Air Changes per Hour Calculator
Convert fan airflow to air changes per hour. Enter the room's length, width, and ceiling height in feet plus airflow in CFM to get ACH — and see how many CFM you need for a chosen target rate.
Example: with Room length (ft) 20 · Room width (ft) 15 · Ceiling height (ft) 8 · Airflow (CFM) 200 · Target rate (for sizing) Living areas (6 ACH) → Air changes per hour: 5.00 ACH.
- Room volume2,400 cu ft
- CFM needed for target240 CFM for 6 ACH
Computed by the calculator below using its default values. Change any input to see your own numbers.
ACH = CFM × 60 ÷ room volume in cubic feet. One air change means a volume of air equal to the whole room has moved through it.
What air changes per hour actually measures
ACH is how many times per hour a fan moves a room's worth of air. A 20 × 15 ft room with 8 ft ceilings holds 2,400 cubic feet; a 200 CFM fan moves 12,000 cubic feet an hour, so it delivers 5 air changes. The formula is just airflow × 60 minutes divided by room volume.
The commonly cited residential targets — 5 to 8 ACH for living spaces, more for kitchens, baths, and workshops — are ventilation conventions from HVAC practice, not code minimums. Building codes work differently: ASHRAE 62.2 sets whole-house mechanical ventilation at roughly 0.35 ACH (with a floor-area-plus-bedrooms CFM formula), because it targets continuous background fresh air rather than spot exhaust. Both views are useful: size spot fans by room ACH, size whole-house ventilation by 62.2.
How it’s calculated
ACH = airflow (CFM) × 60 ÷ (length × width × height in feet). Required CFM for a target = target ACH × volume ÷ 60. Target-rate presets (5–10 ACH by room) are widely used HVAC rules of thumb; whole-house fresh-air requirements follow ASHRAE 62.2 (≈0.35 ACH) instead.
Assumes the fan actually delivers its rated CFM — duct length, elbows, and dirty filters commonly cut real-world airflow 20–40% below the label.
Commonly cited air change targets
| Space | Typical ACH | Why |
|---|---|---|
| Bedrooms | 5–6 | Comfort and CO2 control |
| Living areas | 6–8 | General ventilation |
| Bathrooms | 7–8 | Moisture removal |
| Kitchens | 7–8 | Odor, grease, moisture |
| Laundry / workshop | 8–10 | Humidity, dust, fumes |
| Whole house (ASHRAE 62.2) | ≈0.35 | Continuous fresh-air minimum |
HVAC industry rule-of-thumb ranges; whole-house figure per ASHRAE Standard 62.2 conventions.
Common mistakes
- Forgetting ceiling height — ACH uses volume, not floor area, so 10 ft ceilings need 25% more CFM than 8 ft.
- Using the fan's rated CFM as delivered CFM: static pressure from ducting often cuts real airflow by a third.
- Comparing a bath fan's 8 ACH target to the whole-house 0.35 ACH standard — they measure different jobs (spot exhaust vs continuous fresh air).
- Mixing minutes and hours: CFM is per minute, so multiply by 60 before dividing by volume.
Frequently asked questions
What is the air changes per hour formula?
ACH = CFM × 60 ÷ room volume in cubic feet. A 150 CFM fan in a 1,000 cu ft room gives 150 × 60 ÷ 1,000 = 9 air changes per hour.
How many CFM do I need per square foot?
With 8 ft ceilings, 1 ACH is about 0.13 CFM per square foot, so a 6 ACH target works out to roughly 0.8 CFM per square foot. Taller ceilings scale it up proportionally.
What is a good ACH for a house?
For spot ventilation, 5–8 ACH in living spaces and 7–10 in kitchens, baths, and workshops are common targets. For continuous whole-house fresh air, ASHRAE 62.2 lands near 0.35 ACH — a different standard for a different job.
Does higher ACH always mean better air?
Not automatically. Past the point of controlling moisture, CO2, and odors, extra air changes mostly add noise and heating/cooling cost. Filtration quality and where the air comes from matter as much as the count.