Key takeaways
- HVI recommends 1 cfm per square foot for bathrooms up to 100 square feet, and 50 cfm per toilet, shower and tub above that.
- The 2021 IRC table figure is 50 cfm intermittent for a bathroom and 100 cfm intermittent for a kitchen.
- Above 400 cfm, IRC M1503.6 calls for makeup air where a fuel-burning appliance that isn't direct-vent is inside the air barrier.
Choose a bathroom or a kitchen. For a bathroom, enter its length and width, and for a large one, its fixtures. For a kitchen, enter the range width in inches and whether it's against a wall or in an island. The calculator gives the Home Ventilating Institute's (HVI) recommendation and the 2021 IRC table figure side by side.
How the calculator works
Bathrooms up to 100 sq ft: HVI cfm = length × width × 1
Bathrooms over 100 sq ft: HVI cfm = 50 × (toilets + showers + tubs) + 100 × jetted tubs
Range hoods: HVI cfm = range width in feet × 100 (wall) or 150 (island)
HVI's bathroom guideline recommends 1 cfm per square foot for bathrooms up to 100 square feet, which it says is "approximately eight air changes per hour". Above that, it rates each fixture: 50 cfm for a toilet, a shower or a bathtub, and 100 cfm for a jetted tub. HVI's range hood guide gives a recommended and a minimum rate per linear foot of range.
Next to each result, the calculator shows the figure from 2021 IRC Table M1505.4.4. If you answer yes to the fuel-burning appliance question and a kitchen result is over 400 cfm, it flags the makeup air section, M1503.6.
Two different numbers
The code figure and HVI's figure answer different questions. The code table sets a floor for any bathroom or kitchen. HVI's figure scales with the room.
| Room | 2021 IRC table figure | HVI recommendation |
|---|---|---|
| Bathroom or toilet room | 50 cfm intermittent or 20 cfm continuous | 1 cfm per sq ft up to 100 sq ft; per fixture above |
| Kitchen | 100 cfm intermittent or 25 cfm continuous | 100 cfm per ft of range on a wall, 150 in an island |
The IRC table's footnote says these capacities are rated at 0.25 inch of water column static pressure. That matters when you compare fans, because a fan's airflow falls as the duct pushes back. Section M1503.5 says range hoods are sized to the same table.
Bathroom examples
A small bathroom. HVI's own example is an 8 × 5 foot bathroom: 40 square feet, so 40 cfm at 1 cfm per square foot. The IRC table's figure for an intermittent bathroom fan is 50 cfm, which is higher. The calculator shows both, and a 40 cfm fan would sit below the table's cell for an intermittent fan.
A large bathroom. HVI's second example is 20 × 12 feet, or 240 square feet, with a tub, a separate shower and an enclosed toilet. That's over 100 square feet, so each fixture counts:
- Tub: 50 cfm
- Shower: 50 cfm
- Toilet: 50 cfm
- Total: 150 cfm
HVI suggests either a 50 cfm fan over each fixture or one 150 cfm fan that pulls air through the whole room. It says exhaust points are typically over or near the shower or tub and in an enclosed toilet room.
HVI also recommends running the fan for 20 minutes after using the bathroom, and says a timer is a good way to do it. Air has to come in to replace what leaves, so it says bathroom doors need to be undercut to let makeup air in.
Range hood examples
HVI's per-foot rates:
| Range location | HVI recommended | HVI minimum |
|---|---|---|
| Against a wall | 100 cfm per ft | 40 cfm per ft |
| In an island | 150 cfm per ft | 50 cfm per ft |
A 36-inch range against a wall is 3 feet, so:
- Recommended: 3 × 100 = 300 cfm
- Minimum: 3 × 40 = 120 cfm
That matches the brochure's own table for a 36-inch wall hood. The same 36-inch range in an island gives 3 × 150 = 450 cfm recommended and 3 × 50 = 150 cfm minimum. A 48-inch island range gives 4 × 150 = 600 cfm, which is past the 400 cfm trigger described below.
HVI's brochure is dated 2008, and it's the range hood guidance HVI publishes for consumers. It also says hood width should match the cooking range, and the hood should sit at the manufacturer's recommended height.
Makeup air over 400 cfm
Section M1503.6 applies where a fuel-burning appliance that isn't direct-vent sits inside the home's air barrier. In that case, "each exhaust system capable of exhausting in excess of 400 cubic feet per minute" needs makeup air "at a rate approximately equal to the exhaust air rate", with a damper under M1503.6.2.
Whether your appliances fit that description depends on how each one is vented, so read the section itself. The calculator raises the flag but doesn't design the makeup air. That's a question for your building department or an HVAC contractor.
Duct runs and rated airflow
A fan's label rating is measured under test conditions. In your house, the duct's length, width and bends all push back on the fan. HVI's range hood guide says "the duct's efficiency is affected by its width, number of bends, and length". We don't give a maximum duct length, because it's specific to each fan. Read your fan's installation instructions for its duct size and longest allowed run.
For quieter fans, HVI points to the sone rating, where doubling the sones sounds roughly twice as loud.
Common mistakes
- Taking the IRC figure as the target. It's a floor for any bathroom, and HVI's figure is often higher in a big room.
- Counting a big bathroom by area. Over 100 square feet, HVI switches to counting fixtures.
- Ignoring the duct. A long or narrow duct with several bends delivers less than the fan's rating.
- Forgetting where the air comes from. Without a door undercut or other path, the fan can't move its rated air.
- Missing the 400 cfm line. A large hood near a fuel-burning appliance brings in M1503.6.
Frequently asked questions
What size exhaust fan do I need for my bathroom?
Up to 100 square feet, HVI recommends 1 cfm per square foot, so a 7 × 9 foot room is 63 cfm. Over 100 square feet, add 50 cfm per toilet, shower and tub, and 100 for a jetted tub. Compare it with the IRC figure of 50 cfm intermittent.
Is 50 cfm enough for a bathroom?
It's the IRC table's figure for an intermittent bathroom fan. For a room under 50 square feet it's above HVI's figure, and for a larger room HVI's figure is higher.
How many cfm do I need for a range hood?
HVI recommends 100 cfm per foot of range for a wall hood and 150 for an island. A 30-inch wall range works out to 250 cfm recommended and 100 minimum.
When do I need makeup air for a range hood?
Under 2021 IRC M1503.6, when the exhaust can move more than 400 cfm and a fuel-burning appliance that isn't direct-vent is inside the air barrier. Check which edition your area uses.
How long should a bathroom fan run after a shower?
HVI recommends 20 minutes after using the bathroom.
Do I need a fan if my bathroom has a window?
HVI says enclosed toilet rooms must have an operable window or a fan. Your local code decides what counts for the whole bathroom, so ask your building department.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- 2021 International Residential Code, Chapter 15, Exhaust Systems (Table M1505.4.4, M1503.5, M1503.6)
- Home Ventilating Institute: Bathroom ventilation
- Home Ventilating Institute: Range Hoods: A Consumer Guide (PDF, 2008)