What an air change per hour actually measures
ACH normalises airflow by the size of the space it serves. Two hundred and fifty cfm is a lot of air in a bathroom and very little in a warehouse, so the raw airflow number tells you nothing on its own. Divide it by the room's own volume and you get a rate that is comparable between spaces: how many times per hour the fan moves a volume of air equal to the room's.
The arithmetic is a unit conversion. Airflow in cubic feet per minute, multiplied by 60, gives cubic feet per hour. Divide by the room volume in cubic feet and the cubic feet cancel, leaving a pure rate in reciprocal hours. In SI it is simpler still: cubic metres per hour divided by cubic metres, with no conversion constant at all.
What ACH does not measure is how much fresh air each person gets. That is a different question with a different answer, and it is the one modern ventilation standards mostly ask. ASHRAE 62.1 sets commercial ventilation rates from a per-person component plus a per-square-foot component, precisely because occupant-generated contaminants scale with people and material-generated contaminants scale with area, and neither scales with ceiling height. A room with a 20 ft ceiling gets a very generous ACH from a modest fan while its occupants get exactly the same fresh air as they would in an 8 ft room.
ACH remains the right measure when the contaminant is distributed through the volume rather than produced by occupants: dilution of solvent vapour in a shop, heat removal from a machine room, smoke clearance, or humidity control in a grow space. It is also the natural language for exhaust in small rooms, which is why bathroom and kitchen requirements are often written that way.
One air change does not replace the air
The phrase invites a picture of a piston pushing the old air out and pulling new air in. That is not what happens in a real room. Incoming air mixes with what is already there, and some of the newly supplied air leaves again before it has done any work.
For a perfectly mixed space with a contaminant that is not being replenished, the concentration decays exponentially: C(t)/C(0) = e−ACH·t with t in hours. After one nominal air change, e−1 = 0.368, so 37% of the original air is still there. Two air changes leave 13.5%, three leave 5.0%, and it takes about 4.6 air changes to reach 1%.
This matters whenever a clearance time is being specified. If a procedure requires 99% removal, that is 4.6 nominal air changes, not one: at 6 ACH the room needs 46 minutes, not 10. The minutes-per-change output on this page is the nominal figure, 60 divided by ACH, and it is the right number to multiply by the number of changes your requirement calls for.
Real rooms are also not perfectly mixed. Where the supply and the exhaust are close together, part of the airflow short-circuits between them and the effective rate in the occupied zone is lower than the nominal one. Where a room is long and the exhaust is at one end, the far end clears more slowly. Both effects mean the exponential model is the optimistic case, which is why standards that care about clearance specify diffuser placement as well as airflow.
Worked example: a 20 by 15 ft office at 250 cfm
An office measures 20 ft by 15 ft with an 8 ft ceiling, and the supply diffuser has been measured at 250 cfm with a flow hood. The design target for the space is 6 ACH.
- Volume. 20 × 15 × 8 = 2,400 cu ft.
- Delivered rate. 250 cfm × 60 min/h = 15,000 cu ft per hour. Divided by 2,400 cu ft: 6.25 ACH.
- Minutes per nominal change. 60 ÷ 6.25 = 9.6 minutes.
- Airflow the target needs. 6 × 2,400 ÷ 60 = 240 cfm.
- Surplus. 250 − 240 = +10 cfm, so the diffuser is delivering 4% more than the target.
Now check the clearance implication. If something is spilled in that office and you want 99% of it gone, that is 4.6 nominal changes at 9.6 minutes each, or about 44 minutes with the door shut and the fan running. If instead the requirement is 90% removal, that is 2.3 changes, or 22 minutes.
And check the ventilation implication. At 250 cfm this room is well ventilated by ACH, but if it seats eight people, the outdoor air component matters far more than the total supply. Only the outdoor fraction of that 250 cfm counts toward ASHRAE 62.1; the recirculated part dilutes nothing that the room itself is producing.
Choosing a target rate
Take the target from the document that governs the space, not from a general rule. Three families of requirement exist and they are written in different units.
Residential dwelling ventilation. ASHRAE 62.2 sets the whole-dwelling rate from floor area and the number of bedrooms, and it sets local exhaust separately: kitchens at 100 cfm intermittent or 5 ACH continuous, and bathrooms at 50 cfm intermittent or 20 cfm continuous. Note that only the kitchen figure is written as an ACH, and that the calculator's job there is simply to turn 5 ACH into a cfm for the kitchen's own volume.
