When you are trying to choose a home air purifier, two numbers come up again and again: ACH and CADR. Understanding how these relate helps you size a purifier realistically for your room instead of guessing based on marketing terms.
ACH (Air Changes per Hour) describes how many times in one hour the entire volume of air in a room is filtered or replaced. It is a room-focused metric.
CADR (Clean Air Delivery Rate) describes how much filtered air a purifier delivers, usually in cubic feet per minute (CFM). It is a device-focused metric.
ACH is useful when you start from your goals for a specific room. CADR is useful when you compare purifiers. Converting between the two lets you answer questions like:
- How much CADR do I need to reach 4, 5, or 6 ACH in my bedroom?
- Is my current purifier strong enough for my open-plan living area?
- What happens to ACH if I move a purifier from a small room to a larger room?
Why ACH and CADR Matter for Home Air Purifiers
When you are trying to choose a home air purifier, two numbers come up again and again: ACH and CADR. Understanding how these relate helps you size a purifier realistically for your room instead of guessing based on marketing terms.
ACH (Air Changes per Hour) describes how many times in one hour the entire volume of air in a room is filtered or replaced. It is a room-focused metric.
CADR (Clean Air Delivery Rate) describes how much filtered air a purifier delivers, usually in cubic feet per minute (CFM). It is a device-focused metric.
ACH is useful when you start from your goals for a specific room. CADR is useful when you compare purifiers. Converting between the two lets you answer questions like:
- How much CADR do I need to reach 4, 5, or 6 ACH in my bedroom?
- Is my current purifier strong enough for my open-plan living area?
- What happens to ACH if I move a purifier from a small room to a larger room?
The Basic ACH to CADR Conversion Formula
The relationship between ACH and CADR is based on simple volume and flow calculations. The key is to keep units consistent and to work step by step.
Step 1: Calculate Room Volume
First, estimate your room volume in cubic feet. In the U.S., this is usually straightforward:
Room volume (cubic feet) = length (ft) × width (ft) × ceiling height (ft)
Example room:
- Length: 12 ft
- Width: 10 ft
- Ceiling height: 8 ft
Room volume = 12 × 10 × 8 = 960 cubic feet.
Step 2: Understand the ACH to CADR Formula
ACH tells you how many times per hour the room volume is cleaned. CADR tells you how many cubic feet per minute are cleaned. There are 60 minutes in an hour, so:
ACH = (CADR × 60) ÷ room volume
Rearranging this to solve for CADR:
CADR (CFM) = (ACH × room volume) ÷ 60
This formula assumes the purifier is running on a steady setting and that air mixing in the room is reasonably good. In real homes, furniture, doors, and room shape can influence mixing, but the formula still gives a practical planning estimate.
Step 3: Choose an ACH Target
Homes do not have a single universal “correct” ACH. Reasonable example targets people often consider include:
- 2–3 ACH: Light background air cleaning in low-use rooms
- 4–5 ACH: Common target range for bedrooms and living rooms when air cleaning is a priority
- 6+ ACH: Often considered for situations where faster particle reduction is desired in a specific room
These are planning examples, not strict rules. The right choice depends on your comfort goals, noise tolerance, and how closed-off the room is from the rest of the home.
Example values for illustration.
| Step | What to do | Why it matters | Notes |
|---|---|---|---|
| 1 | Measure room length and width | Defines basic floor area | Round to nearest half-foot for simplicity |
| 2 | Measure ceiling height | Changes the total air volume | Many homes are 8–9 ft but verify |
| 3 | Calculate room volume (L × W × H) | Core input for ACH and CADR math | Keep in cubic feet for U.S. units |
| 4 | Pick an ACH target range | Aligns air cleaning with your goals | Common examples: 3, 4, 5, or 6 ACH |
| 5 | Use CADR = (ACH × volume) ÷ 60 | Translates room goal to purifier size | Do this for the low and high ends of your ACH range |
| 6 | Compare with purifier specs | Checks if a purifier is likely adequate | Remember CADR is often listed for smoke, dust, and pollen |
| 7 | Plan placement and fan speed | Affects real-world ACH in the room | Avoid blocking intake and outlet; consider door position |
Example: ACH to CADR Conversion for a Bedroom
Working through a concrete example helps make the formula feel less abstract. Consider a fairly typical U.S. bedroom.
Example 1: 12 × 10 Bedroom, 8 ft Ceiling
Room size:
- Length: 12 ft
- Width: 10 ft
- Ceiling height: 8 ft
Room volume: 12 × 10 × 8 = 960 cubic feet.
Goal: Plan for around 5 ACH as a mid-range target.
