Room Volume and Air Changes
Airflow needed = room volume × air changes per hour. A room 4 × 3 m with a 2.4 m ceiling is 28.8 m³; at 6 ACH it needs 172.8 m³/h, which is about 102 CFM. Ventilation is based on volume, not floor area — so a tall room needs proportionally more air for the same result.
What "air changes per hour" means
ACH is simply how many times the entire volume of air in a room is replaced in an hour. At 6 ACH, the air in the room is swapped out six times — once every ten minutes.
The relationship is a single line, in both directions:
airflow (m³/h) = volume (m³) × ACH and ACH = airflow ÷ volume
It is not a clean sweep. "Six air changes" does not mean the old air is fully gone six times over. Incoming air mixes with what is already there, so some fresh air leaves before doing any work and some stale air lingers. ACH is a throughput measure, not a purity guarantee — which is why targets are set with a comfortable margin.
Working out the room volume
For a rectangular room it is just length × width × ceiling height. The part people get wrong is using floor area instead of volume.
| Ceiling height | Volume | Airflow at 6 ACH | In CFM |
|---|---|---|---|
| 2.4 m (standard) | 28.8 m³ | 172.8 m³/h | 102 |
| 2.7 m | 32.4 m³ | 194.4 m³/h | 114 |
| 3.0 m (period) | 36.0 m³ | 216.0 m³/h | 127 |
| 4.0 m (loft/warehouse) | 48.0 m³ | 288.0 m³/h | 170 |
Identical floor plans, but the 4 m room needs 67% more airflow than the standard one. Any rule of thumb quoted "per square metre" quietly assumes a ceiling height, and undersizes every tall room it meets.
For a sloped ceiling, split the space into a box up to the lowest ceiling point plus a triangular prism for the slope above, and add the two.
How many air changes does a room need?
| Space | ACH | Why |
|---|---|---|
| Bedroom | 3–5 | CO₂ and moisture from occupants |
| Living room | 3–6 | General occupancy |
| Bathroom | 6–8 | Clearing shower moisture |
| Kitchen | 8–12 | Cooking moisture, grease, odour |
| Utility / laundry | 6–10 | Drying moisture load |
| Home office | 4–6 | CO₂ affects concentration |
| Workshop / garage | 10–20 | Fumes, dust, solvents |
| Server / plant room | 15–30 | Heat removal |
These are typical design figures. Building regulations in your jurisdiction take precedence, and they often specify a fixed extract rate for a room type — for example a set litres-per-second figure for a bathroom — rather than an ACH.
Converting between m³/h and CFM
Fans are rated in whichever unit their market uses, so you will meet both.
| From | To | Multiply by |
|---|---|---|
| m³/h | CFM | 0.5886 |
| CFM | m³/h | 1.699 |
| m³/h | L/s | 0.2778 |
| L/s | m³/h | 3.6 |
| CFM | L/s | 0.4719 |
A rough field check: CFM ≈ m³/h ÷ 1.7. So a fan rated 250 m³/h is about 147 CFM, and a 100 CFM fan is about 170 m³/h.
Worked example: sizing a bathroom fan
A bathroom 2.5 × 2.0 m with a 2.4 m ceiling, targeting 8 ACH:
| Step | Working | Result |
|---|---|---|
| Volume | 2.5 × 2.0 × 2.4 | 12.0 m³ |
| Airflow at 8 ACH | 12.0 × 8 | 96 m³/h |
| In CFM | × 0.5886 | 56.5 CFM |
| In L/s | × 0.2778 | 26.7 L/s |
| +25% for duct losses | × 1.25 | 120 m³/h / 71 CFM |
So specify a fan rated at roughly 120 m³/h, not 96. That margin is not padding — see below.
Why the fan on the box is not the fan on the wall
A fan's rated airflow is measured on a test rig at zero static pressure — no duct, no bends, no grille. Every real installation is worse:
| Factor | Effect |
|---|---|
| Long duct run | Friction loss rises with length |
| Bends and elbows | Each 90° bend costs meaningful flow |
| Flexible ducting | Ribbed inner wall — much worse than rigid |
| Concertinaed flexible duct | Can halve the flow on its own |
| External grille / cowl | Adds resistance; louvres add more |
| Undersized duct | Steep penalty — never reduce below the fan's spigot |
Add 20–30% when specifying. Use rigid duct where you can, keep the run short and straight, and never crush or concertina flexible hose to fit — that is the most common reason a correctly-sized fan underperforms.
