Quick answer
Air changes per hour = airflow in CFM × 60 ÷ room volume in cubic feet. 200 CFM in a 1,800 ft³ room gives 6.67 ACH, or one full air change every 9 minutes.
Enter the airflow and room volume. The calculator returns ACH and minutes per air change.
Related tools: Air Density Calculator: Density of moist air from pressure, temperature and humidity.
How to use the Air Changes per Hour Calculator
- Enter airflow (CFM) (above 0).
- Enter room volume (ft³) (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Airflow | A number, in CFM | > 0 |
| Room volume | A number, in ft³ | > 0 |
Formula and method
Formula
ACH = CFM × 60 ÷ room volume
IEEE-754 double precision; shown to the decimals indicated.
Uses delivered or measured airflow.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Airflow = 200; Room volume = 1800.
| Result | Value |
|---|---|
| Air changes per hour | 6.667 |
| Minutes per full air change | 9 |
Example 2
Inputs: Airflow = 50; Room volume = 960.
| Result | Value |
|---|---|
| Air changes per hour | 3.125 |
| Minutes per full air change | 19.2 |
Example 3
Inputs: Airflow = 1000; Room volume = 12000.
| Result | Value |
|---|---|
| Air changes per hour | 5 |
| Minutes per full air change | 12 |
The first line is the headline result (air changes per hour). Below it the air Changes per Hour Calculator lists minutes per full air change, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
Typical targets
Homes often run 0.35 to 1 ACH from ventilation and leakage; kitchens and bathrooms need more; hospital rooms are designed for 6 to 12 or higher. Targets depend on the standard that applies.
Room volume
Volume is length × width × height: a 15 × 12 ft room with 10 ft ceilings is 1,800 ft³. Air purifiers use CADR in place of CFM, a related number.
How the result changes with airflow
Holding every other input at the first worked example, this table changes only airflow. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Airflow | Air changes per hour | Minutes per full air change |
|---|---|---|
| 100 | 3.333 | 18 |
| 200 | 6.667 | 9 |
| 400 | 13.333 | 4.5 |
| 800 | 26.667 | 2.25 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Airflow: entering "0" shows "Airflow must be greater than 0." beside that field and no result.
- Room volume: entering "0" shows "Room volume must be greater than 0." beside that field and no result.
Choosing between the Air Changes per Hour Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Window Air Conditioner Size Calculator](/engineering/window-air-conditioner-size-calculator/) | room area, area unit and sunlight | Recommended capacity |
| [CFM Calculator](/engineering/cfm/) | use | Airflow |
| [Air Density Calculator](/science/air-density/) | pressure, temperature and relative humidity | Air density |
| [Drops per Minute Calculator](/science/drops-per-minute/) | volume, time and drop factor | Drip rate |
| [Dew Point Calculator](/science/dew-point/) | air temperature and relative humidity | Dew point |
Assumptions and limitations
- Mixing efficiency is not considered
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for airflow and room volume to confirm this.
Frequently asked questions
How many CFM for 6 ACH?
CFM = ACH × room volume ÷ 60, so 1,800 ft³ at 6 ACH needs 180 CFM.
Does ACH mean air is replaced?
Mixing means not all air is replaced.
Metric version?
Use airflow in m³/h divided by room volume in m³; the result is the same ACH.
What does the Air Changes per Hour Calculator calculate?
Air changes per hour from airflow and room volume. It returns Air changes per hour and Minutes per full air change.
What formula does the Air Changes per Hour Calculator use?
ACH = CFM × 60 ÷ room volume. Uses delivered or measured airflow.
What do I need to enter in the air Changes per Hour Calculator?
You enter Airflow (CFM) and Room volume (ft³).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Mixing efficiency is not considered.
What is an example of a air Changes per Hour result?
With airflow 200 and room volume 1800, the air changes per hour is 6.667.
Related calculators
- Air Density Calculator — Density of moist air from pressure, temperature and humidity.
- Drops per Minute Calculator — IV drip rate in drops per minute from volume, time and drop factor.
- Dew Point Calculator — Dew point from temperature and relative humidity.
- Acceleration Calculator — Average acceleration from change in velocity over time.
- Sunrise and Sunset Calculator — Approximate sunrise, sunset, solar noon and day length for a date and location.
- Enthalpy Calculator — Enthalpy change of a reaction from enthalpies of formation (Hess's law), or ΔH = ΔU + pΔV at constant pressure.
Sources
- OpenStax, University Physics Volume 1 (Rice University): mechanics
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.