Understanding your result
The headline is the fan size to buy, with the required airflow and which rule set it in the sub-line. The table shows all three rules for your room, the code minimum, the HVI area rule for rooms up to 100 sq ft or the fixture rule above that (the one that does not apply is shown for information), and the 8 air changes per hour rule, and takes the largest as the requirement. The stats repeat the requirement, the fan size and the duct’s equivalent length.
The duct rows are what make the fan work in practice. The equivalent length adds the straight duct, 10 ft for each elbow and 15 ft for the wall or roof cap; past about 25 ft of 4-inch duct, the notes recommend a fan whose rating is given at 0.25 in wc of static pressure (HVI publishes both figures) or a 6-inch duct, and the velocity row shows when 4-inch duct is simply too small for the fan.
For the rest of the house’s moisture and odour control, the range hood calculator handles the kitchen, the whole-house ventilation calculator the continuous outdoor air that a bathroom fan on a timer can also supply, and the dew point calculator shows at what humidity the mirror and the cold window start to condense. The air changes calculator converts any fan’s CFM to ACH for any room.
How we calculate this
The International Residential Code’s M1507 sets the minimum mechanical exhaust for bathrooms at 50 CFM intermittent or 20 CFM continuous. The Home Ventilating Institute’s guidance adds the area rule for small bathrooms and the fixture rule for large ones, and 8 air changes per hour is a long-standing rule of thumb that HVI also cites; where they disagree, the larger figure clears steam faster and the calculator uses it. The duct allowances are typical fan-maker figures for 4-inch duct, and the velocity is airflow divided by the duct’s cross-section. The rules are collected on the ventilation and exhaust minimums page, and the unit conversions page covers CFM to m³/h for metric mode.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Code minimum | 50 CFM intermittent, 20 CFM continuous | International Residential Code, section M1507 |
| Area rule | 1 CFM per sq ft, minimum 50, for rooms up to 100 sq ft | Home Ventilating Institute |
| Fixture rule | 50 CFM per toilet, shower, tub; 100 per jetted tub, for rooms over 100 sq ft | Home Ventilating Institute |
| Air-change rule | 8 ACH | HVI and common design practice |
| Fan sizes | 50, 80, 110, 150, 200, 300, 400 CFM | Residential bathroom fans sold in the US |
| Duct allowances | 10 ft per 90° elbow, 15 ft for the cap, 25 ft threshold for 4-inch duct | Typical fan manufacturer installation guidance |
| Run-on time | 20 minutes | HVI and ENERGY STAR recommendations |
Assumptions last reviewed October 8, 2026.
The calculator does not model the fan curve, so it cannot say exactly how much a given fan will deliver through your duct; HVI-certified ratings at 0.1 and 0.25 in wc are the figures to compare. It does not address makeup air (a bathroom fan rarely needs it), the noise rating, or the continuous-ventilation alternative in which a quiet fan runs all day at 20 CFM and boosts on demand. The guide to bathroom and kitchen exhaust rules covers all of that, and the guide to humidity and comfort explains why 20 minutes of run time matters more than 20 extra CFM.
Two worked examples
A typical full bathroom
10 × 8 ft, 8 ft ceiling, toilet, shower and tub, 10 ft of 4-inch duct with two elbows to a wall cap.
- Area 80 sq ft, volume 640 cu ft
- Code minimum 50 CFM; area rule 80 CFM (room ≤ 100 sq ft); 8 ACH = 640 × 8 ÷ 60 = 85 CFM
- Required: 85 CFM → next fan size 110 CFM
- Duct: 10 + 2 × 10 + 15 = 45 ft equivalent; 1,261 FPM in 4-inch duct
The 8 air change rule edges out the area rule here, and the 110 CFM fan is the right size, but the 45 ft equivalent run of 4-inch duct is the real problem: a fan rated 110 CFM at 0.1 in wc may deliver 60 to 70 through it. Choose a fan rated at 0.25 in wc, or run 6-inch duct, which brings the velocity to 560 FPM.
A large master bathroom with a jetted tub
14 × 11 ft, 8 ft ceiling, toilet, shower, tub and jetted tub, 20 ft of 6-inch duct with three elbows to a roof cap.
