Sizing a bathroom exhaust fan by CFM

The question on site is simple: this bathroom, which fan? Fans are sold by CFM — cubic feet per minute of air moved — and the wrong number shows up months later as a fogged mirror that never clears, peeling paint, or mould in the window corners. Sizing takes a minute and has a method.

The square-footage rule

For bathrooms up to about 100 square feet, the common rule is 1 CFM per square foot, with a 50 CFM minimum. A standard 5 × 8 ft bathroom is 40 sq ft, so the minimum governs: 50 CFM is the floor, not the target. An 8 × 10 ft bathroom is 80 sq ft and calls for 80 CFM.

Over 100 square feet the rule switches to counting fixtures. The common allowance is 50 CFM for each toilet, shower and bathtub, and 100 CFM for a jetted tub. A large ensuite with a shower, a freestanding tub and a toilet adds up to 150 CFM, which in practice means either one high-capacity fan or two fans placed where the moisture is made.

The same arithmetic is sometimes run as an air-change calculation — room volume in cubic feet, multiplied by 8 air changes per hour, divided by 60. With an 8 ft ceiling it lands on the same answer as the square-footage rule, which is why the shortcut works.

When to size up

The rule assumes a small room, a flat 8 ft ceiling and a short duct. Move away from any of those and the number should move too.

• High or vaulted ceilings. The rule sizes for floor area, but the fan is emptying a volume. A 10 ft ceiling holds a quarter more air than an 8 ft one; scale the CFM up in roughly the same proportion, or run the air-change calculation with the real volume.
• Long or twisting duct runs. Every foot of duct and every elbow costs airflow. If the run to the exterior is long, has more than a couple of bends, or is flex duct rather than smooth pipe, go up a size.
• Enclosed toilet or shower rooms. A door between the fan and the moisture defeats the fan. A separate water-closet or shower compartment commonly gets its own fan, sized at 50 CFM, rather than relying on one unit down the hall.
• Steam showers and big soaker tubs. These produce moisture faster than the fixture counts assume. Size generously and control the fan with a timer so it keeps running after the room is empty.

Rated CFM is not installed CFM

The number on the box is measured on a test rig at low static pressure. The installation in the ceiling has real resistance: the duct, the elbows, the wall or roof cap, and any backdraft damper all push back, and delivered airflow falls as resistance rises. An 80 CFM fan on a long, kinked flex run can move half its rating.

This is why an oversized fan on a short, smooth run beats an undersized one every time. The oversized fan loafs along, quiet, and still clears the room; the undersized fan runs flat out and loses. Sizing up one step from the rule-of-thumb number costs little and covers the difference between the test rig and your attic.

The duct itself matters as much as the fan. Keep the run as short and straight as the framing allows, use smooth pipe over flex where you can, and match the duct diameter to the fan outlet — necking a 6-inch outlet down to a 4-inch duct throws away the capacity you paid for. The duct must terminate outdoors through a proper cap, never into the attic or soffit cavity, where the moisture condenses on cold sheathing and does exactly the damage the fan was meant to prevent.

Sones: the number that decides whether the fan gets used

Sones measure perceived loudness. Under 1.0 is barely audible — a quiet fridge in the next room. Around 1.5 is noticeable but easy to live with. At 3 or 4 sones the fan is the loudest thing in the room, and that is the fan the homeowner switches off after thirty seconds.

That matters because a fan only removes moisture while it is running. A loud fan that gets turned off performs worse than a smaller quiet one that runs its full cycle. For ensuites and bathrooms off bedrooms, specify 1.0 sone or lower. Bigger is not louder: many current 110 CFM fans run under 1 sone, quieter than the 50 CFM builder units they replace.

Run time does the real work

Most of the moisture is cleared in the 20 to 30 minutes after the shower ends, not during it. A fan wired to the light switch stops the moment the room goes dark, which is the moment the job is half done. Put the fan on its own timer switch, or use a model with a built-in humidity sensor that runs until the room is actually dry.

What to order

Most residential bathrooms land between 80 and 150 CFM once duct runs and ceiling heights are counted honestly — browse the range in bathroom fans and filter by CFM and sones. Timer and sensor controls to run the fan properly are in wiring devices, worth adding to the same order.

Check before you install. This guide summarises common practice to help you specify parts. The Ontario Building Code, the Ontario Fire Code and the Canadian Electrical Code are the authority, they are revised periodically, and your local authority having jurisdiction can be stricter. Confirm current requirements and have the work done by a licensed electrician.