SizingKitHVAC · Ventilation
Air changes per hour calculator, and what ACH is not
This converts between air changes per hour and CFM in both directions for a room of any volume, and puts the result beside the two calculations the ventilation standards actually specify: the ASHRAE 62.2 whole-dwelling rate and the ASHRAE 62.1 breathing-zone rate. The ACH table by space is printed with a basis column, because five of its eleven rows are trade figures no standard publishes, five are requirements out of ASHRAE 62.2 or ASHRAE 170, and one is arithmetic on the bathroom rule. It is free, needs no signup, and computes in the tab.
- 100% free
- No signup
- 11 spaces, each with its basis
- ASHRAE 62.2 and 62.1 routes
- ACH and CFM both directions
The room, and which way you are working
The first two routes convert between an airflow and a rate. The other two are the calculations the ventilation standards actually specify, reported in air changes so you can see how far the two ideas are apart.
Square feet. Only the volume the air actually mixes through counts — a sealed closet inside the room is not part of it.
Doubling this halves the air-change rate for the same airflow, which is the whole weakness of the unit.
Measured at the grille or taken off the schedule. Supply air that recirculates counts toward a total air-change rate; only outdoor air counts toward a ventilation one.
Air changes
4.69 ACH
Airflow
150 CFM
Metric
70.8 L/s
- Volume
- 1,920 ft³ / 54.4 m³
- Floor area
- 240 ft² / 22.3 m²
- One air change takes
- 12.8 min
ACH = CFM × 60 ÷ volume. This is a definition, not a standard: it restates an airflow against the room it enters, and it holds whatever the room is for.
4.69 ACH lands on kitchen, continuous exhaust, bedroom in the table below — read the basis column before you treat that as agreement.
Air changes per hour by space — with what each figure is
Three kinds of number sit in the ACH tables that circulate online, and they are printed together without distinction everywhere else. Five of these rows come out of a standard, one is arithmetic on a rule, and five have no published origin at all.
| Space | ACH | What it is | Source |
|---|---|---|---|
| Dwelling, whole house | 0.35 | Standard | ASHRAE 62-1989 minimum, 0.35 ACH but not less than 15 CFM per person. Superseded: current practice is ASHRAE 62.2's Qtot formula, which is not an air-change rate. |
| Kitchen, continuous exhaust | 5 | Standard | ASHRAE 62.2 Table 5.1, continuous option, on the kitchen volume. |
| Hospital operating room | 20 | Standard | ASHRAE Standard 170, Table 7.1 — 20 total air changes per hour, of which at least 4 outdoor, at positive pressure to adjoining spaces. |
| Airborne infection isolation room | 12 | Standard | ASHRAE Standard 170, Table 7.1 and CDC guidelines — 12 total air changes per hour for new construction, of which at least 2 outdoor, at negative pressure. |
| Hospital patient room | 6 | Standard | ASHRAE Standard 170, Table 7.1 — 6 total air changes per hour, of which at least 2 outdoor. |
| Bathroom | 7.5–8 | Derived | The HVI 1 CFM/ft² rule at an 8 ft ceiling is 60/8 = 7.5 ACH, which the trade rounds to 8. Not a requirement — the code minimum is a flat 50 CFM. |
| Kitchen, residential | 7–8 | Rule of thumb | Widely circulated trade table. No standard publishes it. |
| Bedroom | 5–6 | Rule of thumb | Widely circulated trade table. No standard publishes it. |
| Living and general residential space | 6–8 | Rule of thumb | Widely circulated trade table. No standard publishes it. |
| Office | 6–8 | Rule of thumb | Widely circulated trade table. ASHRAE 62.1 governs an office and specifies CFM per person and per ft², not ACH — see ASHRAE_62_1_OUTDOOR_AIR. |
| Restaurant dining room | 8–12 | Rule of thumb | Widely circulated trade table. ASHRAE 62.1 governs and specifies CFM per person and per ft², not ACH. |
- Standard — A requirement out of a published standard.
- Derived — Arithmetic on a rule that lives elsewhere in this table.
- Rule of thumb — No standard publishes it. It circulates through HVAC references without an origin.
An air-change rate is a way of stating an airflow relative to a volume, not a design method. Two rooms at 6 ACH with the same volume and different shapes ventilate differently, because what matters is whether the air reaches the occupants, and ACH says nothing about that. Where a standard governs — a house, an office, an operating room — use the standard's own units.
