Flex Duct Static Pressure Calculator
Estimate flex duct pressure drop from diameter, airflow, actual stretch, bends, compression, liner roughness, and grille or filter losses.
▶Flex Duct Presets
Load a common branch or return run, then adjust the stretch and fitting details to match the installed duct.
⚙Duct Inputs
Calculation Breakdown
▦Flex/Duct Grid
▣Diameter and Velocity Reference
| Duct diameter | Area sq ft | Comfort CFM at 700 fpm | High CFM at 900 fpm |
|---|---|---|---|
| 4 in round flex | 0.087 | 61 CFM | 79 CFM |
| 5 in round flex | 0.136 | 95 CFM | 123 CFM |
| 6 in round flex | 0.196 | 137 CFM | 177 CFM |
| 7 in round flex | 0.267 | 187 CFM | 241 CFM |
| 8 in round flex | 0.349 | 244 CFM | 314 CFM |
| 10 in round flex | 0.545 | 382 CFM | 491 CFM |
▣Stretch and Installation Penalties
| Installed condition | Typical input | Friction effect | Use when |
|---|---|---|---|
| Pulled tight and supported | 95-100% stretch | Lowest flex loss | Good straps and straight path |
| Normal attic branch | 85-92% stretch | Moderate loss | Some waves or gentle sag |
| Loose or serpentine run | 70-84% stretch | High loss | Extra length left in place |
| Compressed or pinched | 1.45x to 1.75x sag | Very high loss | Crushed insulation or oval duct |
▣Bend Equivalent Length Guide
| Bend quality | Equivalent length | Example for 6 in duct | Example for 8 in duct |
|---|---|---|---|
| Broad sweep | 6 diameters | 3.0 ft each | 4.0 ft each |
| Normal sweep | 10 diameters | 5.0 ft each | 6.7 ft each |
| Tight bend | 16 diameters | 8.0 ft each | 10.7 ft each |
| Kinked turn | 24 diameters | 12.0 ft each | 16.0 ft each |
▣Loss Component Reference
| Component | Typical range | What raises it | Calculator field |
|---|---|---|---|
| Flex duct friction | 0.04-0.25 in.wg | High CFM, small duct, loose stretch | Diameter, CFM, length |
| Supply grille or boot | 0.03-0.12 in.wg | Small face area or restrictive pattern | Grille and boot loss |
| Return grille/filter | 0.05-0.35 in.wg | Loaded filter or high face velocity | Filter/accessory loss |
| Design margin | 5-20% | Unknown fittings or field variation | Design margin |
ℹPractical Tips
Before you resize or balance your HVAC branch, try this calculator: Use a flex-duct static-pressure calculator. It will help you compare accessory losses, compression, bend losses, stretch, airflow, and more by diameter. You turn on the air handler and the room at the end of the run still never gets cool enough.
…yet the room never quite gets cool enough. Usualy that means static pressure inside the flex duct is higher then it should be. Here’s how to understand that resistance so you can fix the system.
How to Fix Your HVAC Flex Duct Problems
Duct with a flex section will perform different than all metal duct with smooth interior. Technicians will walk on it and they will move it in and around joist bays. Each sag and each wrinkle creates another place for turbulence to occur.
The more turbulence, the more pressure the blower has to fight against. Pulling the flex duct nice and tight eliminates much of that penalty. Friction can easily jump 30; 40 % if the duct is left loose or sagging.
Airflow speed depends on diameter. Smaller diameter = higher speed. Higher speed = louder noise at register.
Is your speed comfortable? Feed the calculator your installed diameter and target CFM, and the calculator will return whether the air is moving too fast or not. Also high speed will push dirt into the filter.
Resistance increases with bends. A small duct bent into a tight kink is equivalent to having more duct of that size. Each bend adds equivalent length, so three or four tight bends can turn a 25-foot duct into 45 effective feet.
Use the calculator to try various bend radii. Rerouting one sharp turn into a broader sweep reduces pressure drop noticeable. Sharp turns is best avoided where practical.
When compressed, the duct shape are altered. Its no longer round but oval. Oval is much more resistant.
The same holds for crushed insulation. Hard to spot when you install drywall. Tight bends can double the pressure drop in a loosely hanging duct.
Inspect it all with a flashlight. Additional losses (Filters), Grilles
Dirty filter uses up static pressure. Restrictive grille reduces air flow.
Both of these losses increase as the filter collects more dirt. Other accessories such as furnitures in front of registers also restricts air flow. Accessory losses need to be tracked independently.
Why? So you dont blame the ductwork for what is happening at the grille. There are unknowns in installations.
Length you can measure; but there could be leakage. Margin for design allow for the uncertainty. Normal variation is covered by a 10% margin.
Older homes does better with 20% margin. The difference between what’s on paper and how it is in real life is protected by margin. The system involve air velocity (which has to be adjusted for different lengths), friction (which depends on duct size), and more power from the blower.
More air flow into one room will create turbulence elsewhere. Test options prior to construction. The calculator allows you to adjust the percentage of stretching to see whether the larger diameter is worth the extra material and tighter clearances.
The most widely used material for air movement is flex duct. Flex duct is great if handled well. Keep it as straight as possible, support it every five feet, and avoid sharp turns.
The equipment will run correctly as designed with proper installation. If installed poorly, the blower has to work harder. Use the information in the calculator to guide you to making the correct decisions.
