SizingKitHVAC · Exhaust
Bathroom fan CFM calculator, and the duct that undoes it
This gives both figures a bathroom fan is chosen on: the flat 50 CFM an inspector checks, and the larger number the Home Ventilating Institute rules ask for — by floor area up to 100 ft², by fixture count above it. Then it takes the run you are actually connecting, with its length, its elbows and its flex, and totals the static pressure the fan will see against the 0.1 in. w.c. its rating was measured at, because 80 CFM on the box down 25 ft of 4 in flex is 3.4 times that pressure and not 80 CFM in the room. It is free, needs no signup, and runs entirely in the tab.
- 100% free
- No signup
- Code and HVI side by side
- Static pressure from your own run
- 3 to 8 in duct
The room, and how the fan runs
Two answers come out of this: the flat figure a code inspector checks, and the larger figure the Home Ventilating Institute sizing rules give. They disagree for almost every real bathroom, and the bigger one is the one that clears the mirror.
The code minimum and the sound limit both change with this. Continuous asks less air and much less noise, because it never stops.
The HVI area rule assumes 8 ft. Above that the rule is scaled here so the air-change rate it implies still holds.
An enclosed water closet inside a larger bathroom is normally given its own fan rather than counted in this footprint.
Past 100 ft² the HVI method stops using area and starts counting what is in the room.
Fixtures in the room
Used above 100 ft², where the area rule stops. Home Ventilating Institute, Home Ventilation Guidelines: for a bathroom larger than 100 ft², add the allowance for each fixture present rather than using the 1 CFM/ft² rule.
50 CFM each
50 CFM each
50 CFM each
100 CFM each
Size the fan at
80 CFM
Metric
37.8 L/s
Code minimum
50 CFM
- Floor area
- 80 ft²
- HVI, by floor area
- 80 CFM
- The other HVI method
- 150 CFM by fixtures
- Air changes that works out at
- 7.5 ACH
ASHRAE 62.2 Table 5.1; IRC Table M1505.4.4. A code minimum is indifferent to the room it is protecting: the same 50 CFM satisfies a powder room and a bathroom four times the size, which is why the figure beside it is usually larger and is the one that actually clears the moisture. Either number still has to be achieved through the duct rather than on a test rig, and that is what the panel below works out.
What the duct does to that number
A certified rating is measured at 0.1 in. w.c. of static pressure. Describe the run you are actually connecting and the panel below says how far past that test point the fan is being asked to work.
Going up one size is the cheapest fix available: the friction rate falls with the fifth power of the diameter.
The friction equation is written for galvanized. Flex carries a multiplier on top of it, in the field beside this one.
Default 2 — The friction equation on this site is for galvanized steel at 0.0003 ft absolute roughness; flexible duct pulled drum-tight runs at roughly twice that rate for the same diameter. Only true when the flex is stretched tight and supported. Left with slack in a joist bay it is several times the galvanized rate, and there is no multiplier that describes a compressed one — the geometry is not repeatable enough to have a number.
Measured along the duct, not across the attic. Slack counts as length and then costs you again as bends.
Count, then the coefficient. Default 0.2 — A round smooth-radius 90° elbow at a centerline radius of about 1.5 diameters, the geometry of a factory formed adjustable elbow. ASHRAE's duct fitting database is the authority for the exact figure and is not reproduced on this site.
Default 1.2 — Six times the smooth elbow, which is the whole argument for sweeping the turns. A flex duct crushed into a right angle is worse again and has no coefficient at all, because the shape is not repeatable.
In inches of water column, and deliberately zero until you fill it. A termination with a gravity backdraft damper is often the largest single loss in the run, and its figure belongs to whoever made the cap. Left at zero, the total below is duct only and understates what the fan sees.
Static pressure on the fan
0.34 in. w.c.
