How strong does an air purifier need to be for your room? The answer lives in a number called CADR, or clean air delivery rate. The Air Purifier Size Calculator finds the minimum CADR for your room and cleaning goal.
You enter length, width, ceiling height, and a target number of air changes per hour. The tool returns the CADR in cubic feet per minute, plus room area, air volume, and hourly turnover. It suits allergy sufferers, pet owners, and anyone shopping for a purifier.
Table of Contents
- What the Calculator Does
- How to Use It, Step by Step
- The CADR Formula
- Worked Example
- How to Read the Results
- Choosing a Purifier With the Right CADR
- Limitations and Assumptions
- FAQs About the Air Purifier Size Calculator
What the Calculator Does
The tool updates as you type or change the air change setting. It shows a main result, a basis line, and four cards.
- Minimum Clean Air Delivery Rate (CADR): the main result, in cubic feet per minute (CFM).
- Basis line: shows the two CADR values the tool compared, one from air changes and one from floor area.
- Floor Area: length times width, in square feet.
- Total Air Volume: floor area times ceiling height, in cubic feet.
- AHAM Recommended Coverage: the room size that the CADR shown can cover.
- Hourly Air Turnover: cubic feet of air the purifier must clean each hour.
- Copy Purifier Requirements and Reset buttons.
How to Use It, Step by Step
- Enter Room Length (Feet) and Room Width (Feet). The defaults are 16 and 14.
- Enter Ceiling Height (Feet). The default is 8.
- Pick a Target Air Changes Per Hour (ACH) from the menu. The default is 4.
- Read the CADR figure and its basis line first, then the four cards below.
- Press Copy Purifier Requirements for a text summary, or Reset to restore the defaults.
Length and width must each be 1 to 1,000 feet. Ceiling height must be 7 to 20 feet. A blank or out-of-range entry shows a short message, and every result reads N/A until you fix it.
The CADR Formula
The tool works out two CADR values and recommends the larger one. The first comes from your air changes per hour. The second comes from the AHAM two-thirds rule.
Volume × ACH ÷ 60vsFloor Area × 2/3=Larger Value = CADR (CFM)
- Floor area = length × width.
- Air volume = floor area × ceiling height.
- Hourly turnover = air volume × ACH.
- ACH-based CADR = hourly turnover ÷ 60, rounded up to a whole CFM.
- AHAM CADR = floor area × 2/3, rounded up to a whole CFM.
- Coverage = recommended CADR × 1.5, rounded, shown as "Up to" a number of square feet.
Rounding up keeps the minimum from ever sitting below the value it came from.
Worked Example
Jordan has spring allergies and wants a purifier for a 12 by 11 foot bedroom. The ceiling is 8 feet, and Jordan picks 4 ACH.
| Result | How It Is Found | Output |
|---|
| Floor Area | 12 × 11 | 132 sq ft |
| Total Air Volume | 132 × 8 | 1,056 cu ft |
| Hourly Air Turnover | 1,056 × 4 | 4,224 cu ft / Hr |
| ACH-Based CADR | 4,224 ÷ 60 = 70.4, rounded up | 71 CFM |
| AHAM CADR | 132 × 2/3 = 88 | 88 CFM |
| Minimum CADR | Larger of 71 and 88 | 88 CFM CADR |
| AHAM Coverage | 88 × 1.5 = 132 | Up to 132 sq ft |
The basis line reads "Larger of 4 ACH (71 CFM) and AHAM 2/3 Rule (88 CFM)." Here the floor area rule sets the answer.
Now take a 20 by 15 foot living room with 9-foot ceilings at 6 ACH. The air change value is 270 CFM, and the AHAM value is 200 CFM. The tool shows 270 CFM CADR, with coverage up to 405 sq ft.
How to Read the Results
Minimum Clean Air Delivery Rate is the lowest CADR to look for on the box. The basis line tells you which rule set it, so you can see why the number moved.
Floor Area, Total Air Volume, and Hourly Air Turnover show the math behind the result. Hourly turnover always uses your chosen ACH, even when the AHAM value wins.
AHAM Recommended Coverage turns the CADR back into a room size. Because the tool never goes below the two-thirds rule, coverage is always at least your floor area.
Choosing a Purifier With the Right CADR
AHAM, the appliance group behind CADR ratings, uses a two-thirds rule of thumb. It suggests a smoke CADR of at least two-thirds of the floor area. That rule assumes an 8-foot ceiling.
Labels often list three CADR numbers, for smoke, dust, and pollen. Many shoppers use the smoke number, since it is usually the lowest. Running the purifier on a lower, quieter speed also cuts real airflow.
To get the room's cubic feet for other projects, try the room volume calculator. For cooling the same space, see the air conditioner BTU calculator. Better air mixing can help too, so check the ceiling fan size guide.
A purifier running all day uses power, which you can estimate with the appliance running cost tool. More room tools are in the home and construction calculator hub.
Limitations and Assumptions
- The tool assumes a simple box-shaped room with a flat ceiling.
- Only three ACH choices are offered: 2, 4, and 6.
- The coverage line uses a fixed 1.5 factor from the two-thirds rule, not a model's tested rating.
- The ACH labels describe cleaning levels only. They are not medical advice for any health condition.
- Open floor plans, doors, and HVAC airflow are not modeled.
FAQs About the Air Purifier Size Calculator
What is CADR on an air purifier?
CADR stands for clean air delivery rate. It measures how many cubic feet of filtered air a purifier delivers each minute.
How many air changes per hour should I pick?
The menu offers 2 ACH (Light Air Cleaning), 4 ACH (Moderate Air Cleaning), and 6 ACH (Frequent Air Cleaning). More changes clean the air faster but need a stronger unit.
Why does the result stay the same when I switch between 2 and 4 ACH?
In rooms with normal ceilings, the floor area rule often gives the larger value. At 2 ACH it always wins, and at 4 ACH it wins up to 10-foot ceilings.
Does ceiling height change the CADR I need?
Only when the air change value is the larger one. At 6 ACH, a taller ceiling always raises the CADR. At lower settings, height may not change it.
Can one purifier clean a whole house?
Most portable units are sized for one room. For several rooms, run the tool for each space or use more than one purifier.
This calculator gives planning estimates only and is not professional engineering, electrical, or building code advice. Reviewed for general accuracy by Prof. Dr. Khalil Mudassar, PhD.