Understanding your result
The headline is either the air changes per hour the room gets from the airflow you entered, or the airflow it needs for the target ACH you chose; the sub-line gives the volume and the minutes per air change, which is the more intuitive figure for exhaust fans (how long until the steam is gone) and purifiers (how long until the air has been through the filter once). The stats repeat volume, the result and the minutes.
The table ends with six rows, one for each common target, giving the CFM this particular room needs for each: ASHRAE 62.2’s whole-house outdoor air rate expressed per room, bedroom and living room supply air, the HVI bathroom exhaust rule, kitchen exhaust while cooking, and the AHAM air purifier rule. Those rows are the point of the calculator: they show at a glance that the 100 CFM that is plenty for an air purifier is far more than the room’s share of ventilation air and far less than a bathroom fan.
For the whole house’s outdoor air rate, use the whole-house ventilation calculator; for a bathroom, the bathroom exhaust fan calculator; for a purifier, the air purifier CADR calculator; and for the conditioned supply air each room gets from a central system, the CFM per room calculator.
How we calculate this
An air change is one room volume of air moved in or out; the conversion between CFM and ACH is only the room’s volume and the sixty minutes in an hour. The targets in the table are from their sources: ASHRAE 62.2’s whole-house rate converted to about 0.35 ACH for a typical house; typical supply air rates for bedrooms and living rooms from the room loads the central AC calculators produce; the Home Ventilating Institute’s 8 ACH bathroom rule and 15 ACH kitchen rule; and the Association of Home Appliance Manufacturers’ 2/3 rule for air purifiers, which at an 8 ft ceiling equals 4.8 ACH. The ventilation and exhaust minimums page lists them with their sources, and the unit conversions page covers CFM to m³/h.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Whole-house outdoor air | about 0.35 ACH | ASHRAE 62.2 (0.03 CFM/sq ft + 7.5 CFM per bedroom + 1) in a typical house |
| Bedroom supply air | about 5 ACH | Typical room loads at 400 CFM per ton |
| Living room supply air | about 7 ACH | Typical room loads at 400 CFM per ton |
| Bathroom exhaust | 8 ACH | Home Ventilating Institute recommendation |
| Kitchen exhaust | 15 ACH | HVI recommendation for kitchens while cooking |
| Air purifier | 4.8 ACH | AHAM 2/3 rule: smoke CADR ≥ ⅔ × floor area, 8 ft ceiling |
Assumptions last reviewed October 8, 2026.
The calculator assumes the air mixes fully in the room, which it does not; a bathroom fan at one end of a long room clears steam from the shower end slowly, and short-circuiting between a supply and a return in the same corner can halve the effective air change. It also does not distinguish between outdoor air, recirculated air and exhaust, except through the targets. The guide to whole-house ventilation explains what counts as ventilation air, and the guide to bathroom and kitchen exhaust covers the exhaust rules.
Two worked examples
A bedroom with 100 CFM of supply air
14 × 12 ft, 8 ft ceiling, 100 CFM from a central register.
- Volume: 14 × 12 × 8 = 1,344 cu ft
- ACH: 100 × 60 ÷ 1,344 = 4.46 air changes per hour
- One air change every 13.4 minutes; 0.60 CFM per sq ft
- For comparison this room needs 8 CFM for its share of whole-house ventilation, 112 CFM for 5 ACH of supply, 179 CFM for 8 ACH of exhaust and 108 CFM for the air purifier rule
The room gets a little under the typical 5 ACH of supply air, which is fine for a bedroom with average exposure; the same 100 CFM would be an unusually strong air purifier and far more outdoor air than the room’s share of ventilation.
The fan a small bathroom needs for 8 ACH
9 × 6 ft, 8 ft ceiling, target 8 air changes per hour.
- Volume: 9 × 6 × 8 = 432 cu ft
- CFM: 8 × 432 ÷ 60 = 58 CFM
- One air change every 7.5 minutes; 1.07 CFM per sq ft
A 58 CFM requirement means the code-minimum 50 CFM fan is slightly short and an 80 CFM fan is the practical choice. The bathroom exhaust fan calculator reaches the same answer by its three rules and adds the duct check that decides whether the fan delivers its rating.
