What air changes per hour means
Air changes per hour (ACH) is calculated by dividing a ventilation system's airflow rate, in cubic meters per hour, by the volume of the room or space it serves: ACH = airflow (m³/h) ÷ volume (m³). A result of 4 ACH means a volume of air equal to the entire room is theoretically supplied or removed four times over the course of one hour.
ACH is a useful planning figure, but it is a theoretical average, not a guarantee of uniform air quality throughout the room. The calculation assumes the incoming air mixes evenly with the air already present; in practice, airflow can 'short-circuit' between a supply and a nearby return or exhaust, or leave poorly mixed pockets in room corners, so a calculated ACH that looks adequate on paper does not automatically mean every part of the room is equally well ventilated.
Converting between CFM and m³/h
Ventilation and fan equipment datasheets commonly state airflow in cubic feet per minute (CFM) in the US, and in cubic meters per hour (m³/h) in much of the rest of the world, so converting between the two is a frequent step before an ACH calculation can be done consistently. The exact conversion factor is 1 CFM = 1.699010796 m³/h, derived directly from the exact foot-to-meter definition (1 ft = 0.3048 m, so 1 ft³ = 0.3048³ m³ = 0.028316846592 m³) multiplied by 60 minutes per hour.
Because the underlying foot and meter definitions are exact, this conversion factor is exact to as many decimal places as needed -- rounding it too aggressively (for example to 1.7) introduces a small but avoidable error into an ACH calculation, particularly when converting a large airflow figure.
Typical ventilation targets: ASHRAE 62
There is no single universal 'correct' ACH that applies to every room -- recommended ventilation rates are set out in engineering standards published by ASHRAE, primarily ANSI/ASHRAE Standard 62.2 for residential low-rise buildings and ANSI/ASHRAE Standard 62.1 for commercial and institutional buildings, both of which are widely referenced in US building codes. Rather than specifying one flat ACH figure for every space, ASHRAE 62.2 bases required continuous whole-house mechanical ventilation airflow on a home's floor area and number of bedrooms, reflecting that ventilation need scales with both space size and expected occupancy.
A commonly cited reference point for quick planning -- and the default used on this site's air changes calculator -- is 6 ACH, but this is a general planning convention, not a code requirement for any particular room type. Bathrooms and kitchens, which generate more moisture and odors, are typically ventilated at higher rates than general living spaces under residential ventilation standards, and the applicable figure for a specific room should ultimately come from the relevant ASHRAE standard or the local building code rather than a single remembered number.
Airtightness and ventilation are complementary, not substitutes
Older, leakier buildings receive a degree of uncontrolled 'natural' ventilation through cracks, gaps and general air leakage in the building envelope, which historically supplied some fresh air exchange even without a deliberate ventilation system. Modern construction practice deliberately reduces this uncontrolled leakage -- sealing gaps, upgrading windows, adding continuous air barriers -- because a tighter envelope reduces the energy needed for heating and cooling.
This is where the building-science principle often summarized as 'build tight, ventilate right' applies: sealing a building's envelope without also adding deliberate, controlled ventilation removes the uncontrolled air exchange the space used to rely on, which can trap moisture, indoor pollutants, and combustion byproducts that would previously have escaped through natural leakage. Airtightness and mechanical ventilation address two different problems -- energy loss and controlled fresh-air exchange -- and a well-designed building needs both, not one instead of the other.
Worked example: sizing a bathroom exhaust fan
Consider a bathroom measuring 3 m by 2 m with a 2.4 m ceiling, giving a volume of 3 x 2 x 2.4 = 14.4 cubic meters. Bathrooms are typically ventilated at a higher rate than general living spaces, so this example uses a target of 8 ACH: the required airflow is 8 x 14.4 = 115.2 m³/h.
Converting that required airflow to CFM, the unit commonly printed on fan datasheets in the US, divides by the exact factor: 115.2 ÷ 1.699010796 ≈ 67.8 CFM. A fan should be selected with a rated airflow at or above this figure, keeping in mind that ducting length, bends and static pressure in the actual installation typically reduce delivered airflow below a fan's nameplate rating.
| Step | Calculation | Result |
|---|---|---|
| Room volume | 3 m × 2 m × 2.4 m | 14.4 m³ |
| Required airflow at 8 ACH | 8 × 14.4 m³ | 115.2 m³/h |
| Converted to CFM | 115.2 ÷ 1.699010796 | ≈67.8 CFM |
Common ACH mistakes
A few errors recur when working with ACH figures in practice: assuming a single target ACH (such as the commonly cited 6) applies equally to every room type, when actual recommended rates vary by application and are set by standards like ASHRAE 62.2 or local code; confusing a fan's rated nameplate airflow with its lower actual installed airflow after accounting for ducting losses; and mixing up CFM and m³/h figures when comparing a datasheet rating against a calculated requirement without converting them to the same unit first.
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How do I calculate air changes per hour (ACH)?
Divide the ventilation airflow rate, in cubic meters per hour, by the room's volume in cubic meters: ACH = airflow (m³/h) ÷ volume (m³). For example, a 14.4 m³ bathroom served by a 115.2 m³/h fan achieves 115.2 ÷ 14.4 = 8 ACH.
How do I convert CFM to m³/h?
Multiply the CFM figure by the exact factor 1.699010796, which comes from the exact foot-to-meter conversion (0.3048³ m³ per cubic foot) multiplied by 60 minutes per hour. For example, 100 CFM converts to 100 × 1.699010796 ≈ 169.9 m³/h.
What is a good ACH for a room?
There is no single universal target -- recommended ventilation rates vary by room type and are generally set out in standards such as ASHRAE 62.2 (residential) or ASHRAE 62.1 (commercial), or in local building code, rather than one fixed number. A commonly cited default used for general planning is around 6 ACH, but bathrooms and kitchens typically warrant higher rates.
Does sealing a house tighter make ventilation less important?
No -- the opposite. A tighter building envelope reduces the uncontrolled natural air leakage that older buildings relied on for some fresh-air exchange, so deliberate mechanical ventilation becomes more important, not less, once a building is well sealed. This is the reasoning behind the building-science principle 'build tight, ventilate right.'
What is ACH50, and is it the same as the ACH used for ventilation sizing?
ACH50 is a measure of building airtightness, obtained from a blower door test that depressurizes a building to a standardized 50-pascal pressure difference and measures the resulting airflow; it describes how leaky the envelope is under test conditions, not the airflow a ventilation system delivers in normal use. It is a related but distinct concept from the ventilation-sizing ACH calculated from a fan's actual airflow and room volume.
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- ASHRAE -- ANSI/ASHRAE Standard 62.2, Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings.
- ASHRAE -- ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality (commercial/institutional).
- National Institute of Standards and Technology (NIST) -- Special Publication 811, exact foot-to-meter conversion underlying the CFM-to-m³/h factor.
- RESNET -- ANSI/RESNET/ICC 380 Standard for testing building airtightness (blower door testing, ACH50).