HVAC Calculators · Calculator
Air Changes per Hour Calculator: ACH from CFM and CFM from ACH
Air changes per hour (ACH) is how many times a room's full volume of air is moved in an hour: ACH = CFM × 60 ÷ room volume in cubic feet. A 15 × 12 ft bedroom with an 8 ft ceiling holds 1,440 cubic feet, so a 100 CFM fan gives about 4.2 ACH, just above the typical 4 ACH for a bedroom. The calculator also turns a blower-door ACH50 result into an estimate of everyday natural leakage: ACH50 ÷ N, so 7 ACH50 in a mixed climate (N = 18) is about 0.39 ACH.
Air changes per hour calculator
What ACH measures
Air changes per hour describes how fast air in a space is moved, replaced or filtered relative to the size of the space. It is used in several different ways, and it helps to be clear which one you mean:
- Exhaust or supply ventilation: a fan pulls stale air out and outdoor air replaces it.
- Recirculating filtration: an air purifier or HVAC filter cleans the same air repeatedly. Here the useful number is based on clean air delivery rate, not raw fan airflow.
- Air leakage: blower door results are reported as ACH50, the leakage at 50 pascals of pressure. That is a test condition, not the leakage in normal weather.
The formula
Volume (ft³) = Length × Width × Ceiling height
ACH = CFM × 60 ÷ Volume
CFM = ACH × Volume ÷ 60
Share of room-type target = ACH ÷ Recommended ACH × 100%
Natural ACH ≈ ACH50 ÷ N
The 60 converts cubic feet per minute to cubic feet per hour. In metric, ACH = m³/h ÷ room volume in m³. One CFM is about 1.70 m³/h.
The room-type select sets a typical recommended ACH for that kind of space. The calculator compares your fan's ACH with it and shows the CFM that would reach it (recommended ACH × volume ÷ 60).
The blower-door inputs are a separate, whole-house calculation. N is the climate factor from the Lawrence Berkeley Laboratory (LBL) infiltration method. It runs from about 14 in very cold, windy regions, where stack and wind effects drive more leakage, to about 22 in mild ones. Those values assume a single-storey house with normal shielding; for a two-storey or exposed house, pick a lower N.
One air change does not mean every molecule has been replaced. With well-mixed air, one air change removes about 63% of an airborne contaminant, two remove about 86%, and three about 95%. That is why targets of four to six ACH are common for rooms where you want quick clean-up.
Worked example
A 15 × 12 ft bedroom with an 8-foot ceiling and a 100 CFM fan:
- Volume: 15 × 12 × 8 = 1,440 ft³.
- ACH: 100 × 60 ÷ 1,440 = 4.17 air changes per hour.
- To reach 6 ACH: 6 × 1,440 ÷ 60 = 144 CFM.
- Room type bedroom, typical 4 ACH: 4.17 ÷ 4 = 104%, so the fan meets it; 4 × 1,440 ÷ 60 = 96 CFM would be enough.
- One air change every 60 ÷ 4.17 = 14.4 minutes.
Separately, the house had a blower-door test of 7 ACH50 and sits in a mixed climate (N = 18): 7 ÷ 18 = 0.39 natural ACH, a fairly leaky but common result for an older home.
These numbers match the calculator's defaults.
Typical ACH targets
| Space or purpose | Typical ACH |
|---|---|
| Whole-house ventilation, modern code home | About 0.3–0.5 (outdoor air) |
| Bedroom or living room filtration | 4–5 |
| Allergy or asthma sufferers, wildfire smoke | 5–6 or more |
| Bathroom exhaust (HVI 1 CFM/sq ft rule) | About 7.5 at an 8 ft ceiling |
| Kitchen exhaust while cooking | 10–15 |
| Workshop with fumes or dust | 6–12, depending on the process |
These are typical planning ranges, not code values. Residential ventilation codes (IRC, ASHRAE 62.2) express requirements in CFM based on floor area and bedrooms rather than as ACH. For bathrooms, see the bathroom fan CFM calculator.
Getting the inputs right
- Use the delivered airflow. Fan ratings are at low resistance. A long duct can cut real airflow considerably; see duct sizing.
- Use CADR for purifiers. A purifier's raw fan CFM overstates cleaning; its CADR already accounts for how well the filter captures particles.
- Open floor plans: treat connected spaces as one volume, or the result will look better than reality.
- Vaulted ceilings: use the average height.
ACH50 and how tight your house is
Energy auditors use a blower door to depressurize a house to 50 pascals and measure the airflow needed, then convert it to ACH50. Older homes often test well above 10 ACH50, typical new construction in many states must be at or below 3–5 ACH50 depending on climate zone, and very tight homes go below 1.
Because 50 Pa is far more pressure than wind and stack effect normally produce, ACH50 overstates everyday leakage. The LBL method divides it by an N-factor, roughly 14–22 depending on climate, building height and wind exposure. The result is an estimate of average natural infiltration, not a measurement; real leakage swings with weather from day to day. Homes that estimate below about 0.35 natural ACH generally need mechanical ventilation, but they also need less heating and cooling capacity. Pair this with the insulation R-value chart when you are planning upgrades, and with the CFM calculator for room airflow.
ACH for ventilation vs filtration
It is easy to mix up two different jobs that both get quoted in ACH. Ventilation swaps indoor air for outdoor air, which dilutes carbon dioxide, odors, moisture and gases from furniture and cleaning products. Filtration recirculates indoor air through a filter, which removes particles such as dust, pollen and smoke but does nothing for gases or CO₂.
Whole-house ventilation rates are low because every cubic foot of outdoor air has to be heated or cooled. A typical modern code home might bring in outdoor air at around a third to a half of an air change per hour, often through an exhaust fan, a supply duct to the return, or a heat- or energy-recovery ventilator. Filtration rates can be much higher because the air never leaves the house, so there is no energy penalty beyond fan power. When you set a target in the calculator, decide first which of these you are trying to achieve, then measure the airflow that actually does that job: outdoor-air CFM for ventilation, CADR or filtered CFM for cleaning.
In humid weather, extra outdoor-air ventilation adds moisture that your air conditioner must remove. Keep ventilation near the code minimum and use filtration for particle control.
Frequently asked questions
How many air changes per hour is good for a bedroom?
For air cleaning, four to five per hour is a good target, and the bedroom option in the room-type list uses 4 ACH. For outdoor-air ventilation, much less is normal; whole-house systems usually deliver well under one air change per hour.
How do I convert ACH to CFM?
Multiply ACH by the room volume in cubic feet and divide by 60. Six ACH in a 2,000 ft³ room is 6 × 2,000 ÷ 60 = 200 CFM.
Is ACH50 the same as ACH?
No. ACH50 is measured under a 50-pascal test pressure. Natural leakage is far lower, typically estimated as ACH50 divided by an N-factor of about 14 to 22 depending on climate, height and exposure. Enter both in the blower-door fields of the calculator to get the estimate.
Last reviewed 2026-10-28