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Bathroom and Kitchen Exhaust Fan CFM Calculator

Choose the room and how the fan will run to see the minimum exhaust rate, the size to buy and, for powerful range hoods, whether make-up air is required.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

Result

Minimum delivered exhaust
50CFM
Recommended fan rating
70CFM

At least 50 CFM delivered; buy a fan rated about 70 CFM or more.

  • Rated CFM is measured at a test pressure; long or crushed ducts reduce delivered airflow. Check the code your jurisdiction has adopted.
Airflow for 5 air changes per hour
80CFM
Room volume
960ft³
Show the working
  1. Code minimum (IRC, as summarised by DOE’s Building America Solution Center), bathroom, intermittent
    50 CFM
  2. ENERGY STAR advice: buy a fan rated at least 70 CFM, because installed fans deliver less than their rating
    70 CFM rated

Minimum exhaust rates

DOE’s Building America Solution Center summarises the residential code minimums for local exhaust:

  • Bathroom: 50 CFM intermittent or 20 CFM continuous
  • Kitchen: 100 CFM intermittent or 25 CFM continuous

ENERGY STAR also recommends that kitchen exhaust provide at least 5 air changes per hour of the kitchen volume, so the tool shows the larger of the two for kitchens. Your adopted code governs.

Rated versus delivered airflow

Fans are rated at a test pressure. Real ducts with bends, long runs or crushed flex deliver less. For bathrooms, ENERGY STAR suggests buying a fan rated at least 70 CFM to reliably deliver 50 CFM intermittently, and more than 50 CFM for a 20 CFM continuous fan. Use smooth, short, sealed ducts to the outdoors.

Make-up air for big range hoods

Any exhaust fan running above 400 CFM needs a make-up air system that starts automatically with it and supplies air at the same rate. Without it, a powerful hood can depressurise the house and pull combustion gases back down the flue of a furnace, water heater or fireplace that is not sealed-combustion.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Room
Bathroom or toilet room
Fan operation
Intermittent (switched on when needed)
Room length
12 ft
Room width
10 ft
Ceiling height
8 ft
Fan you are considering (rated CFM, 0 to skip)
0 CFM

Result

Minimum delivered exhaust
50 CFM
Recommended fan rating
70 CFM
Airflow for 5 air changes per hour
80 CFM
Room volume
960 ft³

At least 50 CFM delivered; buy a fan rated about 70 CFM or more.

Assumptions

  • Minimums as summarised by DOE’s Building America Solution Center: bathrooms 50 CFM intermittent or 20 CFM continuous; kitchens 100 CFM intermittent or 25 CFM continuous; ENERGY STAR recommends at least 5 air changes per hour for kitchen exhaust.
  • Any exhaust fan over 400 CFM needs make-up air that runs with it at an equal rate.

Limitations

  • Code editions and local amendments differ; your adopted code governs.
  • Does not size range hoods by burner input or hood width; follow the range maker’s instructions.

Frequently asked questions

Is a bigger bathroom fan always better?

Not much beyond what is needed to clear moisture; larger fans are louder and waste conditioned air. Choose a quiet fan (3 sones or less intermittent, 1 sone or less continuous).

Where should the exhaust go?

Directly outdoors through a short, smooth, sealed duct — not into an attic or crawl space.

What if my hood is exactly 400 CFM?

The make-up air requirement applies to fans operating above 400 CFM, so 400 CFM or less does not trigger it.

Sources

  1. Bathroom Exhaust Fans — U.S. DOE Building America Solution Center (PNNL). Per ASHRAE 62.2 (2010) and IRC 2021 M1507, bathroom fans ≥ 50 cfm intermittent or ≥ 20 cfm continuous; ENERGY STAR recommends a fan rated at least 70 cfm for intermittent use and more than 50 cfm for continuous use; ≤ 3 sones intermittent, ≤ 1 sone continuous.
  2. Kitchen Exhaust Fans — U.S. DOE Building America Solution Center (PNNL). Per IRC 2015, kitchen fans ≥ 100 cfm intermittent or ≥ 25 cfm continuous; ENERGY STAR recommends ≥ 5 air changes per hour of the kitchen volume; any exhaust fan operating above 400 cfm needs a make-up air system that starts automatically and supplies air at an equivalent rate.
  3. Air Change Rates in Rooms — The Engineering ToolBox. ACH = CFM × 60 / room volume.

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