- Area 154 sq ft, volume 1,232 cu ft
- Fixture rule (room > 100 sq ft): 50 + 50 + 50 + 100 = 250 CFM; 8 ACH = 164 CFM
- Required: 250 CFM → next fan size 300 CFM
- Duct: 20 + 3 × 10 + 15 = 65 ft equivalent; 1,528 FPM in 6-inch duct
In a large bathroom the fixtures drive the requirement, and the jetted tub alone adds 100 CFM. A 300 CFM fan is usually an inline unit in the attic with two ceiling grilles, one over the shower and one near the tub, which also spreads the pickup across the room. The 65 ft run calls for the fan’s rating at 0.25 in wc.
Where to find your inputs
Room dimensions. Floor length and width and the ceiling height; for a bathroom with a separate toilet compartment, size that compartment’s own fan separately at 50 CFM.
Fixtures. Count what is there. A shower over a tub counts as both a shower and a tub in the HVI rule.
Duct run. Measure or estimate the path from the fan to the cap, count every 90° bend (two 45° bends count as one 90°), and note the duct diameter at the fan’s outlet.
Fan ratings. The HVI-certified CFM at 0.1 and 0.25 in wc and the sone rating are on the box and in the HVI directory.
Common mistakes
- Buying the fan by its zero-static rating. Through 30 ft of 4-inch duct a 110 CFM fan may move 60. Check the rating at 0.25 in wc.
- 4-inch duct on a big fan. Above 110 CFM use 6-inch; the velocity and noise in 4-inch are too high.
- Venting into the attic. It rots the roof deck. Terminate outdoors with a damper and insulate the duct.
- Switching the fan off at the door. Run it 20 minutes after the shower; a timer or humidity switch does it for you.
- A loud fan. A 3-sone fan never gets used. Choose 1.0 sone or less.
- Sizing by the room alone in a big bathroom. Over 100 sq ft the fixtures set the airflow, and a jetted tub adds 100 CFM.
Questions people ask
- What size exhaust fan do I need for a bathroom?
- For a bathroom up to 100 sq ft, the Home Ventilating Institute recommends 1 CFM per square foot with a minimum of 50 CFM, and many designers also check 8 air changes per hour; an 8 × 10 ft bathroom with an 8 ft ceiling needs 80 CFM by the area rule and 85 by the air-change rule, so a 110 CFM fan. For bathrooms over 100 sq ft, add 50 CFM for each toilet, shower and tub and 100 for a jetted tub. The code minimum is 50 CFM intermittent or 20 CFM continuous.
- Is a 50 CFM fan enough for a small bathroom?
- It meets the International Residential Code minimum for any bathroom, and it is adequate for a half bath with no shower or a very small full bath of 40 to 50 sq ft with a short duct. For a typical 5 × 8 full bath (40 sq ft, 320 cu ft) a 50 CFM fan gives 9 air changes per hour at its rating, which is enough if the duct is short and straight; on 25 ft of 4-inch duct with two elbows it may deliver only 30, which is not.
- How long should the bathroom fan run after a shower?
- About 20 minutes, which is long enough to bring the humidity back down and dry the surfaces; HVI and ENERGY STAR both recommend it. A timer switch, a humidity-sensing switch or a fan with a built-in delay does it without anyone remembering. Running the fan during the shower and switching it off at the door is the commonest reason a correctly sized fan still leaves mould on the ceiling.
- Can I vent a bathroom fan into the attic?
- No. The IRC requires exhaust to terminate outdoors, not in an attic, soffit or crawl space, because the warm moist air condenses on the roof deck and insulation and grows mould and rots wood. Use rigid or smooth flex duct to a roof or wall cap with a backdraft damper, insulate the duct where it passes through unconditioned space so condensation does not run back to the fan, and keep it as short as possible.
- What duct size does a bathroom fan need?
- 4-inch for fans up to about 80 to 110 CFM on short runs, 6-inch for anything larger or for runs beyond about 25 ft equivalent. A 110 CFM fan moves air through 4-inch duct at over 1,200 feet per minute, which is noisy and restrictive; the same fan in 6-inch duct runs at 560 FPM. Each 90° elbow counts as about 10 ft of 4-inch duct and the wall or roof cap about 15 ft. Many fans accept both sizes with an adapter; use the larger.
- What do sones mean on a bathroom fan?
- Loudness. One sone is about the sound of a quiet refrigerator; a fan rated 1.0 sone or less is quiet enough to leave running for 20 minutes without wanting to switch it off, which is the whole point. Older fans were 3 to 4 sones. Quiet fans tend to be larger and better built, and the quietest arrangement is an inline fan in the attic with the grille in the ceiling. Noise, not CFM, is why most bathroom fans are not used.