Use the formula:
CADR = (ACH × room volume) ÷ 60
CADR needed for 5 ACH:
- CADR = (5 × 960) ÷ 60
- CADR = 4,800 ÷ 60
- CADR ≈ 80 CFM
So, in this example, a purifier delivering about 80 CFM of clean air could provide roughly 5 ACH in that 12 × 10 × 8 bedroom, assuming reasonable air mixing and doors mostly closed.
Checking Different ACH Targets for the Same Room
You can repeat the calculation for a range of ACH values to see what feels realistic for noise, size, and budget.
For the same 960 cubic foot room:
- 3 ACH: CADR = (3 × 960) ÷ 60 = 2,880 ÷ 60 ≈ 48 CFM
- 4 ACH: CADR = (4 × 960) ÷ 60 = 3,840 ÷ 60 ≈ 64 CFM
- 6 ACH: CADR = (6 × 960) ÷ 60 = 5,760 ÷ 60 ≈ 96 CFM
From these examples, the same room could be served by a range of purifier sizes depending on how many air changes you are aiming for and how loud you are willing to let the purifier run.
Example: ACH to CADR Conversion for an Open Living Area
Larger rooms and open-plan spaces need more airflow to reach the same ACH as a small bedroom. The same formula applies; only the room volume changes.
Example 2: 20 × 18 Living Room, 9 ft Ceiling
Room size:
- Length: 20 ft
- Width: 18 ft
- Ceiling height: 9 ft
Room volume: 20 × 18 × 9 = 3,240 cubic feet.
Goal: Compare 3 ACH and 5 ACH targets.
3 ACH target:
- CADR = (3 × 3,240) ÷ 60
- CADR = 9,720 ÷ 60
- CADR ≈ 162 CFM
5 ACH target:
- CADR = (5 × 3,240) ÷ 60
- CADR = 16,200 ÷ 60
- CADR ≈ 270 CFM
This illustrates why a purifier that feels powerful in a small bedroom can seem less effective in a large, open room: the room volume is several times larger, so you need much more CADR to reach the same ACH.
Multiple Purifiers in One Space
In many homes, using several moderate-strength purifiers instead of a single large one is more flexible. CADR is roughly additive when units are in the same space with good air mixing.
For example, if you want about 270 CFM of clean air for that 3,240 cubic foot living room (≈5 ACH), you might reach it with:
- One purifier at about 270 CFM, or
- Two purifiers at about 135 CFM each, or
- Three purifiers at about 90 CFM each
In practice, noise, layout, and access to outlets will guide how you combine units. The math just gives you a total CADR target to work toward.
How Room Size, Ceiling Height, and Doors Affect ACH
ACH calculations make a few simple assumptions, but real homes can differ. Here are factors to keep in mind.
Ceiling Height Changes Required CADR
If two rooms have the same floor area but different ceiling heights, the taller room has more air volume, so it needs more CADR for the same ACH.
Example floor area: 12 × 12 = 144 square feet.
- 8 ft ceiling: Volume = 144 × 8 = 1,152 cubic feet
- 10 ft ceiling: Volume = 144 × 10 = 1,440 cubic feet
At 5 ACH:
- 8 ft ceiling: CADR = (5 × 1,152) ÷ 60 ≈ 96 CFM
- 10 ft ceiling: CADR = (5 × 1,440) ÷ 60 ≈ 120 CFM
So the same purifier will deliver fewer air changes per hour in a room with higher ceilings, unless you compensate with more CADR.
Room Shape, Doors, and Open-Plan Layouts
The simple ACH formula treats a room like a single box with well-mixed air. Actual conditions vary:
- Closed doors: Help keep the purifier’s effort focused on that room.
- Open doors and hallways: Effectively connect volumes together, spreading out the delivered CADR.
- Odd shapes and alcoves: May get less air circulation unless the purifier is placed thoughtfully.
If you run a purifier in a bedroom with the door wide open to a hallway or another room, the real-world ACH in the bedroom alone will be lower than your calculation based on bedroom volume only, because some cleaned air drifts out and new unfiltered air comes in.
Fan Speed and Real-World CADR
Published CADR ratings are typically measured at higher fan settings. If you run your purifier on a lower speed for noise reasons, the effective CADR is usually lower. Some manufacturers provide airflow or CADR estimates for multiple fan speeds; if not, you can treat the published rating as a best-case and understand that quieter settings may deliver fewer ACH than your calculation.
