Short answer

Multiply the room's volume (floor area × ceiling height) by the number of clean air changes per hour you want. The result is the clean air delivery rate (CADR) the purifier needs, in m³/h. For most homes and offices, 5 air changes per hour is a sensible target: a 20 m² room with a 2.5 m ceiling (50 m³) needs about 250 m³/h, or roughly 150 CFM. Size for the fan speed you will actually use, not the maximum.

Step 1 — Calculate room volume, not just floor area

Room volume (m³) = length (m) × width (m) × ceiling height (m)

Floor area is how rooms are usually described, but air purifiers clean air, and air fills the whole volume. A 25 m² room with a 2.4 m ceiling holds 60 m³; the same floor area under a 3.2 m ceiling holds 80 m³ — a third more air to clean.

For irregular rooms, split the space into rectangles, calculate each one and add them together. For sloped ceilings, use the average height. If a room is permanently open to another space (an open-plan kitchen and living area, for example), treat the connected spaces as one volume.

Step 2 — Choose a target air change rate

Air changes per hour (ACH) express how many times, in theory, a room's full volume of air passes through a filter each hour. When we talk about purifiers, we mean clean air changes — the equivalent volume of particle-free air delivered.

Clean ACHTypical useNotes
2–3Background filtration in well-ventilated rooms with low particle sourcesNoticeable but slow effect; long clean-up after spikes
4–5General target for homes, offices and classroomsU.S. CDC ventilation guidance suggests aiming for 5 or more
6–8Bedrooms of allergy-sensitive people, busy meeting rooms, outdoor smoke episodesFaster recovery after cooking, cleaning or visitors
10+Specialised settings (clinics, temporary high-exposure situations)Needs professional design alongside ventilation

ACH from a purifier adds to the air changes from ventilation and natural settling, so the room total is higher than the purifier alone. However, planning on the purifier's contribution keeps the calculation conservative.

Step 3 — Calculate the required CADR

Required CADR (m³/h) = room volume (m³) × target clean ACH
CADR in CFM = CADR in m³/h ÷ 1.699   |   CADR in m³/h = CFM × 1.699
Example 1 — bedroom. 4.0 m × 3.5 m, ceiling 2.6 m → volume 36.4 m³. Target 6 ACH for an allergy-sensitive sleeper → 36.4 × 6 ≈ 218 m³/h (≈ 128 CFM). Because the purifier will run at night, choose a unit that delivers about 220 m³/h on a setting quiet enough for sleep.
Example 2 — living room. 6.0 m × 4.5 m, ceiling 2.7 m → volume 72.9 m³. Target 5 ACH → 365 m³/h (≈ 215 CFM). If the room opens onto a 20 m² kitchen with the same ceiling (54 m³), the combined volume is 127 m³ and the requirement rises to about 635 m³/h — a strong case for two units, one in each zone.

For quick numbers, use our air purifier size calculator, which also estimates how long a unit takes to clear 90% of airborne particles.

Quick reference table

Floor areaCeilingVolumeCADR for 3 ACHCADR for 5 ACH5 ACH in CFMCADR for 8 ACH
12 m²2.5 m30 m³90 m³/h150 m³/h88240 m³/h
16 m²2.5 m40 m³120 m³/h200 m³/h118320 m³/h
20 m²2.6 m52 m³156 m³/h260 m³/h153416 m³/h
25 m²2.7 m67.5 m³203 m³/h338 m³/h199540 m³/h
35 m²2.7 m94.5 m³284 m³/h473 m³/h278756 m³/h
50 m²3.0 m150 m³450 m³/h750 m³/h4411,200 m³/h

Many purifiers carry a recommended room size instead of, or next to, a CADR. In the AHAM system used in North America, the suggested room size in square feet is approximately the smoke CADR (in CFM) × 1.55. This rule assumes a ceiling height of 8 ft (about 2.4 m) and delivers close to 5 air changes per hour.

