Exhaust Fan CFM Calculator

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.

There is a second published rule for bathrooms, 8 air changes an hour, and the two do not agree. At a standard 8-foot ceiling the air-change rule asks for slightly more: a 10 by 12 bathroom is 120 CFM by floor area and 128 CFM by air changes. The gap widens with ceiling height, and a room with a 10-foot ceiling comes out at 160 CFM by air changes against the same 120 by floor area. The floor-area rule simply has no way to know about the extra volume. The calculator runs both and takes the larger, which is what a careful sizer does.

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

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 an 8-foot ceiling and a jetted tub works out like this:

• Floor area rule: 120 sq ft x 1 CFM per sq ft = 120 CFM
• Air change rule: 960 cu ft x 8 ACH / 60 = 128 CFM
• Take the larger, 128, then add 50 CFM for the jetted tub = 178 CFM at the grille

That last figure is what has to arrive in the room, not what the box says. Smooth rigid duct with one 90-degree elbow costs about 15%, so the fan itself needs a rating of 178 / 0.85 = 210 CFM. Swap that rigid duct for flexible and the same installation needs 178 / (0.85 x 0.75) = 280 CFM, which is a different fan and a different price. Duct choices move the answer more than most people expect, and they are decided on site by whoever is doing the run.