Against the rating point
3.4 ×
- Duct velocity
- 917 ft/min
- Velocity pressure
- 0.052 in. w.c. / 13.1 Pa
- Straight run
- 0.214 in. w.c. / 53.3 Pa
- Elbows
- 0.126 in. w.c. / 31.3 Pa
- Termination
- not entered
- Sound limit, intermittent
- 3 sones
This run puts 0.34 in. w.c. on a fan whose airflow was certified at 0.1 — 3.4× the test point. The fan is operating that far back on its curve, so the CFM on the box is not the CFM in the room. Read the fan's own pressure-airflow curve at this static pressure, or fix the run: a size up in duct, a sweeping elbow instead of a sharp one, or the same fan mounted inline nearer the termination. ASHRAE Standard 62.2 sound requirement for a continuously operated local exhaust fan. A fan remote-mounted with at least 4 ft of duct between it and the grille is exempt from the sound requirement, which is why an inline fan in the attic is the usual way to meet it.
Bathroom exhaust: what is required, and what is recommended
The top rows are code and are the same whatever the room is like. The bottom rows are Home Ventilating Institute guidance, which exceeds the code and is what a large bathroom needs to work.
| Basis | Applies to | Rate |
|---|---|---|
| Code minimum | Any bathroom, intermittent operation | 50 CFM |
| Code minimum | Any bathroom, continuous operation | 20 CFM |
| HVI, area rule | Up to 100 ft², 8 ft ceiling | 1 CFM per ft² |
| HVI, fixture method | Toilet, in a room over 100 ft² | 50 CFM |
| HVI, fixture method | Shower, in a room over 100 ft² | 50 CFM |
| HVI, fixture method | Bathtub, in a room over 100 ft² | 50 CFM |
| HVI, fixture method | Jetted or whirlpool tub, in a room over 100 ft² | 100 CFM |
| Rating condition | Certified airflow and sound are measured here | 0.1 in. w.c. |
| Sound limit | Intermittent / continuous operation | 3 / 1 sones |
ASHRAE Standard 62.2, Table 5.1 (Local Ventilation Exhaust Airflow Rates), and the International Residential Code, Table M1505.4.4 (Minimum Required Local Exhaust Rates for One- and Two-Family Dwellings).
Area rule: Home Ventilating Institute sizing guidance for bathrooms up to 100 ft², stated at an 8 ft ceiling and scaled here for anything taller. Home Ventilating Institute, Home Ventilation Guidelines: for a bathroom larger than 100 ft², add the allowance for each fixture present rather than using the 1 CFM/ft² rule.
ASHRAE Handbook—Fundamentals, duct design chapter: Δp per 100 ft = 0.109136 × Q^1.9 ÷ D^5.02, for galvanized steel round duct at an absolute roughness of 0.0003 ft and standard air. Flexible duct pulled tight is roughly twice this, and flex left compressed can be several times it — the single most common reason a system that was sized correctly does not move the air.
How to size a bathroom exhaust fan
The room decides the CFM. The duct decides whether you get it.
Measure the room and say how the fan runs
Two walls and the ceiling height. Whether the fan is switched or runs continuously as part of the whole-house ventilation changes both the code minimum — 50 CFM against 20 — and the sound limit it has to meet. Up to 100 ft² the HVI rule sizes on floor area; past that it stops and counts fixtures instead, so the toilet, shower, tub and jetted tub fields are what carry a large bathroom.
Take the larger of the two answers
The code minimum is a compliance floor that does not change with the size of the room, and the HVI figure is what actually clears the moisture. For a small powder room they agree; for a 10 by 8 bathroom the HVI rule asks for 80 CFM against the code's 50, and the plate shows both so you can see which one you are buying to.
Describe the duct before you buy the fan
Diameter, straight length, how many elbows and whether they sweep or turn sharply, plus your cap's own published loss. The second plate totals the static pressure and prints it as a multiple of the 0.1 in. w.c. the fan's airflow was certified at — the number that decides whether a fan rated at 80 CFM is moving 80 CFM or 50.
