Duct Friction Loss Calculator
Estimate duct pressure loss from CFM, duct dimensions, run length, fitting equivalent length, material roughness, air density, velocity pressure, and a target friction rate.
1.Pick a duct scenario
2.Enter airflow and duct data
Duct friction results
3.Duct and material spec grid
4.Reference tables
| Duct Material | Roughness | Multiplier | Best Use |
|---|---|---|---|
| Smooth PVC/plastic | 0.00006 in | 1.00x | Short exhaust sleeves, specialty smooth duct |
| Smooth aluminum | 0.0024 in | 1.00x | Light exhaust duct and short appliance runs |
| Galvanized steel | 0.0060 in | 1.00x | Common HVAC trunks and branches |
| Spiral metal | 0.0048 in | 0.96x | Longer mains where smooth joints help |
| Duct board | 0.0180 in | 1.12x | Low velocity residential trunks |
| Internally lined duct | 0.0300 in | 1.18x | Sound-sensitive return or supply sections |
| Flexible metal | 0.0120 in | 1.80x | Short connections, fully stretched |
| Flexible plastic | 0.0240 in | 2.40x | Short final runs with generous diameter |
| Duct Role | Typical Velocity | Quiet Target | Friction Aim |
|---|---|---|---|
| Bedroom supply branch | 500-700 fpm | Lower noise | 0.06-0.10 |
| Living area supply | 600-900 fpm | Balanced throw | 0.08-0.12 |
| Return branch | 500-800 fpm | Grille quietness | 0.05-0.09 |
| Main supply trunk | 700-1100 fpm | Lower trunk rumble | 0.05-0.10 |
| Bathroom exhaust | 600-1000 fpm | Fan curve match | 0.08-0.18 |
| Kitchen exhaust | 900-1500 fpm | Capture and grease | 0.12-0.30 |
| Fitting | Equivalent Length | Use In Field | Loss Note |
|---|---|---|---|
| Long radius 90 elbow | 8-15 ft | Supply or return | Lower loss than sharp elbows |
| Short radius 90 elbow | 15-30 ft | Tight branches | Use sparingly near fans |
| 45 degree elbow | 4-10 ft | Offset changes | Two may beat one tight 90 |
| Register boot | 10-25 ft | Terminal connection | High loss if neck is small |
| Wye or takeoff | 10-35 ft | Branch split | Smoother entry lowers turbulence |
| Backdraft damper | 15-45 ft | Exhaust runs | Pressure varies by blade design |
| Example Run | Airflow | Duct | Target Rate |
|---|---|---|---|
| Small bath exhaust | 80 CFM | 5 in round | 0.10-0.18 |
| Bedroom supply | 120 CFM | 6 in round | 0.06-0.10 |
| Kitchen hood duct | 400 CFM | 8 in round | 0.12-0.25 |
| Media room return | 650 CFM | 14 x 8 in | 0.05-0.09 |
| Open plan supply | 900 CFM | 16 x 8 in | 0.06-0.10 |
| Whole house trunk | 1200 CFM | 16 in round | 0.05-0.08 |
5.Practical friction loss tips
Ducts are not like water running down a slick tube. Air isn’t liquid, it’s thick; it sticks to duct surfaces which are typicaly plastic or metal, rough on the inside. Air swirls about elbows chaotically. This use up energy and makes noise in your livig room.
If you’ve noticed your HVAC system sounding louder then usual (or if its output seems weaker), it’s most often not because of the fan. It’s friction. Before you cut any metal, the calculator above will run the math for you; so you know where energy is going.
How Air Moves Through Ducts
When air travels through a duct it collides with sides and doesn’t flow very well. That resistance that creates an effect is called pressure drop. Straight ducts resists air flow by their inner diameter and roughness. However, fittings are where issue arises. Tees, elbows, boots and dampers all presents obstacles to your air flow. A single 90 degree elbow can equals up to fifteen or even twenty feet of straight duct work.
To get good measurements, you have to account for equivalent length. This means you must add the actual distance plus extra virtual feet for each transition and turn in the system. This is why many folks gets away with purchasing a 50-foot blower but only put in 40 feet (plus six elbows behind the wall) and think they’re good to go. Seventy feet of resistance kill the airflow entirely.
The other factor here is nature of material itself. Rigid PVC or smooth spiral metal won’t impede airflow very much at all. Standard galvanized sheet metal has no surprises. But flex duct? That’s different. Even if flex duct is stretched taut, those internal ribs interrupt airflow, creating effect of being twice or even three times as long as stated. Pulling flex around hard turns and allowing it to droop in corners throttles your own system. Throttle back the thermostat if you want to save some coin; you’re blocking air from reaching the room anyway. This is a small consideration, but it can throw off a complex whole-house system.
The chart on the page show the impact of material roughness on the friction multiplier. You should also monitor another number called velocity pressure. This indicates how hard the air is blowing. Higher velocity pressure mean louder noise. If your velocity pressure jumps, it indicates your ducts is pushing a lot of air. This means you have too much air going into a given duct and you need a bigger one. If this occurs, you’ll hear whooshing from all your registers.
In contrast, lower velocity result in less friction loss, making it quieter and more efficient. Often, the big trunk returns are bigger (wider) than the supply branches because they’re carrying tons of air very slowly so your house stay nice and quiet. Take note as you play with duct size: What effect does that have on the velocity pressure? And what effect does it have on the friction rate?
Remember: A bigger duct means higher material costs and more ceiling space is needed. However, it also means a lower static pressure load on your blower motor which leads to better efficiency.
At high altitudes, the air is less dense than at sea level. Temperature also affects air density. The colder the air, the more dense it gets; warmer air is less dense. That changes the way the air flow through those same pipes. The hot exhaust air coming off the fan (from the kitchen or bathroom) will be less dense (lighter) than the surrounding air and behave different. Density compensation is used in every calculation. This makes sure you do not oversize fans so they never reach their rated output, and you do not undersize fans so they gets too hot and die early.
“There’s never one right answer when it comes to designing airflows, you can’t get everything you want: low cost, tight spaces, minimum noise and max airflow. You can only prioritize two things. The calculator lets you input your number of fittings and CFM requirement to show you where the rubber meets the road. If the numbers come out poorly, time to go back to the drawing board. See if you can increase the duct sizes, avoid adding an elbow somewhere, smooth out some of the runs, or change materials… But just remember, air wants to resist moving; so plan for that resistance in your design, as there aren’t any easy fixes after drywall goes up. And manage expectations before starting construction.”
