Air Changes per Hour
Work out ACH, required ventilation airflow, or the room volume a given fan can serve — with correct CFM, m³/h and L/s handling built in.
Fills dimensions (example only)
Fills target ACH (Airflow/Volume mode)
Outdoor air vs. recirculated air
Total supply airflow is not the same as fresh outdoor-air ventilation. Enter the outdoor-air portion to see both figures.
Supply vs. exhaust balance
Compare supply and exhaust airflow. This gives an airflow difference, not a verified room-pressure measurement.
No calculation yet
Fill in the room and airflow details, then press Calculate to see ACH, conversions and the ventilation gauge.
Calculation breakdown
Every substitution shown, so the result can be checked by hand — useful for coursework and design review alike.
Run a calculation above to see the full breakdown here.
Theoretical air-change curve
Assumes instantaneous, perfectly uniform mixing — real rooms rarely mix perfectly, so treat this as a reference curve, not a measured outcome.
Curve uses the calculator's most recent ACH value (default 6.00 ACH until you calculate). Remaining fraction = e−ACH × t, t in hours.
Ideal mixing / contaminant reduction estimator
Estimates the time to reach a target relative concentration under ideal, well-mixed conditions only.
- Incomplete air mixing
- Short-circuit airflow
- Recirculation
- Filter efficiency
- Source generation continuing
- Outdoor-air fraction
- Envelope leakage
- Room geometry & obstructions
- Diffuser / equipment placement
ACH vs. time per air change
| ACH | Time per theoretical air change |
|---|
The math behind it
How to use this calculator
- Enter room dimensions, or switch to direct room volume entry.
- Select the correct measurement units for each field.
- Enter the supply, exhaust, or ventilation airflow rate.
- Select the matching airflow unit — CFM and m³/h are handled with the correct time-base conversion automatically.
- Press Calculate.
- Review the ACH value, time per air change, unit conversions, gauge and calculation breakdown.
- Switch to Required Airflow mode when sizing a system for a target ACH.
What this tool offers
Calculation
- ACH, required airflow, and room volume modes
- Automatic metric ⇄ imperial conversion
- CFM, m³/h, and L/s normalization
- Time and seconds per theoretical air change
- Total air volume moved per hour
Analysis
- Outdoor-air vs. total supply ACH
- Supply / exhaust airflow balance
- Ideal-mixing air-change curve
- Contaminant-reduction time estimate
- Step-by-step calculation breakdown
Reference & definitions
Ventilation terms are often used loosely. Getting them straight matters for interpreting any ACH figure correctly.
Airflow terms
Supply air is air delivered into a space. Exhaust air is air removed from it. Recirculated air is returned and resupplied rather than exchanged with the outdoors, so it moves air without necessarily diluting indoor contaminants.
Total ACH reflects all mechanical air movement; outdoor-air ACH reflects only the fresh-air fraction — they answer different questions and shouldn't be read as interchangeable.
Effectiveness & boundaries
Ventilation effectiveness describes how well supplied air actually reaches the breathing zone rather than short-circuiting to the return. Filtration removes particles from recirculated air without adding outdoor air; infiltration is uncontrolled air movement through the building envelope.
This tool does not evaluate compliance with ASHRAE, CDC/healthcare guidance, or local mechanical codes. Confirm applicable requirements with the current published standard and a qualified engineer before relying on any figure here for design or compliance purposes.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| ACH | Air changes per hour | 1/hr |
| Airflow Rate | Volumetric ventilation airflow rate | m3/hr |
| Room Volume | Total room air volume | m3 |
What it means
Air Changes per Hour (ACH) is a measure of how often the air in a space is completely replaced with outdoor air. It is calculated by dividing the supplied airflow rate (m³/h) by the room volume (m³). ACH is a key parameter for designing heating, ventilation, and air conditioning (HVAC) systems and for ensuring acceptable indoor air quality. Recommended ACH values vary by application: residential (0.5-1), offices (1-3), operating rooms (15-20). Higher ACH reduces contaminant build‑up but increases energy consumption. Example: A room of 50 m³ receives an airflow of 100 m³/h. ACH = 100/50 = 2 h⁻¹. This means the air in the room is replaced twice every hour, which is adequate for a typical office.
Worked example
Air Changes per Hour – Two Examples
Real‑World| Parameter | Value |
|---|---|
| Airflow rate | 500 m³/hr |
| Room volume | 100 m³ |
| Parameter | Value |
|---|---|
| Airflow | 1,000 m³/hr |
| Volume | 150 m³ |
Common mistakes
- Airflow rate: The volume of outdoor air supplied to the space – in m³/hr.
- Room volume: The total volume of the room – in m³.
- ACH: Number of air changes per hour – a measure of ventilation rate.
- Typical values: Residential: 0.35‑0.5 ACH; offices: 2‑6 ACH; hospitals: 6‑15 ACH.
- Effective ventilation: This is the theoretical ACH; actual mixing efficiency may be lower.
Applications
Air changes per hour (ACH) is the number of times the entire volume of air within a room is replaced by outdoor or conditioned air in one hour, calculated as airflow rate divided by room volume. This metric is crucial for ventilation design in buildings, ensuring adequate indoor air quality, controlling odours, and removing pollutants. Engineers use ACH to size HVAC systems, to comply with building codes (e.g., ASHRAE 62.1), and to design cleanrooms and healthcare facilities. Higher ACH rates are required in kitchens, laboratories, and waiting rooms to dilute contaminants. By calculating ACH, building professionals can balance energy efficiency with occupant health and comfort.
- Design of ventilation systems for residential, commercial, and industrial buildings
- Compliance with indoor air quality standards (ASHRAE, LEED)
- Assessment of infectious disease transmission risk (e.g., COVID‑19)
- Design of cleanrooms, operating theatres, and pharmaceutical facilities
- Energy performance evaluation and retrofitting of ventilation systems
Related calculators · Air Quality
Recently updated formulas
Suggest an improvement
Found a mistake? Have a better example? Let us know.
You may send up to 3 suggestions per 24 hours.