Duct CFM Calculator
Enter room dimensions and target air changes per hour to calculate the required airflow in CFM. The result links directly to the duct sizing calculator so you can find the right duct diameter in one step.
Formula
CFM = (ACH × L × W × H) / 60- CFM = cubic feet per minute (airflow required)
- ACH = air changes per hour (target ventilation rate)
- L × W × H = room volume in cubic feet
- ÷ 60 = converts hours to minutes
Suggested duct size uses the ACCA Manual D equal-friction formula at 0.08 in.wg/100ft (default residential).
Common Room CFM Targets
| Room type | Typical size (ft) | ACH | Approx. CFM | Duct size |
|---|---|---|---|---|
| Small bedroom | 10 × 10 × 9 | 6 | 90 | 7" |
| Master bedroom | 14 × 16 × 9 | 6 | 202 | 9" |
| Living room | 18 × 20 × 9 | 8 | 432 | 12" |
| Kitchen | 12 × 14 × 9 | 10 | 252 | 10" |
| Bathroom | 8 × 10 × 9 | 10 | 120 | 8" |
Approximate values for planning. Actual design requires ACCA Manual J load calculation.
Related Tools
- Duct sizing calculator — CFM to round/rectangular duct size
- Duct sizing chart — quick reference table
- Flex duct sizing chart
- Return air duct sizing calculator
Frequently Asked Questions
ACCA Manual D residential guidelines: bedrooms 4–6 ACH, living areas 6–8 ACH, kitchens 10–15 ACH (due to heat gain), bathrooms 8–10 ACH. These are starting estimates — a full Manual J load calculation accounts for insulation, windows, solar gain, and local climate.
ACH stands for Air Changes per Hour — how many times the total air volume of a room is replaced with conditioned air in one hour. Higher ACH means more air movement, faster temperature recovery, and better humidity control, but requires larger ductwork.
ACH = (CFM × 60) / Room volume (ft³). Conversely, CFM = (ACH × Volume) / 60. This calculator uses the CFM formula so you can find the required duct size directly.
Use the average ceiling height. For a room with a flat section at 8 ft and a vault to 12 ft, the average is roughly 10 ft. Alternatively, measure or estimate the actual room volume and work backwards: enter a height that gives you the correct cubic footage.