Ductwork Pressure Drop Calculator
Estimate HVAC duct friction loss, fitting loss, velocity, equivalent diameter, density correction, and total static pressure for round or rectangular duct runs.
Pick a common duct run, then adjust the geometry, airflow, material, fittings, density, and friction method to match the job.
Inside diameter. For flex duct, use stretched inner diameter.
Updated from material presets; edit if you have project data.
Use for coils, unusual entries, fire dampers, or measured manufacturer data.
Duct Pressure Drop Results
| Material | Typical roughness | Use case | Pressure drop note |
|---|---|---|---|
| Galvanized steel | 0.0003 in | Most HVAC trunks and branches | Baseline for many duct calculators |
| Spiral seam steel | 0.00018 in | Round commercial duct | Smooth and efficient when sealed well |
| Aluminum duct | 0.0002 in | Light exhaust and specialty runs | Similar to smooth metal at HVAC velocities |
| PVC plastic duct | 0.00006 in | Corrosive exhaust where allowed | Low wall roughness, fittings still matter |
| Duct board | 0.0009 in | Residential trunks and plenums | Higher roughness than sheet metal |
| Flexible duct, stretched | 0.0030 in | Short final connections | Loss rises sharply if sagged or compressed |
| Internally lined metal | 0.0018 in | Sound control sections | Use manufacturer data for critical runs |
| Concrete or masonry chase | 0.0100 in | Large shafts and old chases | Very rough; often needs direct measurement |
| Duct section | Typical velocity | Quiet design | Common friction target |
|---|---|---|---|
| Residential supply branch | 600 to 900 fpm | 500 to 700 fpm | 0.06 to 0.10 in/100 ft |
| Residential return branch | 500 to 700 fpm | 400 to 600 fpm | 0.04 to 0.08 in/100 ft |
| Main trunk | 700 to 1000 fpm | 600 to 800 fpm | 0.05 to 0.08 in/100 ft |
| Commercial branch | 900 to 1400 fpm | 700 to 1000 fpm | 0.08 to 0.15 in/100 ft |
| Kitchen or lab exhaust | 1200 to 2000 fpm | Code and capture driven | Use hood and fan data |
| Fitting | Modeled K | Better option | Design note |
|---|---|---|---|
| 90 degree standard elbow | 0.35 each | Long radius or turning vanes | Often dominates short branch losses |
| 90 degree tight elbow | 0.75 each | Increase radius | Avoid near equipment outlets |
| 45 degree elbow | 0.18 to 0.38 | Use two gentle offsets | Lower than a sharp 90 degree turn |
| Gradual transition | 0.15 each | Keep angle under 15 degrees | Good for reducers and expansions |
| Typical takeoff or wye | 0.55 each | Smooth entry with balancing damper | Actual loss depends on flow split |
| Damper, grille, hood | 0.35 to 0.90 | Use rated device pressure drop | Manufacturer data beats generic K values |
| Rectangular duct | Area | Equivalent round | Good use |
|---|---|---|---|
| 8 x 6 in | 0.33 sq ft | 7.1 in | Small supply branch |
| 10 x 8 in | 0.56 sq ft | 9.2 in | Return or larger branch |
| 12 x 8 in | 0.67 sq ft | 10.1 in | Compact trunk section |
| 16 x 10 in | 1.11 sq ft | 13.2 in | Residential main trunk |
| 18 x 12 in | 1.50 sq ft | 15.2 in | Light commercial duct |
| 24 x 12 in | 2.00 sq ft | 17.5 in | Larger return or supply trunk |
The pressure drop in a duct system is the resistance that the air encounters as it moves through the duct system. There are two main forms of resistance within the duct system. Air loses energy to friction against the walls of the duct.
Additionally, air also loses energy to turbulence as it moves through elbows, transitions and other fitting along the duct system. This calculator will allow you to determine the pressure drop within a duct system. This calculator can assist you in determining the resistance that your duct system will create.
How to Use the Duct Pressure Drop Calculator
To calculate the pressure drop within a duct system, the calculator will require you to enter information regarding the system, such as the shape of the duct system, the airflow that will pass through the duct system, the length of the duct, the materials of the duct system, and the types of fitting that is included within that system. Based on these entry fields, the calculator will provide four different output from the calculator. The first output will be the total pressure drop within the system.
The second output will be the friction rate of the system (per 100 feet of duct). The third output will be the velocity of the air within the duct system. The fourth and final output will be the portion of the total pressure drop that the duct fittings cause.
These various outputs will allow you to determine if the planned duct system is compatible with the capabilities of the equipment that will be installed within the duct system. Each of the input fields for the calculator are important in that each field will impact the resistance that is created by the duct system. For instance, the shape of the duct will create different level of resistance than other shapes.
