Friction loss is the heart of duct sizing: every foot of duct steals a little static pressure from your blower. This calculator solves the real fluid-dynamics equations to give friction loss per 100 feet of round galvanized duct, and warns you when a run is moving air too fast.
How it works
The tool computes velocity from airflow and area, finds the Reynolds number, and solves the Colebrook-White equation for the friction factor before applying Darcy-Weisbach:
velocity (FPM) = CFM / duct_area
Re = velocity_fps × diameter_ft / kinematic_viscosity
1/√f = -2 log10( ε / (3.7 D) + 2.51 / (Re √f) )
ΔP per 100 ft = f × (100 / D) × (ρ × V² / 2), converted to in. w.c.This is the same physics behind the equal-friction duct charts, computed directly rather than read off a curve.
Example
400 CFM through 8-inch round duct runs about 1150 FPM with roughly 0.13 in. w.c. of loss per 100 feet — a bit fast and lossy for residential, so bumping to 9 or 10 inch duct brings velocity and friction back into the comfortable range. Size every run to a consistent friction rate, add equivalent lengths for fittings, and total the worst-case path to pick a blower with enough external static pressure.
Choosing a design friction rate
The friction rate you choose determines the trade-off between duct size and system static pressure. Common design targets:
| System type | Typical design friction rate |
|---|---|
| Residential supply (low noise priority) | 0.06–0.08 in. w.c./100 ft |
| Residential supply (balanced) | 0.08–0.10 in. w.c./100 ft |
| Light commercial | 0.10–0.15 in. w.c./100 ft |
| Commercial VAV / high-pressure | 0.15–0.25 in. w.c./100 ft |
Using a higher friction rate shrinks duct diameter and reduces material cost but increases the blower’s required external static pressure, which means a larger, louder, or less efficient blower. Keeping all branches near the same friction rate also helps with natural balancing — if one branch runs at 0.05 and another at 0.15, the system will never be in balance without dampers.
Adding fitting losses
This tool gives friction for straight duct only. Every fitting adds loss expressed as an “equivalent length” of straight duct:
- Standard 90° elbow: roughly 15–30 ft equivalent length depending on radius
- Tee (straight-through): 10–20 ft
- Tee (branch): 20–50 ft
- Register boot: 15–25 ft
Add the sum of all fitting equivalent lengths to the physical measured length of the run, then multiply by the friction rate per 100 feet to find total straight-plus-fitting head loss for that branch. Sum the longest path (highest total head loss) to select the blower.