Where to find your inputs
Room dimensions. Measure the floor and the ceiling height; for a sloped ceiling use the average height. Closets with open doors count as part of the room.
Airflow. The fan’s rated CFM from its label (in practice it delivers less through a long duct), a register’s CFM from the CFM per room calculator, or an air purifier’s CADR from its label.
Target ACH. From the table in the result for the purpose you have in mind, or from a code or standard that specifies one.
Common mistakes
- Comparing ACH across purposes. 0.35 ACH of outdoor air is right; 0.35 ACH from a bathroom fan is useless. Match the target to the job.
- Using the fan’s label CFM. A 110 CFM fan on 30 ft of 4-inch duct may move 60; use the rating at 0.25 in wc or measure it.
- Forgetting the ceiling. A 10 ft ceiling adds 25% to the volume and cuts the ACH by a fifth.
- Ignoring mixing. A supply and return in the same corner short-circuit; real air change in the room is lower than the arithmetic.
- Chasing high ACH with outdoor air. Every extra air change of outdoor air is air that must be heated or cooled.
- Reading purifier ACH from the “room size” on the box. Use the CADR and this calculator; the box figure often assumes 1 ACH.
Questions people ask
- How do I calculate air changes per hour?
- Multiply the airflow in cubic feet per minute by 60 to get cubic feet per hour and divide by the room's volume in cubic feet. A 14 × 12 ft room with an 8 ft ceiling is 1,344 cu ft; 100 CFM into it is 6,000 cubic feet per hour, which is 4.46 air changes per hour, one change every 13.4 minutes. In metric, divide m³/h by the volume in m³.
- How many air changes per hour should a house have?
- For outdoor air, ASHRAE 62.2's formula of 0.03 CFM per square foot plus 7.5 CFM per bedroom plus one works out to about 0.3 to 0.4 air changes per hour in a typical house, and that is the figure a continuous ventilation fan or ERV should deliver. Older leaky houses had 0.5 to 1.0 ACH of natural infiltration; tight new houses have 0.1 to 0.2 without mechanical ventilation, which is why codes now require it.
- How many air changes per hour does a bedroom need from the air conditioner?
- This is conditioned supply air, not outdoor air, and it is set by the room's heating and cooling load rather than by a ventilation rule; the result is usually 4 to 6 air changes per hour of supply in a bedroom and 6 to 8 in a living room. A 150 sq ft bedroom getting 180 CFM from a central system sees about 9 ACH, which is generous; 100 CFM gives about 5. Low supply ACH shows up as a room that lags the thermostat.
- What ACH should a bathroom exhaust fan give?
- The Home Ventilating Institute recommends 8 air changes per hour for bathrooms, alongside the 1 CFM per square foot rule for small rooms and the fixture rule for large ones; the code minimum is 50 CFM intermittent whatever the room size. An 8 × 10 ft bathroom with an 8 ft ceiling needs about 85 CFM for 8 ACH, which rounds up to a 110 CFM fan. The bathroom exhaust fan calculator applies all the rules at once.
- What ACH does an air purifier need?
- Four to five air changes per hour through the filter is the usual target for a real reduction in fine particles, and the AHAM 2/3 rule (a smoke CADR of two-thirds of the room's square footage, for 8 ft ceilings) delivers about 4.8. For a 168 sq ft bedroom that is a CADR of about 112 CFM. The air purifier CADR calculator sizes the unit; this calculator shows the CFM that 4.8 ACH needs for any room.
- Is more air changes always better?
- No. Outdoor air changes cost energy to heat, cool and dehumidify, so the ventilation rate should meet the standard and not much more; supply air changes above what the load needs mean an oversized system; and exhaust air changes above the rule pull conditioned air out of the house. The right ACH depends on what the air is doing, which is why the calculator lists six different targets rather than one.