Technical specifications
| Code minimum carried | 50 CFM intermittent and 20 CFM continuous, for a bathroom of any size, from ASHRAE 62.2 Table 5.1 and IRC Table M1505.4.4. A compliance floor, not a design: the same 50 CFM satisfies a powder room and a 140 ft² master bath. |
|---|---|
| HVI area rule | 1 CFM per square foot of floor area up to 100 ft², stated at an 8 ft ceiling and scaled here for anything taller. At 8 ft it works out at 7.5 air changes per hour, which is the origin of the trade's 'eight air changes' for a bathroom. |
| HVI fixture method | Above 100 ft² the area rule stops and the allowances are added instead: 50 CFM each for a toilet, a shower and a bathtub, 100 CFM for a jetted or whirlpool tub. Guidance that exceeds code, not code itself. |
| The rating condition | Certified airflow and sound are both measured at 0.1 in. w.c. of static pressure. Everything past that point belongs to whoever ran the duct, which is why the number on the box and the number in the room are different measurements. |
| Duct arithmetic | Straight run from the ASHRAE friction equation over the length, multiplied for flex; elbows as loss coefficient × velocity pressure. 80 CFM in 4 in round is 917 ft/min and 0.428 in. w.c. per 100 ft, while the same air in 6 in is 407 ft/min and 0.056 — a factor of eight for one size. |
| What an ordinary run costs | 80 CFM down 25 ft of 4 in flex with two sharp elbows totals 0.34 in. w.c. before the cap is counted — 3.4 times the pressure the rating was taken at. The identical run in 6 in comes to 0.053, about half the test point. |
| Sound | 3 sones for intermittent operation and 1 sone for continuous under ASHRAE 62.2, measured at the same certified point as the airflow. A fan mounted remotely with at least 4 ft of duct between it and the grille is exempt from the sound requirement, which is why inline fans in the attic meet the continuous limit so easily. |
| Numbers this page does not have | ASHRAE's duct fitting database is not reproduced anywhere on this site. The two elbow coefficients are editable defaults with what moves them stated, and the wall or roof cap loss starts blank for the cap manufacturer's own figure rather than being filled with an invented one. |
Frequently asked questions
Is 50 CFM enough for my bathroom?
It is enough to pass, and for anything over about 50 ft² it is not enough to work. 50 CFM is a flat code minimum that takes no account of how big the room is, how many fixtures are in it or how long the shower is, and the HVI sizing rule — 1 CFM per square foot — overtakes it the moment the floor exceeds 50 ft². A 10 by 8 bathroom asks for 80 CFM on that rule against the code's 50, and the difference is the one people notice as a mirror that stays fogged and paint that peels above the shower.
Why does my new fan not seem to move what the box says?
Because the box figure was measured at 0.1 in. w.c. of static pressure and your duct is almost certainly asking for more. A certified rating is taken on a test rig, and 25 ft of 4 in flex with two sharp elbows puts about 0.34 in. w.c. on the fan before the exterior cap is even counted — over three times the test condition, which drives the fan back along its own pressure-airflow curve. Nothing is faulty; the run is the problem, and the fixes are a larger duct, sweeping elbows instead of sharp ones, or a shorter path to the outside.
Can I use 4 inch duct for a bathroom fan?
You can, and it costs you more than anything else on the job. Friction rate falls with roughly the fifth power of the diameter, so at 80 CFM a 4 in duct runs at 0.428 in. w.c. per 100 ft against 0.056 in a 6 in — a factor of eight — and the velocity drops from 917 ft/min to 407, which is also the difference between a duct you can hear and one you cannot. Many fans ship with a 4 in collar and an adapter to 6 in in the box; fitting the adapter is the cheapest performance available on the whole installation.
How many sones is actually quiet?
1 sone or below is what people describe as quiet, and it is also what ASHRAE 62.2 requires of a fan that runs continuously; 3 sones is the limit for an intermittently switched fan and is clearly audible. A sone is a loudness scale rather than a sound-pressure level, so it does not convert to decibels by a fixed offset the way two pressure levels do — if you are comparing sound pressures instead, the decibel calculator handles that arithmetic. And the rating is taken at the same 0.1 in. w.c. as the airflow, so a fan fighting a bad duct is louder in the room than its rating says.
