Ventilation is specified in two dialects — CFM, the airflow a fan actually moves, and ACH, the air changes per hour a room’s health depends on — and this CFM Calculator translates between them: enter the room’s dimensions and a target ACH to get the required fan size, or enter a fan’s CFM to learn the air changes it delivers.
Worked Example: A 20 × 15 Workshop With 9-Foot Ceilings
The room’s volume is 20 × 15 × 9 = 2,700 cubic feet. Six air changes per hour means moving 6 × 2,700 = 16,200 cubic feet of air every hour, which per minute is 16,200 / 60 = 270 CFM — the fan requirement, before duct losses. The reverse calculation rides in the same result: the 400 CFM fan in the last field delivers 400 × 60 / 2,700 = 8.9 ACH in this room — comfortably above the target, which is the usual and correct direction of error, because the nameplate CFM is measured at zero static pressure and real ductwork taxes it. That tax is the number’s most important caveat: a fan rated 400 CFM blowing through elbows, filters and a louvered vent may deliver 300 or fewer at the grille, so the professional sequence is to compute the requirement here, add the system’s pressure-drop reality (manufacturer fan curves exist for this), and select the fan whose curve crosses your CFM at your static pressure — not the fan whose box merely prints the number. The ACH target itself is a choice with a literature: general living spaces are often designed around 4–6 total ACH equivalents when mechanical ventilation and infiltration combine, bathrooms exhaust intermittently at 50–110 CFM by fixture, kitchens demand 100+ CFM intermittent (or capture-rated hoods), workshops and garages with engines or solvents push toward the 6+ of this example and beyond, and health-care and laboratory spaces live in a different, code-written world of 6–15+ ACH with pressure relationships this page does not pretend to design.
The ACH Idea, and What an Air Change Really Means
An “air change” sounds like the room’s air being swapped wholesale, and the truth is subtler in a way that matters for expectations. Ventilation air mixes rather than pistons: under perfect mixing, one air change replaces about 63% of the original air, two changes about 86%, and the contaminant concentration decays exponentially (each change multiplies what remains by about 0.37) rather than stepping to zero. This is why standards speak in changes per hour against a rate of contaminant generation, and why diluting a strong source (a solvent spill, a smoky pan) is a losing strategy compared to capturing it at the source with local exhaust — the kitchen hood and the fume hood are the same argument wearing different sheet metal. It is also why the calculator’s formula, CFM = volume × ACH / 60, is a sizing identity rather than a purification promise: it guarantees the dilution rate, and the dilution math does the rest over time. Rooms that defeat mixing (dead corners, short-circuiting supply-to-exhaust paths, closed doors starved of return air) underperform their ACH on paper — placement and pathway are the other half of ventilation design, and a fan sized perfectly by this page still needs somewhere for the makeup air to come from: exhaust without a makeup path simply depressurizes the room until the fan’s delivered CFM sags toward whatever the cracks admit, which is the physics behind backdrafting fireplaces and doors that whistle.
Sizing Fans and Ducts From the Number
With required CFM in hand, the practical chain runs: choose the fan by its curve at the system’s estimated static pressure (long, elbowed, filtered runs justify stepping a size up), keep duct velocities civilized (roughly 600–900 feet per minute in residential mains keeps noise down — a 270 CFM requirement through a 6-inch round duct at about 1,375 fpm would sing; an 8-inch duct brings it to about 775 fpm and silence), and remember that flex duct, crushed or sagging, taxes flow far beyond its paper friction. Bathrooms and kitchens add their own sizing traditions on top of ACH logic — per-fixture minimums and capture efficiency for hoods — and whole-house ventilation standards (such as ASHRAE 62.2’s formula blending floor area and bedroom count) produce a continuous CFM figure that this page’s reverse mode can translate into your rooms’ ACH for comparison. The through-line of all of it: compute the room’s volume honestly (vaulted ceilings hold more air than the floor plan suggests — measure to the volume, not the footprint), pick the target from the space’s actual use, respect the static-pressure tax, and the fan you buy will be the fan the room needed.
