Air Changes per Hour Calculator
Convert measured supply or exhaust CFM into room air changes per hour, add portable-air-cleaner CADR as equivalent ACH, compare a target, and display ideal contaminant-decay times alongside an explicit imperfect-mixing multiplier.
Measure the room and clean-air lanes
Air-change and equivalent-clean-air result
The removal times assume the source has stopped and clean air mixes perfectly before the selected multiplier is applied. People, continuing sources, stagnant zones, short-circuit airflow, particle settling, deposition, and re-suspension can change reality.
ACH compares airflow per hour with room volume
The CDC defines air changes per hour as the ratio of air volume flowing through a space during a period to the room volume. If 1,200 cubic feet of clean air enters a 1,200-cubic-foot room in one hour, that is one nominal air change.
U.S. airflow is commonly measured in cubic feet per minute. Multiplying CFM by 60 converts it to cubic feet per hour. Dividing by length × width × height produces ACH. The unit describes turnover rate; it does not mean every molecule is replaced in a neat sequence.
Equivalent ACH, or eACH, expresses filtration or another verified treatment as an equivalent clean airflow divided by room volume. It permits a common arithmetic comparison, but outdoor ventilation and filtration do different things. Particle filters do not remove carbon dioxide or every gas.
Default CDC-style example: why supply and exhaust are not added
The room is 12 × 10 × 10 feet, so volume is 1,200 ft³. Outdoor supply is 65 CFM and exhaust is 80 CFM. The cited CDC FAQ uses the larger 80-CFM value because exhaust exceeds supply and 15 CFM transfers into the room. Under its assumption that transfer air is contaminant-free, clean airflow is 80 CFM.
Adding 65 + 80 would count air crossing the room system twice and incorrectly produce 7.25 ACH. Using 80 × 60 ÷ 1,200 produces 4.0 ACH. A 5-ACH planning target needs 100 CFM, so the displayed gap is 20 CFM.
The transfer-air assumption matters. If adjacent air contains the same contaminant, exhaust does not create 15 CFM of clean dilution. Pressure, door gaps, corridors, supply paths, and source locations require professional airflow assessment.
Keep outdoor air, recirculation, and CADR in the correct lane
| Airflow number | What it may represent | What must be verified |
|---|---|---|
| Outdoor-air supply CFM | Conditioned outside air delivered to the room. | Measure the outdoor fraction, not total HVAC supply that may be mostly recirculated. |
| Exhaust CFM | Air removed from the room with make-up air entering elsewhere. | Identify make-up path, pressure, and whether entering air is clean. |
| Portable-air-cleaner CADR | Tested particle clean-air delivery. | Use smoke CADR for small airborne particles and the actual operating speed. |
| Other equivalent CFM | Professionally established UVGI or treatment equivalence. | Do not invent an equivalence or add a device’s fan CFM without removal efficiency. |
| Total HVAC supply | Mixed outdoor and recirculated air. | Separate outdoor dilution from filtered recirculation to avoid double counting. |
If the same airflow passes through two filters in series, their CADRs cannot simply be added. If two independent portable cleaners operate, their certified clean-air delivery can generally be combined for the same pollutant and operating condition.
A target depends on the space and purpose
CDC/NIOSH says that when possible, workplaces can aim for five or more total ACH/eACH to help reduce airborne viral particles. EPA discusses five ACH from outdoor ventilation alone or equivalent strategies in public indoor spaces. Neither statement makes five a universal code for every room or hazard.
Occupancy, activity, pollutant generation, healthcare use, industrial process, age, vulnerability, room volume, distribution, climate, energy, noise, and local standards matter. Healthcare rooms can have specific outdoor and total ACH, pressure, filtration, and exhaust rules. Kitchens, laboratories, paint use, garages, and combustion appliances need source control and dedicated ventilation.
Use the target box as a comparison after identifying the governing code, standard, or professional objective. Do not select a high number solely because it looks safer; systems must maintain comfort, humidity, pressure, and equipment operation.
