Required CFM, fan rating and duct diameter for a bathroom exhaust fan.
Estimates are planning aids, not a professional quote. Double-check quantities and product coverage — and any local code or permit requirements — before you buy.
⚠ Never vent a bathroom fan into the attic, a soffit under a ridge vent, or a gable — that just moves the moisture problem into the attic instead of outside. The duct has to terminate at a dedicated exterior cap.
| HVAC | $141–$191 |
| Electrical | $8–$11 |
| Total | $149–$202 |
Excludes tools, delivery, tax and permit fees. Labor not included — this is a DIY materials estimate.
These are affiliate links: we may earn a commission if you buy through them, at no extra cost to you. It never changes what the calculator recommends — the shopping list comes from your dimensions, not from what pays.
About this calculator
Enter your bathroom's floor area and fixture count and the Bathroom Exhaust Fan Calculator sizes the fan two ways — the 1 CFM per square foot rule most codes allow for smaller baths, and the fixture-based method (50 CFM per toilet, shower or tub, 100 for a jetted tub) that larger bathrooms use — then takes whichever number is bigger as your true minimum, rounded up to a standard fan rating.
Duct length and elbow count matter just as much as the fan's rating: a bath fan runs on far less static pressure than a furnace blower, so every foot of run and every 90° bend chokes its real airflow. Enter your duct run, elbow count and material and it converts that into an equivalent length, then sizes 4, 5 or 6 in duct so the fan actually delivers its rated CFM at the register.
Step by step
A step-by-step walkthrough to go with your plan above. Follow the product instructions and any local code or permit requirements as you work.
Use the calculator to convert your bathroom's floor area and fixtures into required CFM, then check your planned duct run and elbow count against that CFM to pick a diameter. Getting the duct size right up front avoids buying a fan that never delivers its rated airflow.
Kill the breaker and confirm it with a voltage tester before opening the old housing. If you're replacing an existing fan, note the duct size and route already in place — it may need upsizing to match the new fan.
Secure the housing to the joists at the height and location the manufacturer specifies, keeping it clear enough of the duct takeoff to make a straight, low-bend connection.
Route the shortest practical path to the exterior, keeping elbow count as low as the framing allows since every bend costs airflow. Support flex duct every few feet so it can't sag and lose capacity, and strap rigid sections at the same interval.
Cut the roof, wall or soffit opening sized to the cap, flash and seal it against the weather, and connect the duct with the backdraft damper in line before the cap.
Run cable from the switch location to the fan on a dedicated or GFCI-protected circuit per local code, wiring in a humidity or timer switch now if you want the fan to keep running after you leave the room.
Tape every flex connection with UL181B-FX foil tape, including the fan collar and the termination — an unsealed joint quietly bleeds a big share of the airflow you just sized for.
Restore power, run the fan, and confirm airflow at the cap with a tissue or smoke pencil. Patch and paint the ceiling around the housing once you're satisfied it's pulling properly.
Equip the job
Hand-picked categories that match the shopping list above. Links open a current selection so you can compare brands and prices.
These are affiliate links: we may earn a commission if you buy through them, at no extra cost to you. It never changes what the calculator recommends — the shopping list comes from your dimensions, not from what pays.
As an Amazon Associate I earn from qualifying purchases. We may also earn from other retailer links here. This doesn’t change your price and helps keep these planners free.
Keep planning
Interior & Finishing
Answers
Use whichever of two rules gives the bigger number. For a bathroom under about 100 sq ft, the simple rule is 1 CFM per square foot of floor area, with a 50 CFM floor even for a tiny powder room. For larger bathrooms, add up airflow by fixture instead: 50 CFM each for the toilet, shower and a standard tub, and 100 CFM for a jetted or whirlpool tub. Most inspectors will accept either method, so sizing to the larger of the two keeps you covered regardless of which one gets checked.
It depends on both the fan's CFM rating and how far the air has to travel to get outside. A 4 in duct comfortably carries a 50–80 CFM fan over a short, mostly straight run; a 110 CFM fan usually needs 5 in duct once you add a couple of elbows; and anything approaching 150 CFM, or a long run with several bends, typically needs 6 in. The right way to check is to convert your actual run into an equivalent length (see the next question) and match that against the diameter's real carrying capacity, not just the fan's rated CFM on the box.
Yes, and it's the single most common reason a new fan still leaves the mirror fogged. A bathroom fan has to push air against far less static pressure than a furnace blower does, so length and bends matter disproportionately. Each 90° elbow effectively adds several feet of straight duct worth of resistance, and flex duct's corrugated liner adds friction on top of that per foot. A 110 CFM fan pushing through 40 ft of flex with four elbows can end up delivering closer to half its rated airflow — the fix is a bigger duct diameter or a shorter, straighter run, not a stronger fan.
Anywhere that dumps the moist air fully outside the building envelope: a roof cap, a sidewall louvered cap, or a soffit vent that is not underneath a ridge or gable vent it could just get pulled back into. What's never acceptable is venting into the attic itself, a soffit that feeds attic ventilation, or a chimney — that just relocates the moisture problem into your attic framing and insulation, where it causes rot and mold instead of a fogged mirror.
Most fan housings include a light plastic flapper, but it's often not enough on its own to stop wind-driven air or pests from working past it, especially on longer runs or windy sites. A dedicated spring-loaded backdraft damper at the termination or partway down the duct gives a firmer seal against backflow when the fan isn't running, and it's cheap insurance against cold air infiltration in winter.