About Air Changes per Hour Calculator — ACH from CFM & Room Size
The air changes per hour calculator relates the three quantities of room ventilation: airflow, room volume, and ACH. One "air change" means moving a full room volume of air, so ACH = 60 × CFM ÷ volume in cubic feet (or, in metric, simply m³/h ÷ m³). Solve it forward — how many air changes does my fan or purifier deliver in this room? — or backward: what CFM do I need to hit a target ACH? Enter the room as length × width × height or as a direct volume, in feet or metres.
A caution the honest way: the "recommended ACH by room type" tables that circulate online (bedrooms 5–6, kitchens 7–8, and so on) trace to no published standard, so this tool does not reproduce them. The citable requirements are airflow-based — ASHRAE 62.2 sets a whole-dwelling continuous ventilation rate in CFM plus local exhaust minimums (100 CFM intermittent for kitchens, 50 CFM for baths), and ASHRAE 62.1 sets CFM per person and per floor area for commercial spaces. True ACH mandates exist mainly in healthcare: ASHRAE 170 requires, for example, 20 ACH in operating rooms and 12 in airborne-infection isolation rooms. Use this calculator to convert between the quantities; take the requirement itself from the governing standard for your space.
How It Works
- Choose what to solve: ACH from a known airflow, or the airflow needed for a target ACH.
- Describe the room: length × width × ceiling height (the everyday path), or a direct volume for odd shapes — sum the pieces of an L-shaped room first.
- Enter the airflow (CFM or m³/h) or the target ACH. The imperial/metric toggle converts entries in place, rounded to field precision and clamped, so switching units never invalidates an entry.
- Read the result plus the minutes per complete air change (60 ÷ ACH) — often the more intuitive number: 1.5 ACH means a full change every 40 minutes.
Worked Example
A portable air cleaner moves 200 CFM in a large open-plan space of 8,000 ft³ (say 40 × 25 ft under an 8 ft ceiling). ACH = 60 × 200 ÷ 8,000 = 1.5 air changes per hour — one complete change every 40 minutes. If the goal is the 5 ACH that clean-air guidance for shared rooms often targets, invert the problem: required CFM = 5 × 8,000 ÷ 60 = 667 CFM, telling you one small unit cannot do it and either multiple units or a duct-borne solution is needed.
Air changes per hour by room size at 100 CFM
What a fixed 100 CFM of airflow achieves as the room grows, computed through the calculator with an 8 ft ceiling: the same fan that turns over a small office 7.5 times an hour barely manages 1.25 changes in a large family room. Scale linearly for other airflows — 200 CFM doubles every ACH figure.
| Room (8 ft ceiling) | Volume (ft³) | ACH @ 100 CFM |
|---|---|---|
| 10 × 10 ft (small office) | 800 | 7.50 |
| 12 × 12 ft (bedroom) | 1,152 | 5.21 |
| 15 × 20 ft (living room) | 2,400 | 2.50 |
| 20 × 30 ft (family room) | 4,800 | 1.25 |
Formulas
- Air changes per hour (imperial)
ACH = 60 × CFM / V- Air changes per hour (metric)
ACH = Q / V- Required airflow for a target
CFM = ACH × V / 60 (metric: Q = ACH × V)- Time per air change
t = 60 / ACH minutes
Standards & References
- The ACH identity is definitional (one air change = one room volume of airflow); unit factors are exact: 1 ft³ = 0.028316846592 m³, 1 CFM = 1.69901079552 m³/h (from 1 ft = 0.3048 m, NIST SP 811)
- ASHRAE 62.2 (residential): ventilation prescribed as CFM — a whole-dwelling continuous rate plus local exhaust minimums (kitchen 100 CFM intermittent, bathroom 50 CFM intermittent / 20 continuous) — not as per-room ACH
- ASHRAE 170 (healthcare): the standard that genuinely specifies ACH — e.g. operating rooms ≥ 20 ACH, airborne-infection isolation ≥ 12, general patient rooms ≥ 6
- No primary source publishes a general "recommended ACH by room type" table for homes; treat such tables as unsourced rules of thumb
Frequently Asked Questions
How do I calculate air changes per hour?
Multiply the airflow in CFM by 60 and divide by the room volume in cubic feet: a 200 CFM unit in an 8,000 ft³ space gives 60 × 200 / 8,000 = 1.5 ACH. In metric it is even simpler — airflow in m³/h divided by volume in m³. The calculator also inverts the formula to find the CFM a target ACH requires.
What is a good ACH for a bedroom or kitchen?
Honest answer: no published standard assigns ACH targets to home room types — the bedroom 5–6 / kitchen 7–8 tables online are unsourced folklore. The real residential requirement, ASHRAE 62.2, is airflow-based: a continuous whole-dwelling rate plus local exhaust of 100 CFM (intermittent) at the kitchen and 50 CFM at each bath. Compute your room's actual ACH here, then judge it against the standard that governs your case, not a blog table.
Where do real ACH requirements exist?
Mainly healthcare: ASHRAE 170 mandates minimum total air changes by clinical space — around 20 ACH for operating rooms, 12 for airborne-infection isolation and protective-environment rooms, 6 for general patient rooms. Laboratories and cleanrooms have their own ACH-based design guides. For offices and homes, the governing numbers are CFM-based (ASHRAE 62.1/62.2).
How many CFM do I need for 5 ACH in my room?
CFM = 5 × volume ÷ 60. A 12 × 12 × 8 ft bedroom (1,152 ft³) needs 5 × 1,152 / 60 = 96 CFM; the 5 ACH clean-air target popularized for shared spaces during COVID needs surprisingly serious airflow as rooms grow — 667 CFM for an 8,000 ft³ open plan. That is why big rooms usually need multiple air cleaners or duct-borne ventilation rather than one portable unit.
Does ACH from an air purifier equal ventilation ACH?
Not quite. Ventilation ACH replaces room air with outdoor air, removing CO₂ and everything else; a recirculating purifier filters the same air, removing particles but not CO₂ or humidity. For particle/aerosol purposes people combine them as effective ACH (ventilation + CADR-based air cleaning), using the purifier's CADR rating in CFM in this same formula. For fresh-air requirements, only real ventilation counts.
How does ceiling height change the answer?
Linearly, through the volume: the same 100 CFM that yields 7.5 ACH under an 8 ft ceiling delivers only 6 ACH at 10 ft and 5 at 12 ft, because each air change has more cubic feet to move. This is why tall-ceiling spaces need proportionally more airflow — and why the calculator asks height rather than assuming it (the chart above states its 8 ft assumption).