Two consequences:

  • If your ceiling is higher than 2.4 m, the label overstates the room size the unit can serve. At 3.0 m you need about 25% more CADR for the same effect.
  • Some manufacturers outside the AHAM programme calculate room sizes using as little as 1–2 air changes per hour, which makes a small unit look suitable for a large room. Always convert back to CADR and do your own calculation.

Why real rooms need a margin

The calculation assumes a well-mixed room — cleaned air spreads evenly and every part of the room is served equally. Real rooms have furniture, alcoves, closed cupboards, people moving and doors opening. In practice:

  • Size for the speed you will use. Headline CADR is usually measured at maximum speed with a new filter. If you will run the unit on a medium setting for noise reasons, use the CADR at that setting.
  • Allow for filter loading. Airflow in many units drops as filters collect dust. A 15–25% margin keeps performance acceptable between filter changes.
  • Consider distribution. Long, L-shaped or divided rooms are better served by two smaller units than one large unit at one end. See how airflow affects indoor air quality.

Occupancy-based sizing for shared spaces

For offices, classrooms and waiting rooms, volume-based targets can be complemented with a per-person view. ASHRAE Standard 241 (Control of Infectious Aerosols, 2023) introduced the idea of equivalent clean airflow per occupant, combining outdoor air ventilation and filtration. The detailed targets depend on the space type, but the principle is useful: a small meeting room with many people needs more clean air than its volume alone suggests. Our office planning guide works through an example.

Common sizing mistakes

  1. Using floor area only and ignoring ceiling height.
  2. Comparing airflow (m³/h of air moved) with CADR (m³/h of clean air). A unit with a low-grade filter can move a lot of air and still have a modest CADR.
  3. Using the maximum-speed CADR for a bedroom where the unit will run on low.
  4. Forgetting that an open door or open-plan layout enlarges the volume.
  5. Trusting a "suitable for up to X m²" claim without knowing how many air changes it assumes.
  6. Expecting a purifier to fix stuffiness or high CO₂ — that is a ventilation problem (see PM2.5, VOCs and CO₂).

Key takeaways

  • Required CADR = room volume × target clean air changes per hour.
  • 5 ACH is a good default for homes and offices; go higher for bedrooms of sensitive people or during smoke episodes.
  • Convert CFM to m³/h by multiplying by 1.699.
  • Use the CADR at the speed you will actually run, and add a margin for filter loading and imperfect mixing.

Frequently asked questions

How many air changes per hour do I need?

For general home use, 4–5 air changes per hour of clean air is a common target; the U.S. CDC's ventilation guidance suggests aiming for 5 or more. Rooms with a strong particle source, such as during wildfire smoke episodes or for people sensitive to allergens, benefit from higher rates.

Should I size for floor area or room volume?

Room volume. Two rooms with the same floor area but different ceiling heights contain different amounts of air. Most 'recommended room size' labels quietly assume a ceiling height of about 2.4 m (8 ft).

Is one big purifier better than two small ones?

The total clean air delivery is what matters for the room average. Two smaller units can distribute air better in long or divided spaces and are often quieter at the same total airflow, while one larger unit is simpler to maintain.

Sources and further reading

  1. ANSI/AHAM AC-1 — Method for measuring performance of portable household electric room air cleaners
  2. AHAM Verifide — room air cleaner certification programme
  3. U.S. Centers for Disease Control and Prevention — ventilation guidance for buildings and homes (2023 update recommending 5 or more air changes per hour of clean air)
  4. ASHRAE Standard 241-2023 — Control of Infectious Aerosols (equivalent clean airflow concept)
  5. U.S. EPA — Guide to Air Cleaners in the Home
Educational content. Real-world results depend on the room, the pollutant source, ventilation, installation and maintenance. This guide does not replace project-specific engineering, occupational-health or medical advice. See our editorial policy for how guides are researched and corrected.