Example values for illustration.
| Approx. room size | Ceiling height note | CADR planning idea (for 4–5 ACH) | Notes |
|---|---|---|---|
| 100 sq ft small office | 8 ft typical | About 55–70 CFM | Often feasible with a compact purifier on medium speed |
| 120 sq ft bedroom | 8–9 ft | About 65–90 CFM | Check if desired ACH is reached at a quiet setting |
| 200 sq ft living room | 8–9 ft | About 110–160 CFM | Larger rooms may need higher fan speeds or two units |
| 300 sq ft studio area | 9 ft or higher | Roughly 180–250 CFM | Consider layout; air may not mix evenly around corners |
| Open-plan 400+ sq ft | Varies | Often 250+ CFM | Multiple purifiers placed in different zones can help |
| Hallway plus adjacent rooms | Mixed | No single number | Treat each mostly-closed room separately for ACH planning |
Using ACH and CADR Alongside Other Air Quality Basics
ACH and CADR are key tools for sizing purifiers, but they are only part of the picture for comfortable indoor air.
Filter Types and What They Address
CADR calculations are mostly about particles, such as dust and smoke-sized material. Filter type determines what kinds of contaminants are targeted:
- Particle filters (such as HEPA-type media): Focus on fine particles like dust, dander, and smoke-sized particles.
- Activated carbon filters: Often used to help reduce some odors and certain gases; effectiveness depends on the amount of carbon and contact time.
A higher CADR does not replace the need for the right filter for your goals. For example, if odors are a major concern, you would generally want both adequate CADR and a meaningful amount of carbon media.
Ventilation, Humidity, and Source Control
Purifiers recirculate and clean indoor air; they do not bring in outdoor air or control humidity directly. For many homes, comfort-focused air quality planning also considers:
- Ventilation: Opening windows when outdoor conditions allow, or using mechanical ventilation, can dilute indoor pollutants.
- Humidity management: Keeping indoor humidity in a moderate range with a humidifier or dehumidifier can support comfort and help reduce dampness-related issues.
- Source control: Reducing particle or odor sources indoors (for example, managing dust or smoke sources) decreases the load on your purifier.
ACH and CADR planning fit into this broader picture: once you manage major sources and basic ventilation, a properly sized purifier can help maintain cleaner air within a room.
Noise, Energy Use, and Practical Operation
Running a purifier at the maximum speed all day might reach a higher ACH, but it can be louder and use more energy. Many people find a practical balance by:
- Using higher fan speeds when the room is empty or during specific periods.
- Settling on a moderate speed that is quiet enough for sleep or conversation.
- Positioning the purifier away from beds or seating to soften perceived noise while maintaining airflow.
The ACH to CADR conversion helps you choose a purifier that can hit your target ACH at a speed you are willing to use long term.
Simple Way to Remember the Formula
For everyday use, you can remember the essentials like this:
- Find your room volume in cubic feet.
- Pick a target ACH (for example, 4–6 for many living spaces).
- Use CADR ≈ (ACH × volume) ÷ 60 to estimate the purifier size you need.
Even a rough calculation that is close is usually enough to avoid undersizing a purifier by a large margin and gives you a more grounded sense of how your device should perform in a given room.
Frequently asked questions
How do I calculate the CADR needed to reach a specific ACH in my room?
Use CADR (CFM) = (ACH × room volume) ÷ 60, where room volume is length × width × height in cubic feet. Measure the room dimensions accurately and keep units consistent; the formula assumes reasonable air mixing and doors mostly closed. Use the result as a planning estimate rather than an exact real-world value.
What ACH should I aim for in a bedroom to reduce particles while keeping noise low?
A common practical target for bedrooms is about 4–5 ACH because it balances particle reduction and manageable noise for sleep. Lower targets (2–3 ACH) provide light background cleaning with less noise, while 6+ ACH is used when faster removal is desired. Choose a target based on your comfort, noise tolerance, and room isolation.
Can I combine multiple purifiers to meet an ACH target?
Yes—CADR is roughly additive when units operate in the same well-mixed space, so two or more moderate purifiers can sum to the desired total CADR. Placement and room mixing matter, so distribute units to avoid dead zones. Keep in mind that running multiple units may increase overall noise and energy use.
How do room shape, open doors, or high ceilings affect ACH to CADR conversion?
Taller ceilings increase room volume and therefore require more CADR for the same ACH, while open doors and connected rooms spread cleaned air across larger volumes, lowering real-world ACH in the target space. Irregular shapes and alcoves can reduce effective mixing, so consider placement and possibly multiple units. For connected areas, calculate CADR based on the combined volume you want to treat or close doors to isolate the space.
Will running a purifier on a lower fan speed change the CADR used in the conversion?
Yes—published CADR ratings are often measured at higher fan settings, so operating at lower speeds usually reduces effective CADR and ACH. If quiet operation is important, choose a purifier whose CADR at the setting you expect to use still meets your ACH target. Check manufacturer specifications for multi-speed ratings or use the published CADR as a best-case estimate.
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