Bathrooms and Kitchens: The Intermittent Exceptions
Wet rooms break the ACH habit deliberately — they are sized by fixture and event, not by steady dilution — and the calculator’s reverse mode shows what those traditions mean in ACH terms. An 80 CFM bath fan (a common size) in a small 8 × 5 bathroom with an 8-foot ceiling — 320 ft³ — delivers 80 × 60 / 320 = 15 ACH, which sounds extravagant until the event is a shower: the moisture load arrives in minutes, and guidance such as the Home Ventilating Institute’s (about 1 CFM per square foot of bathroom, or roughly 8 ACH minimum continuous-equivalent) puts the floor near 43 CFM for that room — the 80 is headroom for the event, run on a timer for 20–30 minutes after it. A 300 CFM range hood over a 12 × 12 × 9-foot kitchen (1,296 ft³) is 13.9 ACH while cooking — and still the right design, because capture at the source beats dilution by an order of magnitude: the hood’s job is to intercept the plume before it becomes room air, a job rated by capture efficiency rather than air changes. The lesson for the general calculator: ACH is the correct dialect for continuous, distributed loads (people, off-gassing, background moisture), while point events and point sources get point exhaust — and most real rooms are designed with both, sized by their own logics, exactly as these two rooms are.
After Installation: Measuring What You Actually Got
The sizing chain ends at a grille, and a few checks confirm the delivered airflow resembles the calculated one. The crude test is honest about its crudeness: a tissue or a ribbon at the exhaust grille shows flow exists and its rough enthusiasm, and an incense stick traces the room’s air paths — dead corners and short-circuits announce themselves visibly. A vane anemometer at the grille, multiplied by the grille’s free area, gives a field CFM within shouting distance (grille free area, not nominal size, is the multiplier that matters). The fan laws explain what adjustments do: airflow scales in direct proportion to fan speed, pressure with its square, and power with its cube — so a 10% speed increase buys 10% more air at 33% more power and noise, the arithmetic behind every “just run it faster” disappointment. If the delivered figure falls well short of the design, the suspects, in order, are static pressure (flex duct runs and crushed elbows lead the league), a missing makeup path, a fan curve misread at the selection stage, and only lastly the fan itself. Measure first, and the fix is usually ductwork — the cheapest CFM in the building is the kind that was already paid for and strangled.
Common Mistakes
Sizing by floor area alone and forgetting ceiling height — volume is the input, and a 12-foot ceiling is a third more air than a 9-foot one over the same floor. Trusting the box CFM (free-air rating) as the delivered CFM through real ducts. Exhausting without a makeup-air path and blaming the fan for the physics. Aiming ACH at the room while the contaminant has a source that local exhaust should capture. And confusing CFM with velocity: a small duct can deliver high speed and low volume, and it is the volume that changes the air.
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Frequently Asked Questions
How do I calculate CFM from room size and ACH?
CFM = room volume (ft³) × ACH ÷ 60. A 2,700 ft³ room at 6 ACH needs 2,700 × 6 / 60 = 270 CFM — then size up for duct losses, because nameplate CFM is a free-air figure.
What ACH should I target?
It depends on the space: roughly 4–6 for general occupied rooms, 6+ for workshops and garages with fumes, and code-set higher values for labs and healthcare. Bathrooms and kitchens are usually sized per fixture (50–110 CFM bath fans, 100+ CFM kitchen exhaust) rather than by ACH alone.
How many air changes does my 400 CFM fan give?
ACH = CFM × 60 ÷ volume. In the 2,700 ft³ example room, 400 × 60 / 2,700 ≈ 8.9 ACH on paper — less in practice once duct resistance taxes the fan’s free-air rating.
Does one air change replace all the air?
No. With normal mixing, one air change removes about 63% of the original air; concentration decays exponentially (×0.37 per change). That is why continuous dilution and source capture beat occasional purging.
Why is my exhaust fan underperforming?
The usual suspects: static pressure from ducts, elbows and filters cutting delivered CFM below the box rating, and no makeup-air path, which depressurizes the room and starves the fan. Size from the fan curve, not the box.
What duct size keeps 270 CFM quiet?
Aim for roughly 600–900 fpm in the duct: 270 CFM needs about 0.35 ft² of duct area for ~775 fpm — an 8-inch round duct (0.349 ft²). A 6-inch duct would run near 1,375 fpm and be audible.