Air Purifier CADR Calculator

The Air Purifier CADR Calculator works out the Clean Air Delivery Rate (CADR) your air purifier needs for a specific room. Enter your floor area, ceiling height and how clean you want the air, and it returns the required CADR in both CFM and m³/h, the resulting air changes per hour (ACH), how often the whole room is filtered, and the industry-standard AHAM benchmark. You can also enter a purifier you are considering to instantly see whether it is powerful enough for the room.

What is CADR?

CADR (Clean Air Delivery Rate) is a standardized rating, developed by the Association of Home Appliance Manufacturers (AHAM), that tells you how much clean air a purifier produces. It is expressed as a volume of filtered air per unit of time — cubic feet per minute (CFM) in the United States, or cubic metres per hour (m³/h) in most of the world. A purifier typically carries three CADR numbers: one for smoke, one for dust, and one for pollen. The higher the CADR, the faster and larger the space it can clean.

What are Air Changes per Hour (ACH)?

Air changes per hour is how many times the entire volume of air in a room passes through the purifier's filter in one hour. An ACH of 5 means all the air in the room is cleaned five times an hour — roughly once every 12 minutes. ACH is the number that actually matters for your health: it decides how quickly smoke, allergens and airborne particles are removed. CADR is the tool that delivers your target ACH.

CADR Formula

The required CADR follows directly from your room volume and target air changes per hour:

The factor of 60 converts between per-hour air changes and the per-minute CFM rating. In metric units it is even simpler: required CADR in m³/h equals your target ACH multiplied by the room volume in m³.

Recommended Air Changes per Hour by Use

The AHAM 2/3 Rule

As a quick sanity check, AHAM recommends that a purifier's smoke CADR (in CFM) be at least two-thirds of the room's floor area in square feet. For a 300 sq ft room, that means a smoke CADR of at least 200 CFM. At a standard 8-foot ceiling this rule of thumb delivers about 5 air changes per hour, which is why it works well for allergy and asthma sufferers. This calculator shows the AHAM figure alongside your exact requirement.

How to Choose the Right Air Purifier

How to Use This Calculator

1. Enter your room size: Choose Imperial or Metric units, then enter the floor area and ceiling height (8 ft / 2.4 m is assumed if you leave the height blank).
2. Pick your air quality goal: Select general freshness, allergies, asthma and pets, or wildfire smoke — or choose Custom and type your own ACH.
3. Click Calculate: See the required CADR in CFM and m³/h, the air changes per hour, the minutes per full air change, and the AHAM benchmark.
4. Check a purifier (optional): Enter the CADR of a model you are considering to find out if it is enough, the real ACH it delivers, and the largest room it can properly clean.

Frequently Asked Questions

What is CADR on an air purifier?

CADR (Clean Air Delivery Rate) measures how much clean, filtered air a purifier delivers, in cubic feet per minute (CFM) or cubic metres per hour (m³/h). A higher CADR means the purifier cleans a larger room, or the same room more times per hour. Purifiers are usually rated with separate CADR numbers for smoke, dust and pollen.

How do I calculate the CADR I need?

First find your room volume by multiplying floor area by ceiling height. Then multiply the volume by your target air changes per hour and divide by 60 to get CADR in CFM. For example, a 200 sq ft room with an 8 ft ceiling is 1,600 cubic feet; for 4 air changes per hour you need 4 × 1,600 ÷ 60, which is about 107 CFM.

How many air changes per hour do I need?

About 2 air changes per hour is fine for general freshness, 4 is recommended for allergies and dust, 5 for asthma and pets, and 6 or more for wildfire smoke and airborne viruses. Higher air changes clean the room more often and remove pollutants faster.

What is the AHAM 2/3 rule?

The Association of Home Appliance Manufacturers recommends that a purifier's smoke CADR in CFM be at least two-thirds of the room's floor area in square feet. For a 300 sq ft room that means a smoke CADR of at least 200 CFM. At a standard 8 ft ceiling this rule delivers roughly 5 air changes per hour.

Is a higher CADR always better?

A higher CADR cleans faster and covers bigger rooms, which helps with smoke, allergies and large spaces. The trade-offs are higher price, more noise and greater energy use at top speed. Choosing a purifier rated for a room somewhat larger than yours lets you run it on a quieter medium setting and still hit your target air changes per hour.

Can one air purifier cover my whole home?

Usually not. CADR ratings assume a single closed room with the door shut, because open doorways and hallways let unfiltered air mix back in. For a whole home it is generally better to place a correctly sized purifier in each main room, especially bedrooms and living areas where people spend the most time.

Additional Resources

• Air Cleaners and Air Filters in the Home - US EPA
• Clean Air Delivery Rate - Wikipedia