The material used to construct the system will create different levels of resistance than other materials. Additionally, the temperature of the air within the system and the altitude at which the system is to be installed will alter the density of the air within the system. The density of the air will impact the amount of pressure that is required to move the air at the desired volume within the system.
The calculator allows you to set the density of the air to be either automatically corrected for the temperature and altitude entries, or to manually enter the density of the air. Another main cause of pressure drop within duct systems are the fittings themselves. In many cases, people often dont account for the way that air pressure drops within those fittings.
For instance, using elbows within a duct system can lead to significant drop of pressure. Each of the elbows within the system can be entered into the calculator, as well as the type of each elbow. Using long-radius elbows instead of sharp elbows will reduce the pressure drop within the system.
Additionally, using a gradual transition compared to an abrupt transition will lead to a reduction in the pressure drop caused by those transitions. Thus, this entry field will allow you to see the impact of the various type of fittings that you plan to use within your system prior to purchasing the components themselves. The velocity of the air within the system is another output of this calculator.
Determining the velocity that will exist within the system can help to determine how the system will perform. For instance, if the velocity of the air within the system is too high, then high velocities can create noise and increase the friction within the system. If the velocity of the air within the system is too low, then the duct system may be too large for the area within which it is to be installed.
Therefore, this field will allow you to ensure that the velocity within the system is within the targets for different types of duct system component. In addition to the calculations, there are also reference tables that are included within the calculator. One table includes the different types of materials for duct systems and their roughness.
The roughness of the duct system will impact the friction within the system. A second table includes the targets for friction within duct systems and the targets for the velocity of the air within those duct systems. A third table includes the equivalent sizes of round ducts to the sizes of rectangular ducts.
These tables are provided for your convenience to allow for easier comparisons of duct system components. However, these tables are not a replacement for the calculation that this calculator performs. In the real world, there are additional variable to a system than those included in the calculator.
For instance, any flex duct systems that sag between supports may lead to increased resistance of the system. Similarly, the transitions between components in a system may be tighter than those illustrated by the calculator. Finally, components like coils, filters and grilles will also introduce additional restrictions to the movement of air within the duct system.
However, the calculator calculates the pressure drops that the duct system components cause alone. Thus, you can use the calculator to calculate the expected drop of pressure for the duct system alone, and then add the drops of pressure that are caused by these other components of the system. One of the main inputs required of the system is the target friction rate.
For residential heating and cooling systems, a target friction rate of 0.08 inches of water gauge per hundred feet of duct is commonly specified. This target friction rate helps to balance the size of the ducts against the strength (power) of the fan that must move the air through the duct system. The calculator will output the friction rate that is calculated for the system, as well as indicate whether that value is within or outside of the target friction rate.
This helps to ensure that the system will not contain ducts that are either too small (which will create high air velocities and noise) or too large (which may not have enough space within the ceiling or other area in which it is to be installed). The calculator also includes a field for density of the entering air. In many systems, the air will be heated, which will reduce the density of the air within the system.
In other cases, the air may be within a system at high altitudes, where the atmospheric density of the air is less than at sea level. Therefore, you may choose to use the manual entry mode for the density of the air within the system. The calculator will include an automatic calculation of the density of the air if you choose the automatic mode.
The breakdown of the calculations that the calculator performs will show the equivalent round duct size to the rectangular duct size, the Darcy friction factor, the Reynolds number for the duct system, and the velocity pressure within the system. While the length of the duct and the CFM of the system may be similar for two different duct systems, these factors can help to explain why the pressure drop may be different between the two systems. For many people, when they are planning out their duct systems, they often focus upon the length of the straight duct portions of the system.
It is important to recognize that each of the system’s main components (especially those fittings) are often a major source of the total system pressure drop. Therefore, if the drop of pressure caused by the fittings is more significant than the pressure drop caused by the straight duct portions alone, that component of the system may be better modified than the straight duct portions of the system. Additionally, another field within the calculator is the equivalent length of the duct system.
This value represents the length of straight duct that would create the same drop of pressure as the fittings that are within the system. Thus, each of these fields will allow for you to compare the various types of system fittings to each other. Finally, while this calculator can provide you with a good idea of the total pressure drop that will exist within your system based off the parameters that you enter, the numbers that are provided are a preliminary estimate only.
Your final system will also have to account for variables like fire and smoke damper rules, building codes, hood capture velocities and the manufacturer specifications for each of the system components. While the calculator can assist in the initial design of the system, and ensure that you have a general idea of the expected pressure drops along the system, these numbers are not a replacement for a detailed engineering drawing and review of your system. Thus, the calculator allows you to test various assumptions regarding your system prior to ordering the metal for your system.
