If your mirror still fogs after a shower, the problem is usually not that your bathroom fan is broken. It is that the bathroom fan CFM was never high enough for the room, or that the fan is rated for more air than it can actually move through the duct it was given. Those are two different problems, and fixing the wrong one is how people end up buying a second fan that performs exactly like the first.
Here is how to work out the number you need, and then how to make sure you actually get it.
What CFM means in plain terms
CFM stands for cubic feet per minute. It is the volume of air a fan pulls out of the room every minute. A 50 CFM fan removes 50 cubic feet of air per minute. A small bathroom holds roughly 500 to 600 cubic feet of air, so a 50 CFM fan theoretically swaps the entire room of air about five times an hour.
That word theoretically is doing a lot of work, and we will come back to it.
The two sizing methods, and which one applies to you
The Home Ventilating Institute publishes the sizing guidance most of the industry follows, and it splits at the 100 square foot mark.
Bathrooms up to 100 square feet: one CFM per square foot
Multiply length by width. A 5 by 8 bathroom is 40 square feet, so 40 CFM is the calculated minimum. In practice nobody sells a 40 CFM fan, and 50 CFM is the smallest common size, so a small bathroom lands on 50 CFM by default.
Worked examples:
- 5 by 8 feet (40 sq ft): 50 CFM
- 6 by 10 feet (60 sq ft): 60 to 70 CFM
- 8 by 10 feet (80 sq ft): 80 CFM
- 9 by 11 feet (99 sq ft): 100 CFM
One adjustment the basic formula misses: the square foot rule assumes an 8 foot ceiling. If your ceiling is 9 or 10 feet, you have 12 to 25 percent more air in the room and the same fan will take that much longer to clear it. Multiply your square footage by your ceiling height, divide by 60, then multiply by 8 to get a rate that moves the same volume per hour.
Bathrooms over 100 square feet: add up the fixtures
Once a bathroom is large, floor area stops being a good proxy for how much moisture gets produced. A big room with one shower does not generate more steam than a small room with one shower. So the method switches to counting what is in it:
- Toilet: 50 CFM
- Shower: 50 CFM
- Bathtub: 50 CFM
- Jetted tub: 100 CFM
A primary bathroom with a toilet, a shower and a soaking tub adds up to 150 CFM. That does not have to be one giant fan. Two 75 CFM fans, one over the shower and one near the toilet, will clear the room better than a single 150 CFM unit in the middle of the ceiling, because each one is pulling air from where the moisture and odour actually start.
If your layout includes a separate enclosed toilet compartment, that compartment needs its own fan. A door between the toilet and the rest of the room means the main fan cannot serve it. Our guide to toilet clearance covers how much space those compartments need to work.
Why your fan probably does not deliver its rated CFM
This is the part that gets left out of almost every buying guide, and it is the reason most steamy bathrooms stay steamy after an upgrade.
A fan is rated in a lab against a fixed, low resistance. Your ceiling is not a lab. Every foot of duct, every bend, and every inch of diameter you lose adds resistance, and the fan responds by moving less air. A 110 CFM fan on a long, kinked, undersized run can easily deliver half its rating.
The things that steal airflow, roughly in order
- Undersized duct. If the fan housing has a 6 inch outlet and someone reduced it to 4 inch duct, you have cut the cross sectional area by more than half. This is extremely common and it is the single biggest offender.
- Flexible duct instead of rigid. The ribbed interior of flex duct creates far more friction than smooth rigid pipe over the same distance. Where flex is unavoidable, pull it tight. A sagging, compressed flex run behaves like a much longer duct.
- Elbows. Each 90 degree bend adds resistance roughly equivalent to several extra feet of straight duct. Two or three elbows on a short run can matter more than the run itself.
- Long runs. Venting across an attic to a distant gable is worse than going straight up through the roof, even though it looks tidier.
- A blocked or frozen termination. The backdraft damper at the outside vent gets stuck with lint, paint or a bird nest. Check it. It costs nothing and it is often the whole problem.
- Venting into the attic. This is not a performance issue, it is a damage issue. Dumping warm wet air into an unconditioned attic causes mould and rot. It has to go outside.
