CFM Calculator: Airflow Requirements for Every Room
What Is CFM?
CFM (Cubic Feet per Minute) measures the volume of air flowing through a space per minute. In HVAC, CFM determines how much conditioned air reaches each room, directly affecting comfort, air quality, and energy efficiency.
Key relationships:
- 1 CFM = 1.699 m³/h (cubic meters per hour)
- 400 CFM per ton of cooling (standard rule)
- 1 CFM/sq ft = 3.28 L/s/m²
CFM Requirements by Room Type
| Room Type | CFM/sq ft | Min CFM (small room) | Standard |
|---|---|---|---|
| Living Room | 1.0 | 100 CFM | ASHRAE 62.2 |
| Bedroom | 0.5–1.0 | 75 CFM | ASHRAE 62.2 |
| Kitchen (range hood) | N/A | 100 CFM intermittent | ASHRAE 62.2 |
| Bathroom (exhaust) | N/A | 50 CFM continuous / 80 CFM intermittent | ASHRAE 62.2 |
| Laundry Room | N/A | 100 CFM | IRC M1505 |
| Garage | 1.0 (if conditioned) | 100 CFM | IMC 403 |
| Office | 0.5–1.0 | 50 CFM/person | ASHRAE 62.1 |
CFM Per Ton of AC
The standard airflow for cooling systems is 400 CFM per ton (12,000 BTU/h). This is the nominal airflow across the evaporator coil.
| AC Size | BTU/h | Standard CFM | Acceptable Range |
|---|---|---|---|
| 1 ton | 12,000 | 400 | 350–450 |
| 1.5 ton | 18,000 | 600 | 525–675 |
| 2 ton | 24,000 | 800 | 700–900 |
| 2.5 ton | 30,000 | 1,000 | 875–1,125 |
| 3 ton | 36,000 | 1,200 | 1,050–1,350 |
| 4 ton | 48,000 | 1,600 | 1,400–1,800 |
| 5 ton | 60,000 | 2,000 | 1,750–2,250 |
Note: In humid climates, use 350 CFM/ton for better dehumidification. In dry climates, up to 450 CFM/ton is acceptable.
How to Calculate CFM
Method 1: Area-Based (Simple)
CFM = Area (sq ft) × CFM/sq ft
Example: 250 sq ft living room × 1.0 CFM/sq ft = 250 CFM
Method 2: Air Changes Per Hour (ACH)
CFM = (Room Volume × ACH) ÷ 60
Example: 250 sq ft × 8 ft ceiling = 2,000 cu ft. Target 6 ACH.
CFM = (2,000 × 6) ÷ 60 = 200 CFM
Method 3: Duct Velocity
CFM = Velocity (FPM) × Duct Area (sq ft)
Example: 600 FPM through a 10" × 8" duct (0.56 sq ft).
CFM = 600 × 0.56 = 336 CFM
CFM Chart by Duct Size
| Duct Size (inches) | Area (sq in) | CFM @ 500 FPM | CFM @ 700 FPM | CFM @ 900 FPM |
|---|---|---|---|---|
| 6" round | 28.3 | 98 | 138 | 177 |
| 8" round | 50.3 | 175 | 245 | 314 |
| 10" round | 78.5 | 273 | 382 | 490 |
| 12" round | 113.1 | 393 | 550 | 706 |
| 4×10 rectangular | 40 | 139 | 194 | 250 |
| 6×10 rectangular | 60 | 208 | 292 | 375 |
| 8×12 rectangular | 96 | 333 | 467 | 600 |
Common CFM Mistakes
- Undersizing return air: A 3-ton system needs ~1,200 CFM. A single 14" return duct only delivers ~500 CFM. You need multiple returns or a large trunk.
- Ignoring duct static pressure: Long duct runs with many elbows create pressure drops that reduce actual CFM at the register.
- Not balancing supply and return: Every room needs both supply AND return (or transfer path) for proper airflow.
- Using exhaust-only in tight homes: ASHRAE 62.2 requires mechanical ventilation in homes with <3 ACH50 infiltration rate.
Standards Reference
- ASHRAE Standard 62.2 — Ventilation for Residential Buildings
- ASHRAE Standard 62.1 — Ventilation for Commercial Buildings
- ACCA Manual D — Residential Duct Systems
- IRC Section M1505 — Exhaust Ventilation
Frequently Asked Questions
How many CFM do I need per room?
ASHRAE 62.2 requires 1 CFM per sq ft for living areas, 0.35 ACH whole-house ventilation (whichever is greater), 50 CFM continuous for bathrooms, and 100 CFM intermittent for kitchens. A 200 sq ft bedroom needs about 75–200 CFM depending on occupancy and ventilation strategy.
How many CFM per ton of AC?
The standard is 400 CFM per ton of cooling capacity. This is the airflow across the evaporator coil. In humid climates, reduce to 350 CFM/ton for better moisture removal. In dry climates, 450 CFM/ton is acceptable. Going below 300 CFM/ton risks coil freezing.
What CFM bathroom fan do I need?
ASHRAE 62.2 requires 50 CFM continuous or 80 CFM intermittent for bathrooms up to 100 sq ft. For larger bathrooms, use 1 CFM per sq ft. Always vent to the exterior, never into the attic.
How does duct size affect CFM?
Duct size directly determines CFM capacity at a given velocity. A 6" round duct handles ~100 CFM at 500 FPM. An 8" round handles ~175 CFM. Undersized ducts create high static pressure, noise, and reduced airflow. Use ACCA Manual D for proper duct sizing.
Can I have too much CFM?
Yes. Excessive CFM causes drafts (air velocity >50 FPM at the register is uncomfortable), noise, and energy waste. For supply registers, keep throw velocity under 750 FPM in living areas and 500 FPM in bedrooms.