Air purifiers and CADR
Purifiers are rated by CADR (clean air delivery rate), and the same arithmetic applies — but note the difference: a purifier recirculates and filters rather than bringing in outdoor air. It removes particles; it does not reduce CO₂.
| Room | Volume | At 4 ACH | At 5 ACH |
|---|---|---|---|
| Small bedroom 3 × 3 × 2.4 | 21.6 m³ | 86 m³/h (51 CFM) | 108 m³/h (64 CFM) |
| Bedroom 4 × 3 × 2.4 | 28.8 m³ | 115 m³/h (68 CFM) | 144 m³/h (85 CFM) |
| Living room 5 × 4 × 2.4 | 48.0 m³ | 192 m³/h (113 CFM) | 240 m³/h (141 CFM) |
| Open plan 8 × 5 × 2.7 | 108.0 m³ | 432 m³/h (254 CFM) | 540 m³/h (318 CFM) |
4–5 ACH is the usual target for general air quality; allergy and wildfire-smoke guidance often calls for more. Manufacturers frequently quote a "room size" in square metres — which, again, silently assumes a ceiling height.
More is not automatically better
Every cubic metre extracted in winter is replaced by cold outdoor air that must be heated. Over-ventilating is a direct and continuous heating cost.
The aim is enough air movement to control moisture, CO₂ and pollutants — not the maximum you can install. That is why targets are given as ranges. Heat recovery ventilation (MVHR) changes the economics by reclaiming warmth from outgoing air, which is why airtight modern homes use it rather than simply extracting harder.
Common mistakes
| Mistake | Effect | Fix |
|---|---|---|
| Sizing from floor area | Tall rooms undersized | ACH is based on volume |
| Ignoring duct losses | Fan underperforms | Add 20–30% |
| Using concertinaed flexible duct | Can halve the flow | Rigid duct, pulled taut, short runs |
| Mixing CFM and m³/h | Out by a factor of 1.7 | Convert before comparing |
| Expecting a purifier to cut CO₂ | It cannot | CO₂ needs outdoor air |
| Over-ventilating in winter | Heat straight out the wall | Target the range, not the maximum |
| Forgetting the sloped part of a ceiling | Volume understated | Box + triangular prism |
Frequently asked questions
How do I calculate air changes per hour?
Divide airflow by room volume in consistent units. A fan moving 172.8 m³/h in a 28.8 m³ room gives exactly 6 ACH. Going the other way, multiply room volume by the ACH you want to find the airflow you need.
How many air changes per hour does a room need?
Depends on the room. Bedrooms and living rooms are comfortable at 3–6, bathrooms need 6–8 to clear moisture, kitchens 8–12 for cooking. Workshops with fumes or dust need 10–20. Higher is not automatically better — over-ventilating wastes heat.
How do I convert m³/h to CFM?
Multiply m³/h by 0.5886 for CFM, or CFM by 1.699 to go back. So 100 m³/h is 58.86 CFM, and 100 CFM is 169.9 m³/h. Extractor fans are rated in one unit or the other depending on where they were made.
Does ceiling height matter for ventilation?
Yes — air changes are based on volume, not floor area. A room with a 3 m ceiling has 25% more air to move than the same floor plan at 2.4 m, so it needs 25% more airflow for the same rate. Sizing from floor area alone systematically undersizes tall rooms.
How do I size an air purifier for a room?
Purifiers are rated by CADR. Multiply room volume by your target ACH — commonly 4–5 for general air quality, more for allergies. A 28.8 m³ room at 5 ACH needs a CADR of 144 m³/h, about 85 CFM. Note a purifier filters particles but does not reduce CO₂.
Why does my extractor fan seem too weak?
Rated airflow is measured with no ducting attached. Real installations lose a great deal to duct length, bends, flexible hose and grille resistance — sometimes half the rated figure. Add 20–30% when specifying, use rigid duct where possible, and keep runs short with few bends.
How do I calculate the volume of a room with a sloped ceiling?
Split it into a rectangular box up to the lowest ceiling point, plus a triangular prism for the sloping part above, then add them. For a simple pitched section, the triangular part is ½ × base × rise × length.
Is more ventilation always better?
No. Every cubic metre extracted in winter is replaced by cold outdoor air that must be heated, so over-ventilating is expensive. The aim is enough air movement to control moisture, CO₂ and pollutants without dumping heat — which is why targets are ranges rather than maximums.