Quick answer
In a 20 x 16 x 8 ft room, 200 CFM plus 150 CFM gives 350 CFM combined smoke CADR, or about 8.2 ACH. That is above a 5 ACH clean-air target for this room volume.
How to use this multiple air purifiers ACH calculator
Measure the open room length, width, and ceiling height. Add each purifier type, its smoke CADR, and the quantity you plan to run in that same space. The calculator returns combined CADR, estimated ACH, required CADR for your target, and any CADR gap.
Use this tool when a single purifier is not enough for a large living room, open-plan area, classroom, studio, or smoke-season setup. If you need the CADR for one room from scratch, start with the Air Purifier CADR Calculator. If you already know the combined CADR and want the maximum room area, use the Air Purifier Room Size Calculator.
After checking clean-air performance, compare the operating side: more units can improve CADR but also add electricity and filter cost. Use the Air Purifier Electricity Cost Calculator and Air Purifier Filter Replacement Cost Calculator before buying several purifiers.
Can you add CADR from two air purifiers?
Yes, but only as a planning estimate for the same open room. If two air purifiers run at the fan speed used for their CADR rating, their clean-air delivery can be added to estimate combined ACH. This is useful when one large purifier is expensive, noisy, or hard to place well.
Do not use this as a whole-house shortcut. A purifier in one bedroom should not be counted for another bedroom with the door closed. For divided spaces, calculate each room separately and compare the total electricity and filter cost before buying more units.
Multiple air purifiers ACH formula
room volume = length x width x ceiling heightcombined CADR = sum(CADR per purifier x quantity)actual ACH = combined CADR (CFM) x 60 / room volume (ft3)required CADR = room volume (ft3) x target ACH / 60CADR gap = required CADR - combined CADRCADR is a clean-air flow rate. In imperial units, CFM is cubic feet per minute, so the formula multiplies by 60 minutes per hour. In metric units, combined CADR in m3/h divided by room volume in m3 gives the same ACH estimate.
Assumptions and methodology
This calculator treats multiple purifiers as added clean-air delivery in one shared air volume. That is useful for planning, but it is not a guarantee that every seat or corner receives the same clean air. It is a particle-filtration estimate, not medical, occupational ventilation, or HVAC design advice.
- Add CADR only for purifiers running in the same open space at the fan speed that matches the listed CADR.
- Closed doors, partial walls, hallways, furniture, blocked intakes, and poor placement reduce how well CADR combines.
- Smoke CADR is the preferred input when planning small-particle removal or comparing different purifier models.
- Higher ACH targets clean particles faster but require more CADR or a smaller room volume.
Example calculations
Combined CADR examples
The default setup uses 200 CFM plus 150 CFM in a 20 x 16 x 8 ft room. The room volume is 2,560 ft3, and the combined CADR provides about 8.2 ACH.
One 350 CFM purifier gives the same calculated ACH as two purifiers with the same combined CADR. The practical difference is placement, noise, outlet availability, maintenance, and whether air mixes well across the whole room.
In a 25 x 20 x 9 ft open living area, two 250 CFM units provide about 6.7 ACH. A single 120 CFM purifier in a 20 x 20 x 8 ft room provides only 2.3 ACH, leaving a gap of about 147 CFM for a 5 ACH target.
Multiple air purifiers ACH chart
These examples use smoke CADR, typical room dimensions, and a 5 ACH target. Use the calculator above for your actual ceiling height and purifier count.
| Room type | Example size | Combined CADR | Estimated ACH | Target coverage | Assessment |
|---|---|---|---|---|---|
| Bedroom | 12 x 12 x 8 ft | 120 CFM | 6.3 ACH | 125% | Above target |
| Large bedroom | 15 x 12 x 8 ft | 240 CFM | 10 ACH | 200% | Above target |
| Living room | 20 x 16 x 8 ft | 350 CFM | 8.2 ACH | 164% | Above target |
| Open living area | 25 x 20 x 9 ft | 500 CFM | 6.7 ACH | 133% | Above target |
| Smoke season setup | 20 x 20 x 8 ft | 600 CFM | 11.3 ACH | 225% | Above target |
CADR needed by ACH target
The same 350 CFM setup can be more than enough for everyday use and less generous for a very high smoke-heavy target. The table below uses the default 20 x 16 x 8 ft room.