The practical rule: buy a fan rated somewhat above your calculated number, keep the duct diameter the same as the fan outlet all the way to the exterior, use rigid duct where you can, and take the shortest path out. A 80 CFM fan on a short straight rigid duct will outperform a 110 CFM fan on a long crushed flex run.
Air has to get in, too
A fan cannot push air out of a sealed box. If your bathroom door seals tightly to the floor and there is no other opening, the fan will strain against a partial vacuum and move very little air. You can hear this happening: the fan gets quieter and the tone changes when the door is shut.
The standard fix is an undercut on the door of about three quarters of an inch above the finished floor. If the door is already installed and you do not want to trim it, a transfer grille in the door or the wall does the same job. This one costs nothing to test: run the fan with the door cracked open and see if performance jumps.
Sones: the number that decides whether you use the fan
Sones measure perceived loudness. Lower is quieter, and the scale is roughly linear, so 2 sones sounds about twice as loud as 1 sone.
- 3 to 4 sones: builder grade. Loud enough that people switch it off early.
- 1.5 to 2 sones: noticeable but not annoying. A reasonable target.
- Under 1 sone: effectively background noise. Worth paying for in a primary bathroom or anywhere adjoining a bedroom.
This is not a comfort luxury, it is a performance issue in disguise. HVI recommends running the fan for 20 minutes after the bathroom is used, and nobody leaves a 4 sone fan running for 20 minutes. A quiet fan gets left on. A loud one gets switched off the moment the person leaves, which is exactly when the room still has the most moisture in it.
A countdown timer switch is the cheapest upgrade in this entire article. Set it, walk away, and the room finishes drying itself. A humidity sensing switch does the same thing automatically and is worth it in a bathroom with no window.
Where to put the fan
Placement matters almost as much as capacity. Steam rises and drifts, so the fan should sit between the moisture source and the door, pulling air across the room rather than letting it escape into the hallway.
- Put the fan near or just outside the shower, not in the far corner.
- A fan rated for use inside a shower enclosure must be on a ground fault protected circuit and listed for damp or wet locations. Check the label.
- Avoid putting the fan directly next to the door. It will pull hallway air straight out and leave the shower end of the room untouched.
- In a wet room, where the whole space gets wet, ventilation needs to be sized generously and placed to move air across the entire floor. We go into the tradeoffs in our guide to wet room ideas.
If you are planning the room from scratch, settle the shower footprint before the fan, because the shower position dictates where the fan belongs. Our guide to shower size covers the minimums that actually work, and bathroom vanity sizes covers the other fixed dimension that shapes a layout.
Frequently asked questions
Is a higher CFM always better?
No. An oversized fan in a small bathroom pulls conditioned air out of the house faster than you want, which costs money in heating and cooling, and in a tightly sealed home it can draw air backwards down a water heater or fireplace flue. Size to the room, then make sure the ductwork lets the fan perform.
What size fan do I need for a 5 by 8 bathroom?
Forty square feet calculates to 40 CFM, so the common 50 CFM fan is the right choice. Step up to 70 or 80 CFM if the ceiling is higher than 8 feet or the duct run is long.
How long should I run the bathroom fan after a shower?
About 20 minutes after you leave the room. A timer switch is the easiest way to make that happen consistently.
Can a bathroom fan vent into the attic?
No. It has to terminate outside through a roof cap, a wall cap or a soffit vent designed for exhaust. Venting warm moist air into an attic leads to condensation, mould and eventually rot in the roof structure.
Do I still need a fan if my bathroom has a window?
In most jurisdictions a window can satisfy code, but in practice a fan is far more reliable. Nobody opens a window in January, and a closed window on a cold morning is where condensation collects fastest.
The short version
Work out your bathroom fan CFM with one CFM per square foot under 100 square feet, or by adding up fixtures above it. Then spend your attention on the duct, because a modest fan with a short, straight, correctly sized rigid duct beats a powerful fan strangled by 4 inch flex and three elbows. Buy something under 2 sones, put it on a timer, and make sure the door lets air in. That combination is what actually clears the steam.