What ASHRAE 62.1 asks for instead
A per-person rate for the people and a per-area rate for the building, added. No row of it is an air-change rate, which is the clearest statement available that ACH is not how commercial ventilation is specified.
| Category | Rp | Ra | Default density |
|---|---|---|---|
| Office space | 5 CFM/person | 0.06 CFM/ft² | 5 / 1,000 ft² |
| Conference / meeting room | 5 CFM/person | 0.06 CFM/ft² | 50 / 1,000 ft² |
| Classroom, age 9 and over | 10 CFM/person | 0.12 CFM/ft² | 35 / 1,000 ft² |
| Lecture hall, fixed seats | 7.5 CFM/person | 0.06 CFM/ft² | 150 / 1,000 ft² |
| Retail sales floor | 7.5 CFM/person | 0.12 CFM/ft² | 15 / 1,000 ft² |
| Restaurant dining room | 7.5 CFM/person | 0.18 CFM/ft² | 70 / 1,000 ft² |
ASHRAE Standard 62.1, Table 6-2 (Minimum Ventilation Rates in Breathing Zone), Ventilation Rate Procedure. Rp is CFM per person, Ra is CFM per ft², and the density column is the standard's default occupant density per 1000 ft² for use when the design count is unknown.
This is the breathing-zone rate Vbz. The outdoor air the air handler must bring in is larger: 62.1 divides Vbz by a zone air distribution effectiveness Ez, which is 1.0 for ceiling supply of cool air and as low as 0.8 for ceiling supply of warm air, and a multiple-zone system then goes through the standard's system ventilation efficiency calculation on top of that. Only a handful of categories are reproduced here — take the rest from the standard.
How to work out air changes per hour
The arithmetic takes ten seconds. Knowing whether the rate you are comparing it against is a requirement or a rumor is the part worth doing.
Give the space its volume
Floor area and ceiling height, because that is the shape the ventilation standards ask their questions in and it is what the two standard routes need anyway. Only the volume the air actually mixes through counts, so a closed pantry or a mechanical closet inside the room is not part of it, and a room open to a hallway is larger than its own footprint.
Work in the direction you need
Enter an airflow to get the rate it represents, or enter a rate to get the airflow that achieves it. Both are the same identity rearranged — 60 minutes an hour multiplied by the airflow, divided by the volume — so neither route involves any property of the air and neither changes with altitude or temperature.
Check the rate against what kind of number it is
The printed table carries a basis column: five rows are requirements out of ASHRAE 62.2 or ASHRAE 170, one is arithmetic on the HVI bathroom rule, and five are the trade table that circulates without a source. If the space is a dwelling or a commercial zone, run the 62.2 or 62.1 route instead and compare — an air-change figure that disagrees with the governing standard by a factor of ten is the normal result, not an error.
Technical specifications
| The conversion | ACH = CFM × 60 ÷ volume, and CFM = volume × ACH ÷ 60, both directions from a floor area and a ceiling height. Volume is reported in ft³ and m³ and the airflow in CFM and L/s. |
|---|---|
| ASHRAE 62.2 whole-dwelling rate | Qtot = 0.03 × floor area + 7.5 × (bedrooms + 1), Eq. 4.1a. A 2,000 ft² three-bedroom house comes to 90 CFM, which at an 8 ft ceiling is 0.34 air changes per hour. |
| The 62.2 edition that changes the answer | The area term was 0.01 CFM/ft² in 62.2-2013 and earlier against 0.03 from 62.2-2016 on. The same house computes 50 CFM under the old edition and 90 under the current one — an 80% difference from a single coefficient, and both figures are printed. |
| ASHRAE 62.1 rates carried | Six occupancy categories with a per-person rate, a per-area rate and the standard's default density. An office is 5 CFM per person plus 0.06 CFM/ft² at 5 people per 1,000 ft², so 1,000 ft² of it needs 85 CFM in the breathing zone. |
| Healthcare rows | ASHRAE 170 Table 7.1: an operating room at 20 total air changes per hour with at least 4 outdoor and positive pressure, an airborne infection isolation room at 12 with at least 2 outdoor and negative pressure, and a patient room at 6 with at least 2 outdoor. |
| The bathroom row is derived, not required | 7.5 ACH, which is 60 ÷ 8 — the HVI rule of 1 CFM per ft² at an 8 ft ceiling — and the origin of the trade's 'eight air changes'. What a code actually requires of a bathroom is a flat 50 CFM regardless of its size. |
| Rows with no standard behind them | Five of the eleven: residential kitchen 7–8, bedroom 5–6, living space 6–8, office 6–8 and restaurant dining 8–12. They are in the table because people search for them and labeled so that nobody quotes one at a plans examiner. |
| The superseded figure still in circulation | 0.35 ACH but not less than 15 CFM per person, ASHRAE 62-1989. It is the source of nearly every 'houses need 0.35 air changes' claim online and it was replaced by a formula that is not an air-change rate at all. |
Frequently asked questions
Is there a code minimum air changes per hour for a house?
No — the requirement is a CFM, not a rate. ASHRAE 62.2, which the residential codes point at, asks for 0.03 CFM per square foot of floor area plus 7.5 CFM per bedroom-plus-one, and that total is a flow rather than a proportion of the volume. The widely quoted 0.35 air changes per hour comes from ASHRAE 62-1989 and has been superseded for over two decades; it survives online because it is easier to repeat than a formula, and because 0.35 happens to land near the right answer for a house of average height.
How many air changes per hour should a bedroom have?
