Room CFM from air changes
The core CFM formula in HVAC ventilation is air changes per hour. You decide how many times an hour the room's air should be replaced (the ACH), and the airflow follows from the room volume:
So a 200 ft² bedroom with an 8 ft ceiling (1600 ft³) at 6 ACH needs (1600 × 6) ÷ 60 = 160 CFM. Guideline air-change rates vary by room: living spaces and offices around 6, kitchens and bathrooms around 8, laundry and utility rooms higher. These are general comfort/ventilation guidelines — not a quote of any specific copyrighted standard — and your local code may set its own minimums.
| Room type | Guideline ACH |
|---|---|
| Living room / bedroom | 6 |
| Office | 6 |
| Kitchen | 7–8 |
| Bathroom | 8 |
| Laundry / utility | 8–10 |
| Whole house (fresh air) | 4–6 |
CFM per square foot
People often want CFM "per square foot" rather than per air change. The two are linked through ceiling height: CFM per ft² = (ceiling height × ACH) ÷ 60. With an 8 ft ceiling at 6 ACH that's 0.8 CFM/ft²; bathrooms at 8 ACH and 8 ft work out near 1 CFM/ft², which is why "1 CFM per square foot" is the common shorthand. The per-square-foot tab uses your real ceiling height so the figure is honest rather than a flat guess.
Bathroom, kitchen and whole-house fans
Exhaust fans are sized by the air they move, in CFM. For a bathroom, two methods agree closely:
- 1 CFM per square foot of floor area, with a minimum of 50 CFM for any bathroom.
- 8 air changes per hour: CFM = (area × ceiling height × 8) ÷ 60 — this matters when ceilings are above 8 ft.
A 100 ft² bathroom needs about 100 CFM either way. Add roughly 50 CFM for each extra tub, shower or jetted bath beyond the first, and add about 20% for long or elbow-heavy duct runs, because a fan rated at 0 static delivers less once it's pushing through real duct. The tab applies the 50 CFM floor automatically and flags when ceiling height pushes the volume method above the simple rule.
For a kitchen range hood a common starting point is about 100 CFM per foot of cooktop width for electric and ~150 for gas. A whole-house fan is sized very differently — on the order of 2–3 CFM per square foot of house, enough for roughly 30–60 air changes an hour to flush heat quickly.
Air change rate from a known CFM
To check whether an existing or proposed fan is enough, run the formula backwards: ACH = (CFM × 60) ÷ room volume. A 60 CFM fan in a 480 ft³ bathroom gives (60 × 60) ÷ 480 = 7.5 ACH — just under the usual bathroom target of 8.
Register and return-grille size
A register or return-air grille is sized so the air doesn't move too fast across its face (which is noisy). The free-area method is: free area (ft²) = CFM ÷ face velocity (fpm). Supply registers are commonly sized around 500–700 fpm and returns lower, around 300–500 fpm. Because the vanes block part of the opening, divide the free area by the grille's free-area fraction (often around 0.7) to get the gross face size. The register tab reports the required free area and an approximate square size.
Worked examples
Example 1 — bedroom at 6 ACH
200 ft² × 8 ft = 1600 ft³. At 6 ACH: (1600 × 6) ÷ 60 = 160 CFM. Try ?mode=ach&volume=1200&ach=6 to load a 1200 ft³ room at 6 ACH (120 CFM).
Example 2 — small bathroom fan
A 40 ft² bathroom by the 1 CFM/ft² rule is 40 CFM, but the 50 CFM minimum applies, so size a 50 CFM fan. Load it with ?mode=bathfan&area=50.
CFM & ventilation FAQ
What is the formula to calculate CFM for a room?
CFM = (room volume in ft³ × air changes per hour) ÷ 60. Room volume is length × width × ceiling height, and ACH is how many times you want the room's air replaced each hour. For example a 1200 ft³ room at 6 ACH needs (1200 × 6) ÷ 60 = 120 CFM. The air-change tab does this and lets you pick a guideline ACH by room type.
How many CFM do I need per square foot?
It depends on the room. A rough general comfort figure is about 1 CFM per square foot for living spaces with standard ceilings, but the honest way is to convert target air changes to CFM: CFM per ft² = (ceiling height × ACH) ÷ 60. With an 8 ft ceiling at 6 ACH that's 0.8 CFM/ft²; bathrooms use about 1 CFM/ft². The per-square-foot tab works from your actual ceiling height and ACH.
How do I size a bathroom exhaust fan?
Two methods agree closely. The simple one is 1 CFM per square foot of floor area, with a minimum of 50 CFM for any bathroom. The volume method is 8 air changes per hour: CFM = (floor area × ceiling height × 8) ÷ 60, which matters for high ceilings. A 100 ft² bathroom needs about 100 CFM either way. Add roughly 50 CFM for each extra tub, shower or jetted bath, and add ~20% for long or twisty duct runs. The fan tab applies the 50 CFM floor automatically.
How do I work out the air change rate (ACH) of a room?
Rearrange the room formula: ACH = (CFM × 60) ÷ room volume in ft³. If a 60 CFM fan serves a 480 ft³ bathroom, that's (60 × 60) ÷ 480 = 7.5 air changes per hour. The ACH-from-CFM tab does this so you can check whether a fan meets a target like 8 ACH.
What size register or return grille do I need for a given CFM?
Grille size comes from face velocity: free area (ft²) = CFM ÷ face velocity (fpm). Supply registers are often sized around 500–700 fpm and returns lower, around 300–500 fpm, to stay quiet. Because vanes block part of the opening, divide the free area by the grille's free-area fraction (often ~0.7) to get the gross size. The register tab does this and reports an approximate square size.
Is this CFM calculator free and private?
Yes. It's free, needs no signup or download, and runs entirely in your browser, so nothing you enter leaves your device. You can copy a shareable link that reopens the calculator with your numbers, for example a room volume and air-change target already filled in.