Commercial ventilation. ASHRAE 62.1 uses a per-person plus per-area formula for outdoor air, not an ACH. If someone hands you an ACH figure for an office or a classroom, ask what it was derived from; it is usually a conversion of a 62.1 rate at an assumed ceiling height and occupancy, and it will be wrong for a room with a different height.
Process and hazard ventilation. This is where ACH is genuinely the governing unit: dilution ventilation for solvent vapour, battery rooms, refrigerant machinery rooms under ASHRAE 15, and similar. Those requirements come from the specific standard or from a mass-balance calculation on the contaminant generation rate, and they are the cases where a number like 12 or 20 ACH is meaningful rather than borrowed.
Once you have the airflow, the rest of the system follows. Size the duct that carries it with the duct size calculator, and check that the fan can actually deliver it against the pressure the duct imposes rather than at its free-air rating.
Airflow needed by room volume and target rate
| Room volume (cu ft) | 4 ACH | 6 ACH | 8 ACH | 10 ACH | 12 ACH |
|---|---|---|---|---|---|
| 500 | 33 | 50 | 67 | 83 | 100 |
| 1,000 | 67 | 100 | 133 | 167 | 200 |
| 2,000 | 133 | 200 | 267 | 333 | 400 |
| 4,000 | 267 | 400 | 533 | 667 | 800 |
| 8,000 | 533 | 800 | 1,067 | 1,333 | 1,600 |
| 12,000 | 800 | 1,200 | 1,600 | 2,000 | 2,400 |
A 2,000 cu ft room is 250 sq ft at an 8 ft ceiling. Every value is linear in both volume and rate, so a room twice the size at the same rate needs twice the fan.
Where ACH calculations go wrong
- Using a fan's free-air rating. A bathroom fan rated 80 cfm at 0.0 in.wg may deliver 45 cfm through a real duct with an elbow and a wall cap. Use the rating at 0.25 in.wg, which is the point HVI publishes for residential exhaust, or measure it.
- Treating one air change as complete replacement. One nominal change leaves 37% of the original air in a perfectly mixed room. Multiply by the number of changes your clearance requirement actually implies.
- Applying an ACH target to a tall room. ACH scales with ceiling height while occupant ventilation needs do not. A 20 ft ceiling makes any ACH target easy to hit and tells you nothing about the air the occupants are breathing.
- Counting recirculated air as ventilation. Only outdoor air dilutes contaminants generated inside the room. A supply diffuser delivering 250 cfm of mostly recirculated air provides very little ventilation, whatever ACH it produces.
- Ignoring where the air goes in and comes out. Supply and exhaust close together short-circuit, and the occupied zone sees less than the nominal rate. Rooms designed for clearance specify opening positions for this reason.
- Forgetting makeup air. An exhaust fan can only move the air that something lets in. A 600 cfm range hood in a tight house will not deliver 600 cfm without a makeup air path, and it can depressurise the house enough to backdraft combustion appliances.
Which standard sets your rate
For dwellings, ASHRAE Standard 62.2 governs both the whole-house rate and local exhaust, and states kitchen continuous exhaust as 5 air changes per hour based on the kitchen volume. For commercial and institutional spaces, ASHRAE Standard 62.1 sets outdoor air from occupancy and floor area rather than from an air change rate. For refrigerating machinery rooms, ASHRAE Standard 15 sets ventilation from the refrigerant charge. Local mechanical codes adopt versions of these with amendments, so check the edition your jurisdiction has adopted before designing to a number.
ACH in load calculations and in blower-door work
The same quantity appears in two other places in HVAC, meaning slightly different things each time, and confusing them is common.
In a load calculation, the infiltration rate is quoted in natural ACH: the unintentional leakage through the envelope under normal wind and stack conditions. That figure multiplies into the 1.08 × CFM sensible term and the 0.68 × CFM latent term that drive the design loads, which is exactly how the Manual J load calculator uses it. A natural rate of 0.35 ACH is a typical current house.
In blower-door testing, the result is quoted as ACH50: air changes per hour with the house held at 50 pascals of pressure difference. That is a much larger number, because 50 Pa is far more pressure than weather normally supplies. Converting ACH50 to a natural rate divides by an LBL correlation factor that depends on climate, height and shielding, typically between about 17 and 20. Entering an ACH50 figure into a load calculation as if it were a natural rate overstates infiltration by roughly a factor of twenty.
In ventilation design, ACH is the delivered rate this calculator computes, and it counts only the air a fan actually moves. Keep the three apart: an envelope leakage rate, a pressurised test result, and a mechanical ventilation rate are three different measurements that share one unit. Once you have the mechanical rate, the airflow it implies is the same airflow that feeds the room CFM calculator and the pressure budget in the available static pressure calculator.