Chart
Friction rate chart & calculator
The friction rate is the single input that sinks most duct designs, because people treat it as a constant. It isn't. Derive it properly from your blower's Available Static Pressure and the run's Total Effective Length, then size with it.
Derive the friction rate (Manual D)
Enter ASP and TEL to get the design friction rate, then size duct against it.
| Size \ CFM | 100 | 200 | 400 | 800 | 1,200 |
|---|---|---|---|---|---|
| 6" | 0.082 | 0.290 | 1.05 | 3.95 | 8.65 |
| 8" | 0.020 | 0.071 | 0.252 | 0.924 | 2.00 |
| 10" | 0.007 | 0.024 | 0.084 | 0.304 | 0.651 |
| 12" | 0.003 | 0.010 | 0.035 | 0.123 | 0.262 |
| 14" | 0.001 | 0.005 | 0.016 | 0.058 | 0.122 |
| 16" | 0.001 | 0.003 | 0.009 | 0.030 | 0.063 |
Read down to your duct size and across to the airflow. Cells well above ~0.10 mean a duct that's small for the air it carries — fast and loud. The velocity chart shows the matching air speeds.
What the friction rate really is
Friction rate is the pressure a duct may lose per 100 feet of equivalent length,
in inches of water column. ACCA Manual D defines it as
Friction Rate = (ASP ÷ TEL) × 100,
where Available Static Pressure is what the blower has left for the duct after
the coil, filter, and registers take their share, and Total Effective Length adds
the equivalent length of every fitting to the longest run's straight footage.
Fittings usually dominate the Total Effective Length, which is why a correctly derived friction rate is often well below the 0.10 rule of thumb. Sizing every duct to a too-high friction rate quietly undersizes the whole system, driving up velocity, noise, and static pressure. Use the value you derive here in the console or the CFM-to-duct-size chart, and confirm the resulting velocity stays sensible on the velocity chart.