| Target ACH | Required CADR | Default setup coverage | Use case |
|---|---|---|---|
| 3 ACH | 128 CFM / 217 m3/h | 273% | Everyday particle cleaning |
| 4.8 ACH | 205 CFM / 348 m3/h | 171% | Everyday particle cleaning |
| 5 ACH | 213 CFM / 362 m3/h | 164% | Stronger everyday target |
| 7.5 ACH | 320 CFM / 544 m3/h | 109% | Smoke-heavy or faster filtration |
| 10 ACH | 427 CFM / 725 m3/h | 82% | Smoke-heavy or faster filtration |
When multiple smaller purifiers make sense
Multiple smaller air purifiers can be the better practical choice when a room has several seating areas, noise is a concern, or one large unit would sit far from the source of particles. Place units so intake and outlet airflow is open, not tight against furniture or curtains.
They are not a shortcut for disconnected spaces. If one purifier is in a bedroom and another is in a living room, calculate each room separately. For a whole-home comfort decision, compare room volume and cooling load with tools such as the AC Room Size Calculator.
Related calculators
AC Room Size Calculator
Estimate what room size an air conditioner can cool based on BTU or kW capacity.
Air Purifier CADR Calculator
Estimate the smoke CADR an air purifier needs from room dimensions, ceiling height, and target air changes per hour.
Air Purifier Room Size Calculator
Estimate what room size an air purifier can cover from CADR, ceiling height, clean-air target, and number of units.
Air Purifier Filter Replacement Cost Calculator
Estimate annual air purifier filter replacement cost from filter price, replacement interval, room conditions, and number of units.
Air Purifier Electricity Cost Calculator
Estimate air purifier electricity cost from watts, daily runtime, annual kWh, electricity price, and number of units.
Dehumidifier Size Calculator
Estimate dehumidifier capacity in pints per day or liters per day from room size, dampness level, ceiling height, and space type.
FAQ
Can I add CADR from two air purifiers?
You can roughly add smoke CADR values when the purifiers run at the same time in one open space and airflow is not blocked. Do not add CADR across separate rooms with closed doors or weak air mixing.
How do I calculate ACH for multiple air purifiers?
Add the CADR of each purifier in CFM, multiply the combined CADR by 60, then divide by room volume in cubic feet. In metric units, divide combined CADR in m3/h by room volume in m3.
Is two smaller air purifiers better than one large air purifier?
Two smaller units can be useful when they improve placement, reduce noise at each fan, or cover an open room more evenly. One large unit can be simpler if its CADR is high enough and the room has good air circulation.
Should I use smoke CADR for this calculator?
Use smoke CADR when possible because it is the most useful single CADR number for small particle planning and is easier to compare across models.
What ACH target should I choose?
For everyday particle cleaning, 4.8 to 5 ACH is a practical planning range. Smoke-heavy, wildfire, or fast-cleaning situations often use a higher target, such as 7.5 ACH or a custom value.
Does this work for separate rooms?
No. Treat separate rooms as separate calculations unless there is strong continuous air mixing. A purifier in one bedroom usually should not be counted for another bedroom with the door closed.
Does fan speed affect combined ACH?
Yes. The CADR rating usually applies to a specific fan setting, often high speed. If you run the purifier on a quieter lower speed, the actual CADR and ACH may be lower.
How much extra CADR do I need if I am below target?
Use the CADR gap result. It shows the additional smoke CADR needed to reach the selected target for the current room size and ceiling height.