Enter the required run airflow and the design friction budget in inches water per 100 ft. ACCA Manual D derives friction rate from available static pressure and total effective length; it is not automatically 0.1 for every house. The equation here is an approximate smooth round-duct relationship, not a complete Manual D design or a rule for flexible duct installation.
Fittings such as elbows, tees and dampers add resistance that is accounted for using equivalent lengths, and flex duct with sags performs worse than the friction rate suggests. Total external static pressure sets the maximum friction rate. Design should follow a Manual D duct design, which this tool only approximates.
Round duct diameter (inches) from airflow
| Airflow | 400 |
|---|---|
| Friction rate | 0.1 |
| Round duct diameter (inches) | 9.8 |
| Duct cross-sectional area (sq in) | 75.8 |
Common error: Picking a duct by the size of the register. The airflow and the length of the run set the size, and a large register on a small duct starves the room.
Duct Sizing: formula
ASHRAE Handbook; Fundamentals · ACCA: Friction rate is not a constant
Duct Sizing: changing Airflow
| Airflow | Round duct diameter (inches) | Duct cross-sectional area (sq in) |
|---|---|---|
| 320 | 9 | 64.1 |
| 360 | 9.4 | 70 |
| 400 (example) | 9.8 | 75.8 |
| 440 | 10.2 | 81.5 |
| 480 | 10.5 | 87.1 |
Duct Sizing: compare input ranges
| Input | Tested from | To | Round duct diameter (inches): low input | Round duct diameter (inches): high input | Swing |
|---|---|---|---|---|---|
| Airflow | 360 | 440 | 9.4 | 10.2 | 0.745 (7.6%) |
| Friction rate | 0.09 | 0.11 | 10 | 9.6 | 0.393 (4.0%) |
Duct Sizing: individual input comparisons
| Input | Value | Round duct diameter (inches) | Duct cross-sectional area (sq in) |
|---|---|---|---|
| Airflow | 320 | 9 | 64.1 |
| 360 | 9.4 | 70 | |
| 400 | 9.8 | 75.8 | |
| 440 | 10.2 | 81.5 | |
| 480 | 10.5 | 87.1 | |
| Friction rate | 0.08 | 10.3 | 82.9 |
| 0.09 | 10 | 79.1 | |
| 0.1 | 9.8 | 75.8 | |
| 0.11 | 9.6 | 73 | |
| 0.12 | 9.5 | 70.5 |
Round duct diameter (inches): Airflow against Friction rate
| Airflow / Friction rate | 0.08 | 0.1 | 0.12 |
|---|---|---|---|
| 320 | 9.4 | 9 | 8.7 |
| 360 | 9.9 | 9.4 | 9.1 |
| 400 | 10.3 | 9.8 | 9.5 |
| 440 | 10.7 | 10.2 | 9.8 |
| 480 | 11 | 10.5 | 10.2 |
Duct Sizing: values and definitions
| Quantity and definition | Value used |
|---|---|
| Airflow — Required airflow, in cubic feet per minute. | 400 |
| Friction rate — Design friction rate, in inches of water per 100 feet of duct (0.1 is a common equal-friction design value). | 0.1 |
| Round duct diameter (inches) | 9.8 |
| Duct cross-sectional area (sq in) | 75.8 |
Airflow: half and double
| Airflow | Round duct diameter (inches) |
|---|---|
| 200 | 7.6 |
| 400 | 9.8 |
| 800 | 12.8 |
A duct has to carry the air a room needs without more friction than the fan can overcome. At 400 cubic feet per minute and a friction rate of 0.1 inch of water per 100 feet, the round duct comes to about 9.8 inches, with a cross-section of about 75.8 square inches. A smaller duct raises the velocity, the noise and the pressure the fan must supply, while a bigger one costs more and takes room in the ceiling.
The friction rate is a design choice that balances the two.
Choose the friction rate from the fan's available static pressure and the total run length, and add the loss from fittings, which can be as large as the straight duct.
See total pressure on the static pressure calculator, airflow for room changes on the CFM and air changes page and cooling needs on the HVAC tonnage calculator. The SEER upgrade savings calculator covers efficiency, and the circle and rectangle area pages handle duct cross-sections.
Does a 9.827-inch result establish a nominal duct selection?
No. It is an approximate equation result. Material, fittings, noise, available pressure and actual product sizes need separate design checks.
Does 400 CFM imply one ton of cooling here?
No. Airflow and equipment capacity must follow the actual load and manufacturer’s operating requirements; this tool does not establish a universal per-ton airflow.