Exhaust Fan CFM Calculator
Calculate the exhaust fan CFM rating needed for your bathroom, kitchen, or utility room based on room dimensions and ventilation requirements.
Enter your room dimensions and type to calculate the exhaust fan CFM needed.
Understanding Exhaust Fan Sizing
Exhaust fans are rated in CFM (cubic feet per minute), which measures how much air the fan moves. The correct fan size depends on the room type, room size, and the number of air changes per hour required by building codes and good ventilation practice. An undersized fan fails to remove moisture, odors, and pollutants effectively. An oversized fan wastes energy and can create excessive negative pressure in the home, potentially pulling combustion gases back through water heaters and furnaces.
Bathroom Exhaust Fan Sizing:
For bathrooms, the standard rule is 1 CFM per square foot of floor area, with a minimum of 50 CFM. For bathrooms with a jetted tub, steam shower, or spa features, add 50 CFM above the calculated amount.
| Bathroom Size | Required CFM | Metric (L/s) |
|---|---|---|
| Up to 50 sq ft (4.6 m²) | 50 CFM | 24 L/s |
| 50 - 100 sq ft (4.6 - 9.3 m²) | 1 CFM per sq ft | 0.5 L/s per sq ft |
| Over 100 sq ft (9.3 m²) | Add 50 CFM per fixture | Add 24 L/s per fixture |
| With jetted tub | Add 50 CFM | Add 24 L/s |
| With steam shower | Add 50 CFM | Add 24 L/s |
Kitchen Exhaust Fan Sizing:
Kitchen ventilation depends on the range type and cooking style. Range hoods should provide 100 CFM per linear foot (30 cm) of range width for wall-mounted hoods, and 150 CFM per linear foot for island-mounted hoods (which lose efficiency due to cross-drafts). A 30-inch (76 cm) range needs at least 250 CFM for a wall hood or 375 CFM for an island hood.
For high-BTU professional-style ranges, the rule changes to 1 CFM per 100 BTU of burner output. A range with 60,000 BTU total burner capacity needs a 600 CFM exhaust hood.
Air Changes per Hour (ACH) Method:
For other rooms, calculate CFM using the air changes per hour method:
CFM = (Room Volume in cubic feet x ACH) / 60
| Room Type | Recommended ACH | Example (10x12x8 ft room) |
|---|---|---|
| Bathroom | 8 ACH | 128 CFM |
| Kitchen | 15 ACH | 240 CFM |
| Laundry room | 8 ACH | 128 CFM |
| Garage (workshop) | 6 ACH | 96 CFM |
| Basement / utility | 4 ACH | 64 CFM |
| Home gym | 6 ACH | 96 CFM |
Noise Levels (Sones Rating):
Exhaust fan noise is measured in sones. Lower is quieter. A rating of 1.0 sone is about the sound of a quiet refrigerator. Most budget fans are rated at 3 to 4 sones, while premium quiet fans operate at 0.3 to 1.0 sones.
- 0.3 - 0.5 sones: Nearly silent, premium models
- 0.5 - 1.0 sones: Very quiet, barely noticeable
- 1.0 - 2.0 sones: Quiet, comfortable for most spaces
- 2.0 - 3.0 sones: Moderate, noticeable but not loud
- 3.0 - 4.0 sones: Standard budget fans, clearly audible
Duct Size for Exhaust Fans:
The exhaust duct must match the fan outlet size. Never reduce the duct size below the fan outlet. Use smooth rigid duct when possible because flexible duct reduces effective CFM by 20 to 30 percent due to increased friction. Keep duct runs as short and straight as possible. Each 90-degree elbow reduces effective CFM by about 15 percent.
Practical Example:
A master bathroom measuring 10 feet by 12 feet (120 square feet or 11.1 square meters) with a jetted tub needs 120 CFM base plus 50 CFM for the jetted tub, totaling 170 CFM. Choose a fan rated at 180 to 200 CFM to account for duct friction losses. With a 6-inch (150 mm) duct run of 10 feet and one elbow, the effective delivery would be approximately 150 to 170 CFM.