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Calculate cubic feet per minute (CFM) airflow for ventilation, HVAC systems, and exhaust fans using multiple methods.
Minimum: 0 ACH = 0 CFM
Maximum: 0 ACH = 0 CFM
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CFM (Cubic Feet per Minute) is a measurement of airflow volume. It represents how many cubic feet of air pass through a given space in one minute. CFM is the standard measurement for:
Air conditioners and furnaces are rated by how much air they can move to heat or cool your home.
Bathroom and kitchen fans are rated in CFM to ensure adequate moisture and odor removal.
Fresh air requirements for buildings are specified in CFM per person or per square foot.
CFM = (Length × Width × Height × ACH) ÷ 60
Where ACH = Air Changes per Hour
CFM = Duct Area (sq ft) × Velocity (FPM)
Where FPM = Feet per Minute
CFM = BTU ÷ (1.08 × ΔT)
Where ΔT = Temperature difference in °F
CFM = Number of Occupants × CFM per person
Typical: 15-20 CFM per person (ASHRAE 62.1)
ACH (Air Changes per Hour) indicates how many times the entire volume of air in a room is replaced in one hour.
| Room Type | Min ACH | Max ACH | Notes |
|---|---|---|---|
| Living Room | 4 | 8 | General comfort |
| Bedroom | 4 | 6 | Quiet operation preferred |
| Bathroom | 8 | 10 | Moisture control |
| Kitchen | 10 | 15 | Cooking odors/heat |
| Office | 6 | 10 | Per ASHRAE standards |
| Workshop | 8 | 12 | Dust/fume control |
| Restaurant | 15 | 20 | High occupancy |
| Warehouse | 1 | 4 | Low occupancy |
20' × 15' living room with 8' ceiling, 6 ACH
Volume = 20 × 15 × 8 = 2,400 cu ft
CFM = (2,400 × 6) ÷ 60 = 240 CFM
8' × 10' bathroom (80 sq ft)
Minimum: 50 CFM (for bathrooms ≤100 sq ft)
Recommended: 80 CFM (1 CFM per sq ft)
For shower/tub: Add 50 CFM = 130 CFM total
24,000 BTU cooling load, 20°F temperature difference
CFM = BTU ÷ (1.08 × ΔT)
CFM = 24,000 ÷ (1.08 × 20)
CFM = 24,000 ÷ 21.6 = 1,111 CFM
Verification: 2 tons × 400 CFM = 800 CFM (minimum)
| Application | CFM Requirement | Notes |
|---|---|---|
| Small Bathroom (≤50 sq ft) | 50 CFM | HVI minimum |
| Medium Bathroom (50-100 sq ft) | 50-100 CFM | 1 CFM per sq ft |
| Large Bathroom (>100 sq ft) | 100+ CFM | Add 50 CFM per fixture |
| Kitchen Range (30” wide) | 250-300 CFM | Standard gas/electric |
| Kitchen Range (36” wide) | 360-400 CFM | 100 CFM per linear ft |
| Laundry Room | 100-150 CFM | For dryer venting issues |
| Garage (2-car) | 100-200 CFM | Exhaust fumes |
The standard is 400 CFM per ton of cooling capacity. A 3-ton AC system should deliver approximately 1,200 CFM. This can range from 350-450 CFM depending on design conditions.
For bathrooms under 100 sq ft, a 50 CFM fan is the minimum. For larger bathrooms, use 1 CFM per square foot. Add 50 CFM for each toilet, shower, or bathtub in the space.
ACH = (CFM × 60) ÷ Room Volume (cu ft). For example, 200 CFM in a 2,400 cu ft room = (200 × 60) ÷ 2,400 = 5 ACH.
For wall-mounted hoods, use 100 CFM per linear foot of range width. For island hoods, use 150 CFM per linear foot. A 30” range needs at least 250-300 CFM.
ASHRAE 62.1 recommends 15-20 CFM of outdoor air per person for office spaces. This varies by building type and activity level.