Duct Size to CFM Calculator
Convert HVAC duct diameter to CFM airflow capacity.
Calculate the right duct size for heating and cooling systems by room size.
Enter the room size or select a duct diameter to see airflow capacity and sizing recommendations.
Understanding HVAC Duct Sizing
HVAC duct size directly determines how much conditioned air can flow to each room. An undersized duct restricts airflow, making the room too hot or too cold and forcing the system to work harder. An oversized duct wastes material and can cause noise issues from low air velocity. Proper duct sizing balances adequate airflow with reasonable air velocity and acceptable noise levels for the space being served.
Duct Diameter to CFM Capacity (at 900 FPM velocity):
| Round Duct Diameter | Area (sq in) | CFM Capacity | Metric Equivalent |
|---|---|---|---|
| 4 in (100 mm) | 12.6 | 79 CFM | 37 L/s |
| 5 in (125 mm) | 19.6 | 123 CFM | 58 L/s |
| 6 in (150 mm) | 28.3 | 177 CFM | 84 L/s |
| 7 in (175 mm) | 38.5 | 240 CFM | 113 L/s |
| 8 in (200 mm) | 50.3 | 314 CFM | 148 L/s |
| 9 in (225 mm) | 63.6 | 397 CFM | 187 L/s |
| 10 in (250 mm) | 78.5 | 491 CFM | 232 L/s |
| 12 in (300 mm) | 113.1 | 707 CFM | 334 L/s |
| 14 in (350 mm) | 153.9 | 962 CFM | 454 L/s |
| 16 in (400 mm) | 201.1 | 1,257 CFM | 593 L/s |
Air Velocity Guidelines:
The standard design velocity for residential supply ducts is 600 to 900 feet per minute (FPM), which equals 3 to 4.6 meters per second. Higher velocities move more air through smaller ducts but create more noise. Lower velocities are quieter but require larger ducts.
- Main trunk line: 700 - 900 FPM (3.6 - 4.6 m/s)
- Branch ducts: 600 - 800 FPM (3.0 - 4.1 m/s)
- Supply registers: 500 - 700 FPM (2.5 - 3.6 m/s)
- Return air ducts: 500 - 700 FPM (2.5 - 3.6 m/s)
- Near bedrooms: Under 600 FPM (3.0 m/s) for quiet operation
Room CFM Requirements:
The general rule is 1 CFM per square foot of floor area (5.1 L/s per square meter) for standard 8-foot (2.4-meter) ceilings. Rooms with higher ceilings, large windows, or poor insulation need more.
| Room Type | CFM per sq ft | CFM per sq m | Example (150 sq ft room) |
|---|---|---|---|
| Standard bedroom | 1.0 | 5.1 | 150 CFM |
| Living room | 1.0 - 1.2 | 5.1 - 6.1 | 150 - 180 CFM |
| Kitchen | 1.2 - 1.5 | 6.1 - 7.6 | 180 - 225 CFM |
| Bathroom | 1.0 | 5.1 | 50 - 80 CFM (exhaust) |
| Sunroom / lots of glass | 1.5 - 2.0 | 7.6 - 10.2 | 225 - 300 CFM |
Rectangular Duct Equivalents:
If using rectangular ducts instead of round, you need to find the equivalent round diameter. A common rectangular-to-round conversion: a 10 x 8 inch rectangular duct is equivalent to approximately a 10-inch round duct. The formula for equivalent diameter is:
De = 1.3 x (a x b)^0.625 / (a + b)^0.25
Where a and b are the rectangular duct dimensions in inches.
Friction Loss:
Longer duct runs require larger ducts to overcome friction. Standard residential design uses 0.08 to 0.10 inches of water column friction loss per 100 feet of duct. Each elbow adds approximately 10 to 15 equivalent feet of straight duct to the friction calculation. A typical residential branch run of 25 to 50 feet with two elbows has an equivalent length of about 45 to 80 feet. For runs over 50 equivalent feet, consider increasing the duct by one size.