No standard says. The 5–6 ACH that every table on the internet gives for a bedroom has no published origin — it is a trade figure that has been copied between HVAC references for decades, which is why this page labels it a rule of thumb rather than printing it next to the ASHRAE 170 rows as though they were the same kind of claim. What governs a bedroom in a dwelling is the whole-house rate from 62.2 plus whatever supply air the room's heating and cooling load requires, and those two numbers are usually a factor of ten apart.
Does a room with a higher ceiling need more air for the same ACH?
Yes, proportionally, and that is the clearest illustration of what is wrong with the unit. A 240 ft² room at 6 ACH needs 192 CFM with an 8 ft ceiling and 288 CFM with a 12 ft one, even though the same number of people are breathing in it and the pollutant they generate has not changed. Air changes measure how fast the volume turns over; contaminant dilution depends on how much clean air arrives per person and how well it reaches them, which is exactly why ASHRAE 62.1 is written in CFM per person plus CFM per square foot and mentions no volume at all.
Do recirculated air and outdoor air both count toward the rate?
It depends on which question the rate is answering, and mixing the two is the commonest error here. A total air-change rate — what ASHRAE 170 specifies for a hospital room, and what a filtration or a thermal argument uses — counts all the supply air including what is recirculated. A ventilation rate counts only the outdoor air, because recirculated air brings the room's own contaminants back. A room can sit at 6 total air changes and 0.5 outdoor ones simultaneously, and a design that confuses the two either wastes energy or fails to ventilate.
Why does 62.2 give 90 CFM when 6 air changes per hour gives 1,600?
Because they are answering different questions and only one of them is a ventilation requirement. The 90 CFM is outdoor air, sized to dilute what the occupants and the building materials produce, and it runs continuously or on a schedule. The 1,600 CFM is a total air-change rate on a 16,000 ft³ house, which is roughly what the heating and cooling system moves — mostly recirculated, sized by the load rather than by the people. Both numbers are correct for the same house on the same day.
What air-change rate does an operating room need?
20 total air changes per hour with a minimum of 4 outdoor, at positive pressure relative to adjoining spaces, from ASHRAE Standard 170 Table 7.1 — and healthcare is the one place where a standard genuinely is written in this unit. An airborne infection isolation room is 12 with at least 2 outdoor and negative pressure; a patient room is 6 with at least 2 outdoor. These rows are requirements with a table behind them, which is why the printed sheet marks them apart from the rows that are not.
Can I size an exhaust fan from an air-change rate?
For a bathroom or a kitchen, no — the code sets a fixed CFM and the air-change figure is a back-calculation from it, not the requirement. For a room where no code exhaust rate exists, such as a workshop or a battery room, an air-change rate is a reasonable way to state the design as long as you say what it is based on and remember that it says nothing about whether the air reaches the corner where the contaminant is. The fan that results still has to deliver that flow against the duct it is connected to, which is a separate calculation entirely.
About air changes per hour, and why the standards are not written in them
An air change per hour is a way of restating an airflow against the room it enters: 60 minutes an hour times the flow, divided by the volume. That is all it is — a definition, not a requirement — and it holds whether the room is an operating theater or a garage. It became the unit people search for because it is intuitive and because a table of recommended rates by room type has been circulating through HVAC references for generations. What almost nobody says out loud is where that table came from, and for five of the eleven rows here the answer is that nobody knows.
The gap shows up the moment a real standard is opened. ASHRAE 62.1 specifies commercial ventilation as a per-person rate plus a per-area rate and never mentions volume: an office is 5 CFM for each person and 0.06 CFM for each square foot, because the people and the building materials are what emit, and neither of them emits more because the ceiling is high. ASHRAE 62.2 does the same for a dwelling with Qtot = 0.03 × area + 7.5 × (bedrooms + 1), which for a 2,000 ft² three-bedroom house is 90 CFM — 0.34 air changes an hour, a figure that would look absurdly low next to any table on the internet and is the governing requirement anyway. Healthcare is the exception that proves the pattern: ASHRAE 170 does specify 20 air changes for an operating room, because there the objective genuinely is turning the room's whole volume over fast enough.
So use the unit for what it is good at — expressing an answer you already have, and comparing two rooms of similar shape — and go to the governing standard for the requirement. Where the number you want is the airflow rather than the rate, the CFM calculator reaches it from a load or a duct measurement as well as from a volume. Where the room is a bathroom, the code sets a flat CFM and the rate is a back-calculation from it, which the bathroom fan CFM calculator works through along with the static pressure that decides whether the fan delivers it. Once the flow is settled the duct that carries it comes from the duct size calculator, and the ventilation air you bring in is also a heating and cooling load in its own right — the heat pump sizing calculator is where that side of it lands. None of this is a stamped design; a ventilation design for a healthcare space or a commercial building is an engineer's work and a permit item.
Where the room you typed goes
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
Room dimensions, bedroom counts and occupant numbers are the kind of detail that describes somebody's actual building, and none of it leaves the tab or is written anywhere — the printed table is generated in the browser from figures that exist nowhere else.