How do you calculate friction loss in a duct?

Duct friction loss uses the Darcy-Weisbach equation: dP = f x (L/D) x (rho x V^2 / 2), where f is the friction factor, L is duct length, D is hydraulic diameter, rho is air density, and V is air velocity. For practical HVAC use, this simplifies to: friction rate (IWG/100ft) = 0.0307 x Q^1.852 / (D^4.973) for round ducts, where Q is flow in CFM and D is diameter in inches. This calculator applies the full Darcy-Weisbach formula with the Colebrook-White friction factor for accurate results.

What is the friction rate for residential duct systems?

Standard duct design practice recommends design friction rates of 0.08 to 0.15 IWG per 100 feet for residential duct systems. Lower friction rates (0.08-0.10) result in quieter, more efficient systems with larger duct sizes. Higher rates (0.12-0.15) are used when space constraints limit duct size. The actual friction rate for a specific duct is found by dividing available static pressure by the total effective length of the duct run. Use the static pressure calculator to find available static pressure, then set the friction rate accordingly.

📈 Duct Friction Loss

Friction Loss Calculator

Calculate pressure drop per 100 feet and total pressure drop for round and rectangular ducts. Uses the Darcy-Weisbach method with Colebrook-White friction factors. Verify existing duct runs or confirm new designs.

Unit System:

🌸 Duct Type

📈 Duct Parameters

Airflow *

CFM

Duct Diameter *

Width *

Height *

Duct Length

Duct Roughness i

📈 Friction Loss Results

Parameter	Value	Note

Export:

Duct friction loss fundamentals

Every foot of duct creates pressure drop as air molecules collide with the duct wall. Faster air creates more friction — velocity affects friction loss as velocity squared. A duct carrying twice the CFM doesn't just lose twice the pressure; it loses four times the pressure. This is why oversized ducts are dramatically more efficient: cutting velocity in half reduces friction loss to one-quarter.

Darcy-Weisbach equation

The Darcy-Weisbach equation is the standard for accurate duct friction calculations: dP/L = f x (1/D) x (rho x V^2 / 2). The friction factor f depends on Reynolds number and duct roughness, calculated using the Colebrook-White equation. This calculator iterates to solve the implicit Colebrook-White equation for accurate friction factors across all flow regimes — laminar, transitional, and turbulent.

Round vs. rectangular friction loss

Round ducts are more efficient than rectangular ducts because they have no corners where turbulence intensifies. For rectangular ducts, the hydraulic diameter (D_h = 2HW/(H+W)) is used in place of actual diameter. Aspect ratios above 4:1 become significantly less efficient and should be avoided. Use the rectangular to round calculator to find the equivalent round diameter.

Frequently Asked Questions

How does flexible duct affect friction loss?+

Flexible duct has a much higher roughness than sheet metal — about 10 times higher when fully extended. When compressed or kinked, friction loss increases dramatically: a flex duct compressed to 70% of its extended length has 3-4 times the pressure drop of the same duct fully extended. Standard duct design practice requires flex duct to be fully extended and supported to prevent sagging. Never use flex duct for runs longer than 6 feet. Use this calculator with the "Flexible duct" roughness setting and compare to sheet metal to see the difference.

What friction rate means a duct is sized correctly?+

A duct is sized correctly when its friction rate (IWG/100ft) matches the design friction rate for the system. All ducts in the system should be sized at the same friction rate so they deliver their design CFM simultaneously without balancing dampers. If a specific duct's friction rate is much lower than the design rate, it is oversized — which wastes material but works fine. If it's higher, the duct is undersized and will not deliver design CFM, causing comfort problems in that room.