Duct size calculator
Enter the airflow and the design friction rate. The calculator returns the round duct diameter, the rectangular duct with the same resistance, and the air velocity, using the same equation as a paper ductulator.
- Round duct10 in
- Exact diameter9.8 in
- Rectangular, 8 in high8 × 11 in
- Velocity in 10 in round730 ft/min
A 10 in round duct carries 400 CFM at 0.092 in. w.c. per 100 ft, at or under the 0.10 design rate.
Figures are for smooth galvanized sheet-metal duct. Flexible duct has more resistance, and much more when it is compressed or sagging, so it needs a larger size for the same airflow.
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Duct size chart by CFM
Round sheet-metal duct sizes for common airflows, at the two friction rates most often used in residential design. The rectangular column is an 8-inch-high duct with the same resistance as the round size at 0.10.
| Airflow (CFM) | Round at 0.08 | Round at 0.10 | Rectangular at 0.10 |
|---|---|---|---|
| 50 | 5 in (4.7) | 5 in (4.5) | 8 × 3 in |
| 75 | 6 in (5.5) | 6 in (5.2) | 8 × 4 in |
| 100 | 7 in (6.1) | 6 in (5.8) | 8 × 4 in |
| 125 | 7 in (6.6) | 7 in (6.3) | 8 × 5 in |
| 150 | 8 in (7.1) | 7 in (6.8) | 8 × 5 in |
| 200 | 8 in (7.9) | 8 in (7.6) | 8 × 7 in |
| 250 | 9 in (8.6) | 9 in (8.2) | 8 × 8 in |
| 300 | 10 in (9.2) | 9 in (8.8) | 8 × 9 in |
| 400 | 12 in (10.3) | 10 in (9.8) | 8 × 11 in |
| 500 | 12 in (11.2) | 12 in (10.7) | 8 × 13 in |
| 600 | 12 in (12.0) | 12 in (11.5) | 8 × 14 in |
| 800 | 14 in (13.4) | 14 in (12.8) | 8 × 18 in |
| 1,000 | 16 in (14.5) | 14 in (13.9) | 8 × 22 in |
| 1,200 | 16 in (15.6) | 16 in (14.9) | 8 × 26 in |
| 1,400 | 18 in (16.5) | 16 in (15.8) | 8 × 29 in |
| 1,600 | 18 in (17.4) | 18 in (16.6) | 8 × 33 in |
| 2,000 | 20 in (18.9) | 20 in (18.1) | 8 × 40 in |
The figure in brackets is the exact diameter from the equation. The size in front of it is the next stocked round size.
Main trunk size by system tonnage
A cooling system moves about 400 CFM for each ton of capacity. That gives the airflow the main supply trunk must carry at the air handler.
| System size | Airflow | Round trunk at 0.10 | Velocity |
|---|---|---|---|
| 1.5 tons | 600 CFM | 12 in | 760 ft/min |
| 2 tons | 800 CFM | 14 in | 750 ft/min |
| 2.5 tons | 1,000 CFM | 14 in | 940 ft/min |
| 3 tons | 1,200 CFM | 16 in | 860 ft/min |
| 3.5 tons | 1,400 CFM | 16 in | 1,000 ft/min |
| 4 tons | 1,600 CFM | 18 in | 910 ft/min |
| 5 tons | 2,000 CFM | 20 in | 920 ft/min |
How the calculator works
The calculator uses the equal-friction method, the same one built into a cardboard or plastic ductulator. Every section of duct is sized so that it loses the same pressure per 100 feet of length.
For round galvanized duct carrying air at standard conditions, the friction loss is:
friction = 0.109136 × CFM1.9 ÷ diameter5.02
with friction in inches of water column per 100 feet and the diameter in inches. The calculator solves this for the diameter. This is the equation that reproduces the friction chart in the ASHRAE Handbook of Fundamentals.
A rectangular duct is matched to a round one with the Huebscher equivalent-diameter equation, also from the ASHRAE handbook:
equivalent diameter = 1.30 × (a × b)0.625 ÷ (a + b)0.25
where a and b are the two sides in inches. A rectangular duct needs more cross-section than the round duct it replaces, because it has more wall for the same area.
Choosing the friction rate
The friction rate is not a free choice. It follows from the blower: take the external static pressure the blower can deliver, subtract the pressure lost in the filter, the coil, the registers and other components, and divide what is left over the total effective length of the longest duct run. That calculation is ACCA Manual D.
Manual D expects the result to fall between 0.06 and 0.18 inches of water per 100 feet. Outside that band the blower and the duct layout do not suit each other. When the numbers are not available, 0.10 is a common starting value. A lower rate gives larger, quieter ducts and leaves more margin for a restrictive filter.
Limits of this calculator
- Flexible duct is not sheet metal. The equation assumes smooth galvanized steel. Flex duct has more resistance even when pulled tight, and far more when it sags or is compressed.
- Fittings count. Each elbow, tee and boot adds resistance equal to many feet of straight duct. The friction rate must already account for them.
- Velocity sets the noise. If the velocity shown is high for a living space, choose the next size up.
- A branch is sized for its own room. Use the airflow each room needs, not the system total divided evenly.
Sources
- ASHRAE Handbook of Fundamentals, duct design chapter: the friction chart for round galvanized duct and the equivalent-diameter equation for rectangular duct.
- ACCA, Manual D residential duct design: https://www.acca.org/standards/technical-manuals/manual-d
- ACCA, how Manual D derives the friction rate and its 0.06 to 0.18 range: https://hvac-blog.acca.org/definitive-design-accas-manual-d-revised/
Related
To find the airflow first, use the CFM calculator. To size the equipment the ducts serve, use the AC tonnage calculator or the furnace size calculator.
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Calculator last reviewed on October 5, 2026. Method and sources.