What size bathroom fan do you need

Two published methods, and the square foot that switches between them

Bath fan sizing looks like one calculation and is actually two, with a hard switch between them at 100 square feet. Below that line the room is sized by its floor area. Above it, floor area is abandoned and the fixtures in the room are counted instead.

That switch is the reason the advice online contradicts itself. Half of it describes the small room method, half describes the large room method, and almost none of it says which room it is describing. Find your floor area first, and only then pick the method.

Published planning guideline. Both methods below are published recommendations from an industry body, not code requirements. Nobody inspects them and no building department enforces them. They are what the profession sizes to, which makes them the figures to ask your contractor about.

Method one: 1 cfm per square foot, up to 100 square feet

For a bathroom up to 100 square feet the industry body recommends an exhaust fan providing 1 cfm per square foot of floor area. The same page states that this works out at approximately eight air changes per hour, which is the reason the rule is that simple: it is an air change target wearing a floor area disguise.

Its own worked example is a room 8 feet by 5 feet. That is 40 square feet, and the recommendation is a fan rated at 40 cfm.

One thing the arithmetic does not tell you: a fan rated at 40 cfm is rated at a test pressure, on a bench, with a duct the test specifies. The fan in your ceiling meets a real duct with real bends in it, and what comes out of the building is less than what the label says. That is not an argument for guessing, it is an argument for reading the duct section below before the fan is chosen.

Method two: count the fixtures, above 100 square feet

Above 100 square feet the industry body recommends a ventilation rate based on the number and type of fixtures present, rather than on floor area. Each fixture carries a figure and you add them together.

Its own worked example is a room 20 feet by 12 feet holding a tub, a shower and an enclosed toilet. Fifty plus fifty plus fifty is 150 cfm.

Notice what the two methods do to the same square footage. That 240 square foot room would have come out at 240 cfm under the small room rule, and the fixture count puts it at 150. The methods are not two routes to one answer, they are two different ideas about what a bathroom fan is for, and the published instruction is to use the one that matches the room size rather than the one that gives the number you prefer.

The jetted tub row is the one that catches people. It is double a plain tub, it is the single largest fixture figure published, and it usually arrives in the plan long after the fan was chosen.

The minimum in the code text, and why it is a floor rather than a target

Underneath both recommendations there is a separate and much smaller number, and it is a different kind of number.

Model code figure, verify locally. A mechanical ventilation system of at least 50 cfm, exhausted to the outside, is the figure in the model code text quoted by the planning guidelines, and it is required in many US jurisdictions. What applies where you live is decided by your building department, through your contractor.

Put the three figures beside each other on the small worked example and the relationship is obvious. The 8 by 5 foot room takes 40 cfm by the published recommendation and 50 cfm by the code figure, so here the code figure is the larger of the two. The 20 by 12 foot room takes 150 cfm by the fixture count, three times the code figure. A minimum that is above the recommendation in a small room and far below it in a large one is a floor, not a target, and it was never meant to size anything.

Two figures circulate for bathroom fans and they are not the same kind of number. 50 cfm is a minimum in a model code text. 1 cfm per square foot is a sizing recommendation from an industry body. One is the least allowed, the other is what the room needs.

Where the air leaves the room, and where the replacement air gets in

A correctly sized fan in the wrong place moves the correct volume of the wrong air. The same industry body publishes where the extraction points go: typically over or near the shower or the tub, and in an enclosed toilet compartment.

It also states that an enclosed toilet compartment needs either an operable window or a fan, which is the line people miss when a plan gains a separate compartment late.

And it states the other half of the job, which almost nobody plans for: bathroom doors need to be undercut so that replacement air can get into the room. Air pulled out has to be replaced from somewhere, and a fan sealed into a room with a tight door and a tight threshold is a fan working against itself. The undercut is decided with the door order and the finished floor build-up, not afterwards with a saw.

The duct is a manufacturer figure, and it decides which fans you can fit

The industry body publishes no duct diameter and no maximum duct length. Those are manufacturer figures, they differ sharply from model to model, and they can quietly rule out the fan you have already chosen.

Manufacturer document. One manufacturer publishes a ceiling fan of its own that is adjustable on the unit between 50, 80 and 110 cfm, rated at no more than 0.4 to 0.9 sone across those speeds at the tested pressure, asking for a duct of no less than 4 inches (102 mm) and fitting a joist spacing up to 24 inches (610 mm). Those are that model, from that document.

Manufacturer document. A second manufacturer publishes an older and cheaper model that uses a 3 inch (76 mm) duct and states that its discharge goes through the roof or through a wall. That is a different model from a different document, and a 3 inch duct and a 4 inch duct are not interchangeable in a ceiling that is already framed.

The second manufacturer also publishes the arithmetic for its own product, and it is the most useful thing on this page: add one foot of equivalent duct length for each inch of duct diameter, for every 90 degree elbow. On that rule a 4 inch elbow counts as 4 feet of duct and an 8 inch elbow counts as 8 feet. Again, that is one manufacturer rule for its own fan, and the table in the manual for the fan you buy is the one that counts.

What this means in practice is that a fan is not chosen from a cfm figure alone. It is chosen from a cfm figure, a duct diameter that the ceiling can actually carry for its whole run, and a bend count that somebody has added up.

Where the duct comes out of the building

This is the single most common bathroom ventilation failure and it has nothing to do with the fan. A duct that discharges into an attic, a soffit or a roof space has moved the moisture from a room you can see into a space you cannot.

Many US jurisdictions require the termination to be outside the building rather than into an attic, a soffit or a roof space, and that is verified locally, through your contractor and your building department. The question to ask on the day the ceiling is open is simple: where does this duct come out of the building, and has anybody stood outside and looked at that point.

Photograph the answer. Once the ceiling closes, the only way to check it again is to open the ceiling again.

Noise, and how long the fan runs

Airflow is what a fan is bought for. Noise is why it does not get used, and a fan that does not get used is a fan that moved no air at all.

Manufacturer document. Sone ratings are published per model and tested at a stated static pressure, which is why two figures for the same fan are not a contradiction. One manufacturer publishes no more than 0.4 to 0.9 sone across the three speeds of one of its own fans at 0.25 inches water gauge. That is that model at that test pressure, from that document. Compare fans at the same tested pressure or you are not comparing them.

The industry body recommends leaving the fan running for 20 minutes after the bathroom is used. That is a recommendation about behavior rather than about hardware, but it is worth reading as a hint about the switch: a timer, or a control somebody will actually use, gets more real ventilation out of a room than the next size up of fan does.

The published planning guidelines put ventilation controls between 15 and 48 inches (381 and 1219 mm) above the floor, which is the same range they give for everyday storage. Decide where that switch goes while the wall is open.

The four questions, and what is not your job here

1. Is the fan sized by one of the two published methods, and which one, and is the figure written down on the drawing.
2. Where does this fan discharge, and has anyone seen that point from outside the building.
3. What diameter is the duct, for its whole length, and how many bends are in the run.
4. Is the fan rated for the position it is going in, and is the circuit the one the electrician says it has to be on.

None of the four is a question you answer yourself, and none of them can be answered once the ceiling is closed. Ask them on the day the duct is run, write the answers down with a date, and photograph what you were shown.

Where a fan may be installed, how it is wired and where its duct terminates are decided by your electrician, your contractor and your building department. Nothing on this page is an instruction to carry out any of that work.