pressure than a clean filter. Thus, the static pressure created by a used filter will likely not match the static pressure that the manufacturer publishes in the filter box.
Additionally, the hood from which air enters the duct system can also contribute to the static pressure of the system. A hood that is poorly designed can create large amounts of static pressure in the system. Thus, it is important to add an allowance for velocity static pressure at the end of the calculation.
The calculation will reveal the static pressure losses of the straight duct, the fittings, and the accessories to the system. While many people focus only on the number that is calculated as the static pressure of a system, it is also important to consider the breakdown of that static pressure. For instance, if the static pressure losses from the systems fittings is high relative to the losses on the straight duct sections of the system, then there may be a benefit in using fewer turns in the system.
Similarly, if the velocity of the air in the system is high and the static pressure is high, then increasing the diameter of the duct system will reduce the static pressure. Increasing the diameter of the duct is more effective than adding horsepower to the fan to increase the static pressure. Many systems that are built in the field can differ slightly from the original drawings.
Thus, it is recommended that the technician performs the static pressure calculation for the longest path that air will travel from each of the system hoods to the collector. This will calculate the static pressure that each hood will need to move the air that it does. The technician will then compare the calculated static pressure to the fan curve for that fan at the same airflow.
If the static pressure of the system is less than that provided by the fan, then it will be necessary to either change the fan or the duct system design for the system. The reference tables provide transport velocities for various duct diameters so that it is possible to determine whether chips will remain in the air or if they will settle within the duct. Additionally, the reference tables include static pressure for various fittings in the system and roughness values of various materials.
These tables allow for a sanity check of the duct system when making changes to one of the system variables. For instance, the calculation can be used to make a decision about whether to replace a long flex hose with a straight duct or if changing to a larger trunk for the system will eliminate the static pressure losses of the elbows in the system. Thus, making such changes prior to hanging the duct and installing the fan will allow a technician to manage the static pressure of the system effective.
