About the Air Purifier Size Calculator
This air purifier size calculator works out the clean air delivery rate (CADR) a portable air cleaner needs to keep a room’s air filtered. Enter the room’s length, width and ceiling height in feet or metres and choose how many times per hour you want the air cleaned; the tool returns the required CADR in cubic feet per minute (cfm) — the figure printed on US packaging — and in cubic metres per hour (m³/h), used on many European and Asian models.
It is meant for anyone shopping for a HEPA purifier for a bedroom, living room, nursery or office, and for people dealing with wildfire smoke, pollen, pet dander or dust who want to know whether an existing unit is big enough. It also shows the EPA’s minimum guideline based on floor area alone so you can compare the two approaches.
The EPA and AHAM guideline of roughly two-thirds of the floor area (65 cfm per 100 sq ft) assumes an 8 ft ceiling and gives about 5 air changes per hour. Higher ceilings, smoke events or allergy and asthma concerns justify a higher target. CADR is measured at the purifier’s top fan speed, so if you run it on a quieter setting, choose a model rated above the minimum.
With the default inputs, the minimum cadr is 120 cfm. Change any value above to recalculate instantly.
How to use the air purifier size calculator
- 1Choose feet or metres.
- 2Enter the room length, width and ceiling height.
- 3Set your target air changes per hour (5 is a good general choice).
- 4Read the minimum CADR and compare it with the smoke CADR on purifier packaging.
Formula and method
Air changes per hour (ACH) is how many times the full volume of the room passes through the filter each hour. Multiplying the room volume by the target ACH gives the clean air needed per hour; dividing by 60 converts it to cubic feet per minute, the unit used for US CADR ratings. One cfm equals about 1.699 m³/h.
The EPA guideline (from AHAM) is a minimum CADR of about two-thirds of the floor area in square feet — 65 cfm per 100 sq ft — for an 8 ft ceiling, which works out to roughly 4.9 air changes per hour. This calculator uses the full room volume, so tall ceilings are handled correctly. Use the smoke CADR when comparing models, since smoke represents the smallest and hardest particles to capture.
- CADR
- Clean air delivery rate of the purifier
- ACH
- Target air changes per hour
- Volume
- Length × width × ceiling height
Worked examples
15 × 12 ft bedroom, 8 ft ceiling
The room holds 15 × 12 × 8 = 1,440 cubic feet. Cleaning it 5 times an hour needs 7,200 ft³ per hour, or 120 cfm (about 204 m³/h). That is close to the EPA guideline of 117 cfm for 180 sq ft.
Open-plan living room with a vaulted ceiling during wildfire smoke
A 24 × 18 ft room with a 14 ft ceiling holds 6,048 ft³. Eight air changes per hour for smoke require about 806 cfm — almost three times the floor-area guideline — so two or three large purifiers would be needed.
Metric home office, 5 × 4 m
A 5 × 4 m room with a 2.5 m ceiling holds 50 m³. Five air changes an hour means 250 m³/h of clean air, which is about 147 cfm.
Frequently asked questions
What CADR do I need for my room?+
Multiply the room volume in cubic feet by your target air changes per hour and divide by 60. A 15 × 12 ft room with an 8 ft ceiling at 5 ACH needs about 120 cfm. The quick EPA rule is two-thirds of the floor area in square feet.
What is a good number of air changes per hour?+
About 4–5 ACH suits general use and matches the EPA/AHAM two-thirds rule. People with allergies or asthma often aim for 5–6, and during wildfire smoke or for infection control 6–12 ACH is commonly recommended.
What is the difference between smoke, dust and pollen CADR?+
AHAM tests purifiers against tobacco smoke (smallest particles), dust (medium) and pollen (largest). The smoke CADR is usually the lowest and best represents fine PM2.5 particles, so use it when sizing.
Is a bigger air purifier better?+
Oversizing is fine and lets you run the purifier on a lower, quieter fan speed while still meeting your target. Undersizing means the air is cleaned less often than you expect, especially since CADR is measured on the highest speed.
How do I convert CADR from m³/h to cfm?+
Divide m³/h by 1.699 to get cfm, or multiply cfm by 1.699 to get m³/h. A purifier rated 340 m³/h has a CADR of about 200 cfm.