Removal time uses an ideal exponential-decay model
At 4 ACH, ideal 90% decay is about 34.5 minutes and ideal 99% decay about 69.1 minutes. The CDC table reports the same 69-minute 99% result. A mixing factor of 5 expands the illustration to about 345 minutes.
The source must have stopped. If an infectious person, chemical process, smoke source, or other generator remains, concentration can approach a steady state rather than decaying to zero. Empty-room tables cannot be used to declare an occupied room safe.
Perfect mixing rarely occurs. Supply air can short-circuit to return, furniture can create stagnant zones, and particles can settle or be resuspended. Tracer-gas decay and airflow measurements can characterize actual behavior.
Measure CFM instead of trusting a fan label
Fan nameplate CFM is usually a rated point at a specified static pressure. Duct length, diameter, elbows, grilles, filters, dampers, dirt, installation, and building pressure change delivered airflow. A balancing hood or accepted traverse method provides better evidence.
Portable cleaner CADR is also speed-specific. A device rated on high but operated on quiet low speed delivers less. Dirty filters can change airflow, while bypass or poor fit can reduce effective cleaning. Follow replacement and placement instructions.
Natural ventilation from windows varies with wind, temperature, opening, and flow path. A one-time guess is not a stable ACH. Carbon dioxide can sometimes help assess occupant ventilation patterns, but it is not a direct detector for every pathogen or pollutant.
Map the room before measuring. Record every supply, return, exhaust, transfer grille, open door, and portable cleaner, then note the equipment mode and filter condition. Central systems can cycle, change fan speed, or close outdoor-air dampers, so a reading during one call may not represent the occupied hour. Repeat measurements at the operating condition the result is meant to describe.
For irregular or sloped rooms, divide the space into rectangular prisms and add their volumes. Do not use floor area alone. Large furniture does not normally justify subtracting volume unless a governing method specifically directs it; airflow still has to mix around obstructions, and furnishings can worsen stagnant zones.
More airflow can create new building risks
Outdoor air can introduce heat, cold, humidity, smoke, pollen, or outdoor pollution. HVAC capacity, filtration, freeze protection, condensate, and humidity control matter. During outdoor smoke events, filtration and source control may be preferred to unfiltered outdoor air according to local guidance.
Avoid air cleaners that intentionally generate ozone. EPA identifies ozone as a lung irritant. Device marketing terms such as ionization, plasma, or active oxygen do not replace independent safety and effectiveness evidence.
Frequently asked questions
Should I add supply and exhaust CFM?
Not when they describe opposite sides of the same room balance. The cited CDC example uses the larger value under a clean transfer-air assumption.
Is CADR the same as fan airflow?
No. CADR combines airflow and particle-removal performance. Raw fan CFM without efficiency is not equivalent clean airflow.
Does 5 ACH guarantee protection from infection?
No. It is a useful clean-air goal in cited public/workplace guidance, not a guarantee. Sources, mixing, occupancy, behavior, and vulnerability matter.
Can I use the 99% time to reenter a contaminated room?
Not by itself. The calculation assumes the source stopped and ideal conditions. Hazard-specific professionals and authorities determine safe reentry.
Does a particle filter remove carbon dioxide?
No. Ordinary HEPA or particle filtration does not remove occupant-generated carbon dioxide or many gases.
Why enter a mixing multiplier?
It exposes how imperfect distribution can extend ideal decay time. The correct factor requires measurement and professional judgment.
Related household planning
For an unrelated U.S. It does not calculate HVAC credits, medical risk, ventilation compliance, or clean-air performance.
References
- CDC/NIOSH: ventilation FAQs and ACH example
- CDC/NIOSH: aiming for five or more clean-air changes in workplaces
- U.S. EPA: ventilation, ACH, and equivalent ACH
- CDC: airborne-contaminant removal times by ACH
Educational airflow arithmetic only. Verify clean-air source, measurements, pressure, mixing, hazards, targets, codes, HVAC capability, and professional design.