Where do the elbow numbers in the duct panel come from?
They are this page's own editable defaults, and the honest answer is that the authoritative table is not reproduced anywhere on this site. ASHRAE publishes a duct fitting database of loss coefficients, and the 0.2 used here for a smooth radius 90° elbow and the 1.2 for a sharp or mitered one are order-of-magnitude figures for residential round fittings rather than transcriptions of it — which is why both are fields rather than facts. The figure that will actually be honored is the one in your fan's installation instructions, because that document also carries the fan curve you are comparing against.
Does the fan have to discharge outdoors?
Yes — the mechanical codes require exhaust air to be discharged to the outdoors, and terminating into an attic, a soffit cavity or a crawlspace is not compliant anywhere in the United States. The reason is not paperwork: a bathroom fan moves several pounds of water vapor into the space it discharges into every day, and an attic is exactly where that condenses on cold sheathing. A duct that ends an inch short of a roof cap, or a cap whose damper has been painted shut, produces the same result as no duct at all.
Should a bathroom fan run continuously?
It can, and that is a different sizing problem: continuous operation drops the code minimum from 50 CFM to 20 and tightens the sound limit from 3 sones to 1, because a fan you never switch off has to be one you never notice. Running the bathroom fan continuously is one of the recognized ways of providing whole-house ventilation, in which case the flow it needs comes from the dwelling's total requirement rather than from the room, and that total is a separate calculation on the air changes page.
About bath fan sizing, and why the rating on the box is optimistic
Two organizations answer the question and they answer it differently. The codes — ASHRAE 62.2 Table 5.1 and IRC Table M1505.4.4 — set a flat 50 CFM for an intermittent bathroom fan and 20 CFM for one that runs continuously, and those figures do not move with the size of the room, which is what makes them a compliance floor rather than a design. The Home Ventilating Institute sizes on the room instead: 1 CFM for each square foot up to 100 ft², and above that it abandons area and adds an allowance for each fixture, because a large bathroom's moisture comes from what is in it rather than from its footprint. For anything bigger than about 50 ft² the second number is larger, and it is the one worth buying to.
The harder half is what happens after the fan is chosen. A certified airflow rating is measured at 0.1 in. w.c. of static pressure — a specific, quite gentle test condition — and the run in an ordinary house is nothing like it. At 80 CFM a 4 in duct carries the air at 917 ft/min and drops 0.428 in. w.c. every 100 ft, so 25 ft of flex is 0.21 in. w.c. on its own once the flex penalty is applied, and two sharp elbows add another 0.13 because an elbow's loss is its coefficient multiplied by the velocity pressure, and velocity pressure at 917 ft/min is not small. That is 0.34 in. w.c. before the exterior cap is counted, against a rating taken at 0.1. The fan is not faulty and the label is not a lie — the fan is simply operating a long way back on its own curve, which is a place the label never described.
The same duct in 6 in comes to 0.053 in. w.c. for the identical layout, which is the whole argument for fitting the adapter that came in the box. Where a run is long or twisted enough that no duct size rescues it, the answer is an inline fan mounted near the termination rather than a bigger one at the ceiling. If you want the general version of this pressure-drop arithmetic on a larger duct, that is the duct size calculator; if the airflow you need comes from a load or a measurement rather than from a room's floor area, the CFM calculator reaches it three ways. Running the fan continuously turns it into part of the dwelling's ventilation, which is sized by the air changes per hour calculator rather than by this page, and the sone rating you choose on is a loudness scale that does not translate into the sound-pressure arithmetic on the decibel calculator. The equivalent problem in a water line — flow, length, fittings, pressure lost — is the pipe friction loss calculator. None of this is a stamped design, and where a fan is part of a compliance path for whole-house ventilation, the person signing that off is the one who decides.
What happens to the room you measured
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
The dimensions of somebody's bathroom and the layout of their attic duct are worked out in the tab and kept nowhere, so this can be used standing on a ladder with the fan housing in one hand and nothing about the house is recorded anywhere afterwards.