Ventilation rate and air changes (ACH) calculator
Get the required airflow from room size and air changes per hour, or the ACH from an airflow. It also works out the ASHRAE 62.1 outdoor air by people and area and the heating or cooling load of that outdoor air. An ACH × room volume table is below.
Ventilation airflow and air changes
Air changes per hour n (ACH) is how many times a room's own volume of air is replaced in one hour. The airflow is Q = n × V (m³/h), and the other way round n = Q / V. A room of 100 m² floor area and 3 m ceiling height (300 m³) ventilated at 2 ACH needs 600 m³/h (353 CFM).
Where people are the main pollution source, such as offices, meeting rooms and classrooms, outdoor air is set by occupants and floor area as in the ASHRAE 62.1 ventilation rate procedure. The breathing zone outdoor air Vbz = Rp·Pz + Ra·Az divided by the zone air distribution effectiveness Ez gives the zone outdoor airflow Voz.
Outdoor air has to be cooled or heated to room conditions, so more outdoor air means a larger heating and cooling load. The calculator shows the outdoor-air load (sensible, latent, total) of the same airflow, with the same equations as the quick load calculator.
Airflow by air changes and room volume (m³/h)
Airflow in m³/h (CMH) from Q = n × V. 1 m³/h = 0.5886 CFM; divide by 60 for m³/min. The 0.5 ACH column is the Korean apartment rule.
| Room volume m³ | 0.5 ACH | 1 ACH | 2 ACH | 3 ACH | 5 ACH | 10 ACH | 20 ACH |
|---|---|---|---|---|---|---|---|
| 50 | 25 | 50 | 100 | 150 | 250 | 500 | 1,000 |
| 100 | 50 | 100 | 200 | 300 | 500 | 1,000 | 2,000 |
| 200 | 100 | 200 | 400 | 600 | 1,000 | 2,000 | 4,000 |
| 500 | 250 | 500 | 1,000 | 1,500 | 2,500 | 5,000 | 10,000 |
| 1,000 | 500 | 1,000 | 2,000 | 3,000 | 5,000 | 10,000 | 20,000 |
| 2,000 | 1,000 | 2,000 | 4,000 | 6,000 | 10,000 | 20,000 | 40,000 |
ASHRAE 62.1 outdoor air rates by occupancy
Per person Rp, per area Ra and default occupant density from Table 6-1 (SI edition). 1 L/s = 3.6 m³/h. The last column is Vbz for 100 m² of floor at the default density (before dividing by the distribution effectiveness Ez).
| Occupancy | Rp L/s·person | Rp per person m³/h | Ra L/s·m² | Ra per area m³/h·m² | Density /100 m² | Vbz for 100 m² m³/h |
|---|---|---|---|---|---|---|
| Office space | 2.5 | 9.00 | 0.3 | 1.08 | 5 | 153 |
| Conference/meeting | 2.5 | 9.00 | 0.3 | 1.08 | 50 | 558 |
| Sales (retail) | 3.8 | 13.68 | 0.6 | 2.16 | 15 | 421 |
| Restaurant dining rooms | 3.8 | 13.68 | 0.9 | 3.24 | 70 | 1,282 |
| Classrooms (age 9 plus) | 5.0 | 18.00 | 0.6 | 2.16 | 35 | 846 |
| Lecture classroom | 3.8 | 13.68 | 0.3 | 1.08 | 65 | 997 |
Values can change between editions, so check the edition you apply. Korean apartments need at least 0.5 air changes per hour (Building Equipment Standards Rule, article 11); for other legal rates check the original text of that rule or your local code.
Frequently asked questions
How do I calculate air changes per hour?
Divide the hourly airflow by the room volume: n = Q / V, and the other way round Q = n × V. For example a room of 100 m² floor area and 3 m ceiling height (300 m³) at 2 air changes per hour needs 600 m³/h = 10 m³/min = about 353 CFM.
Should I size by occupants or by air changes?
Where people are the main pollution source, such as offices, meeting rooms, classrooms and restaurants, outdoor air is usually set by occupants and floor area as in ASHRAE 62.1 (Vbz = Rp·Pz + Ra·Az). Air changes depend only on room size, so they suit places with few or changing occupants such as homes, or where a code states a rate. Work out both, then compare the larger value with your code. A 100 m² office at the default 5 people gives Vbz = 2.5 × 5 + 0.3 × 100 = 42.5 L/s = 153 m³/h, about 0.51 ACH with a 3 m ceiling.
What does the Korean 0.5 ACH rule for apartments mean?
Article 11 of Korea's Building Equipment Standards Rule requires new apartment buildings of 30 or more units to have natural or mechanical ventilation able to give at least 0.5 air changes per hour. That is half the dwelling's volume each hour, so a 200 m³ dwelling needs 100 m³/h. Check the original text for the exact scope; other countries set their own rates.
Equations and sources
The equations used by the calculator and the tables above. The outdoor-air load has the same definition as the quick load calculator and the calculator's process card.
| Item | Equation | Notes · basis |
|---|---|---|
| Room volume | V = A · H | A floor area m², H ceiling height m |
| ACH → airflow | Q = n · V | n per hour, Q m³/h |
| Airflow → ACH | n = Q / V | The same equation reversed |
| Breathing zone outdoor air | Vbz = Rp · Pz + Ra · Az | ASHRAE 62.1 ventilation rate procedure; Rp L/s per person, Ra L/s·m², Pz occupants, Az area |
| Zone outdoor airflow | Voz = Vbz / Ez, Q [m³/h] = Voz [L/s] × 3.6 | Ez distribution effectiveness (ceiling cool air 1.0, ceiling warm air 0.8) |
| Airflow units | 1 m³/min = 60 m³/h, 1 CFM = 1.69901 m³/h, 1 L/s = 3.6 m³/h | Same factors as the unit converter |
| Outdoor-air load (cooling) | qs = ṁ · (1.006 + 1.86 Wo) · (to − tr), q = ṁ · (ho − hr) | ṁ = Q / (3600 vo), latent = total − sensible; air states from the calculator engine |
Sources
- ANSI/ASHRAE Standard 62.1, Ventilation and Acceptable Indoor Air Quality: Table 6-1 (minimum ventilation rates, SI edition) and the zone air distribution effectiveness table. Same values as the quick load calculator
- Korea, Building Equipment Standards Rule, article 11 (new apartment buildings of 30 or more units: at least 0.5 air changes per hour), National Law Information Center law.go.kr
- ASHRAE Handbook Fundamentals 2017, chapter 1 Psychrometrics (air states, calculator engine)
Last updated 2026-11-09.