Air Changes per Hour Calculator
Find out how often the air in a room or house is replaced. Calculate air changes per hour from a fan, supply or air purifier rating, the airflow you need for a target ACH, ACH50 from a blower door test, or the minimum ventilation by occupancy under ASHRAE 62.2 and 62.1.
Last updated:
ft² or m² (conditioned floor area for a house)
in the airflow unit above
LBL factor to convert ACH50 to natural ACH; 15–25 typical
1.0 ceiling supply of cool air; 0.8 ceiling supply of warm air
How to use the Air Changes per Hour Calculator
- Choose what to calculate: ACH from airflow, airflow for a target ACH, blower door ACH50, or occupancy ventilation.
- Enter the room size as length × width × height or floor area × ceiling height.
- Enter the airflow, target ACH, CFM50 result, or bedrooms / occupants and space type.
- Read the result plus volume, time per air change and particle clearance times.
- Use the table to compare other air change rates for the same space.
Air changes per hour formulas
- ACH = CFM × 60 ÷ volume (ft³), or ACH = m³/h ÷ volume (m³).
- CFM = volume × ACH ÷ 60 for the airflow needed to reach a target rate.
- ACH50 = CFM50 × 60 ÷ volume from a blower door test; natural ACH ≈ ACH50 ÷ N.
- ASHRAE 62.2: Qtot = 0.03 × Afloor + 7.5 × (Nbr + 1) CFM.
- ASHRAE 62.1: Vbz = Rp × people + Ra × area, then Voz = Vbz ÷ Ez.
- Removal time: t = ln(1 ÷ (1 − E)) ÷ ACH hours for removal efficiency E in a well-mixed room.
ASHRAE 62.1 ventilation rates used
| Space type | People Rp (CFM/person) | Area Ra (CFM/ft²) |
|---|---|---|
| Office space, conference room, lobby | 5 | 0.06 |
| Classroom (ages 5–8 and age 9+) | 10 | 0.12 |
| Lecture classroom | 7.5 | 0.06 |
| Library | 5 | 0.12 |
| Retail sales | 7.5 | 0.12 |
| Restaurant dining, cafeteria | 7.5 | 0.18 |
| Health club / weight room | 20 | 0.06 |
| Hotel guest room, auditorium, place of worship | 5 | 0.06 |
| Daycare (through age 4) | 10 | 0.18 |
Breathing-zone rates from the ASHRAE 62.1 ventilation rate procedure table. Check the edition adopted by your local code and the full standard for system-level corrections.
Particle removal time by ACH
| ACH | 99% removed | 99.9% removed |
|---|---|---|
| 2 | 138 min | 207 min |
| 4 | 69 min | 104 min |
| 6 | 46 min | 69 min |
| 8 | 35 min | 52 min |
| 12 | 23 min | 35 min |
| 15 | 18 min | 28 min |
| 20 | 14 min | 21 min |
Assumes perfect mixing and no ongoing source. Poor mixing, dead zones and people still in the room make real clearance slower.
Frequently asked questions
How do you calculate air changes per hour?
ACH = airflow in CFM × 60 ÷ room volume in cubic feet. A 15 × 12 ft room with an 8 ft ceiling is 1,440 ft³; a 150 CFM fan or purifier gives 150 × 60 ÷ 1,440 = 6.25 air changes per hour. In metric, ACH = m³/h ÷ room volume in m³.
How many air changes per hour do I need?
It depends on the use. Homes need continuous fresh air set by ASHRAE 62.2 (often around 0.35 ACH or more), bathrooms and kitchens are commonly ventilated at about 8 and 15 ACH while in use, and many indoor air quality guides suggest 4–6 or more ACH of clean air in occupied rooms such as classrooms. Local codes may set specific minimums.
What is ACH50?
ACH50 is the number of air changes per hour a house leaks when a blower door holds it at 50 pascals below outdoor pressure. It is CFM50 × 60 ÷ house volume. The 2012 through 2021 IECC limits new homes to 5 ACH50 in climate zones 0–2 and 3 ACH50 in zones 3–8; very tight homes test at 1 ACH50 or less.
How do I convert ACH50 to natural air changes?
A common estimate divides ACH50 by a factor N of about 15 to 25 (the LBL method), using lower values for windy, exposed or tall houses and higher values for sheltered single-story homes. 4 ACH50 ÷ 20 ≈ 0.2 natural ACH. It is only an estimate of average infiltration.
What is the ASHRAE 62.2 ventilation formula?
Since the 2013 edition, ASHRAE 62.2 requires a total continuous ventilation rate of Q = 0.03 × floor area (ft²) + 7.5 × (bedrooms + 1) CFM. A 2,000 ft² three-bedroom home needs 0.03 × 2,000 + 7.5 × 4 = 90 CFM, which may be reduced by an infiltration credit based on a blower door test.
How does air purifier CADR relate to ACH?
CADR (clean air delivery rate) is the equivalent flow of particle-free air, in CFM. Enter it as the airflow to get the clean-air changes per hour: a 200 CFM CADR purifier in a 1,600 ft³ room gives 200 × 60 ÷ 1,600 = 7.5 ACH.
How long does it take to clear the air?
With good mixing, airborne particles decay exponentially: the time to remove 99% is 4.6 ÷ ACH hours and for 99.9% it is 6.9 ÷ ACH hours. At 6 ACH that is about 46 minutes for 99% and 69 minutes for 99.9%, matching the CDC's air change removal table.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.