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AIRFLOWVENTILATIONDUCT SIZING
Calculate room airflow from dimensions and air changes per hour, then estimate duct size, metric airflow and approximate HVAC tonnage.
CFM symbols and meaning
Cubic feet of air moved each minute.
Length × width × height, in ft³.
How often the room air is replaced each hour.
Air speed through the duct, in ft/min.
Cross-sectional area required for the airflow.
CFM formulas
Room volume V = length × width × height
Required CFM Q = (V × ACH) ÷ 60
Duct area A = Q ÷ duct velocity
Round duct diameter = √(4A ÷ π) × 12
For metric results, 1 CFM is approximately 0.471947 L/s or 1.69901 m³/h. The tonnage comparison uses the common rule of thumb of about 400 CFM per cooling ton; actual equipment airflow varies.
How airflow moves through a room
CFM measures quantity, while velocity describes speed. Two ducts can carry the same CFM at different velocities if their cross-sectional areas differ. Lower residential velocities are often quieter; allowable values depend on duct location, pressure and acoustic goals.
Worked example
A room is 20 ft × 15 ft × 8 ft, needs 6 ACH, uses a 700 ft/min target duct velocity and includes a 10% margin.
V = 20 × 15 × 8 = 2,400 ft³
Base CFM = (2,400 × 6) ÷ 60 = 240 CFM
Design CFM = 240 × 1.10 = 264 CFM
The estimated round duct diameter is about 8.3 inches. Select the next practical duct size only after checking friction loss, fittings, static pressure and equipment performance.
Typical ACH planning ranges
| Space | Illustrative ACH |
|---|---|
| Bedroom / living room | 4–6 |
| Office / classroom | 4–8 |
| Kitchen | 7–12 |
| Bathroom | 8–12 |
| Workshop / garage | 6–12+ |
| Storage area | 2–6 |
These are broad planning ranges, not code requirements. Occupancy, contaminants, moisture and local standards may require a different ventilation method.
Measurement and design checklist
- Measure conditioned inside dimensions.
- Use the actual ceiling height.
- Choose ACH for the space use and contaminant level.
- Separate outdoor ventilation from recirculated HVAC airflow when required.
- Check duct velocity, friction rate and equivalent length.
- Include elbows, filters, dampers and registers in pressure calculations.
- Confirm fan performance at the system’s total static pressure.
Frequently asked questions
What does CFM mean?
CFM means cubic feet per minute. It describes the volume of air passing a point during one minute.
Is ACH the same as CFM?
No. ACH is the number of room-volume replacements per hour. CFM is the minute-by-minute airflow needed to achieve that ACH.
How many CFM are needed per HVAC ton?
About 400 CFM per ton is a common starting point, but sensible heat ratio, humidity, coil design and manufacturer specifications can shift the correct airflow.
Can duct diameter be selected from CFM alone?
No. A complete design also checks velocity, friction loss, duct material, equivalent length, fittings, available static pressure and noise.
Does this calculate bathroom exhaust requirements?
It can estimate airflow by ACH, but local codes may prescribe a minimum intermittent or continuous exhaust rate that takes priority.
Common mistakes
- Confusing CFM with feet per minute.
- Using floor area but forgetting ceiling height.
- Applying a high ACH without checking fan pressure.
- Sizing a duct without accounting for friction and fittings.
- Adding excessive safety margin and creating noise.