Air purifiers for a 4-room HDB flat: how many does our model suggest?
Updated · By the purifierplan.com team
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A typical 4-room flat needs about six small purifiers to bring every room to our standard target. But three, used well, get you most of the way for about S$300 less.
- Every room: 6 purifiers, S$1,054 to buy, about S$1,340 over two years. Estimated haze breathed at home falls by about 90%.
- 3 purifiers, one carried between the living room (day) and main bedroom (night): S$757 to buy, about 88% less.
- 1 purifier, carried: S$499, about 50% less.
All figures are estimates from our planner for the layout below, with windows shut. Your flat, your hours at home and today's prices will change them.
The flat we modelled
This is our typical 4-room layout of about 90 m², not a specific HDB design. We assume a 2.6 m ceiling and the hours a household that's out at work or school on weekdays spends in each room.
| Room | Size | Air | Hours a day | Door |
|---|---|---|---|---|
| Living / dining | 4.4 × 5.6 m | 64 m³ | 6 | Open plan |
| Kitchen | 2.4 × 3.6 m | 22 m³ | 2 | Open plan |
| Foyer | 3.0 × 2.4 m | 19 m³ | 2 | Open plan |
| Corridor | 5.6 × 1.1 m | 16 m³ | 2 | Open plan |
| Main bedroom | 3.0 × 4.3 m | 34 m³ | 7 | Shut at night |
| Bedroom 2 | 2.7 × 3.2 m | 22 m³ | 7 | Shut at night |
| Bedroom 3 | 2.7 × 3.2 m | 22 m³ | 7 | Shut at night |
The living room, kitchen, foyer and corridor are open to each other, so the model treats them as one space of 121 m³. A purifier there cleans all four. The bedrooms are separate whenever their doors are shut.
Covering every room
Target: 5 air changes an hour, with each purifier sized to manage it at medium speed (see how sizing works). Picks are ranked by total cost over two years, with extra weight on quietness in bedrooms (the planner's defaults), among models whose figures we've checked.
| Space | Rated output needed | Our pick | Noise there (our estimate) | Buy | Over 2 years |
|---|---|---|---|---|---|
| Living, dining, kitchen, foyer, corridor | 962 m³/h | 3 × Xiaomi Smart Air Purifier 4 Lite Brand's figure | about 56 dB | S$597 | S$755 |
| Main bedroom | 240 m³/h | Xiaomi Smart Air Purifier 4 Lite Brand's figure | about 44 dB | S$199 | S$246 |
| Bedroom 2 | 160 m³/h | Xiaomi Smart Air Purifier 4 Compact Brand's figure | about 45 dB | S$129 | S$169 |
| Bedroom 3 | 160 m³/h | Xiaomi Smart Air Purifier 4 Compact Brand's figure | about 45 dB | S$129 | S$169 |
| Total, 6 purifiers | S$1,054 | S$1,340 |
Prices are the listed prices we recorded on 10 October 2026; the linked listing may differ, so check it before you buy. "Over 2 years" adds replacement filters and electricity at 31.16¢/kWh (SP Group, Q4 2026, with GST). Noise is our estimate at the speed each room needs, not a measurement; it's higher in the living area because three units run together. "Brand's figure" means the output figure is the brand's own, not independently verified.
The living-area figure needs 10% extra because air mixes less evenly across connected spaces. That's an assumption in our model.
What one, two or three purifiers achieve
If six is more than you want to buy, the question becomes where a smaller number does the most good. Our planner places them to maximise how much less haze people breathe at home, weighted by the hours spent in each room.
| Purifiers | Where | Buy | Less haze breathed at home |
|---|---|---|---|
| None | — | S$0 | 0% |
| 1, carried | Levoit Core 600S Tested in the living area by day, main bedroom at night | S$499 | about 50% |
| 2, one carried | As above, plus a Xiaomi 4 Compact in one bedroom | S$628 | about 69% |
| 3, one carried | As above, plus a Xiaomi 4 Compact in each of the other two bedrooms | S$757 | about 88% |
| 6, every room | The full plan above | S$1,054 | about 90% |
Why carrying works: people are in the living room in the day and the bedroom at night, rarely both at once. One larger purifier that follows them covers most of the hours spent at home. It only works if someone actually moves it every evening.
Why the jump from 3 to 6 is small: the carried purifier already cleans the living area while people are in it and the main bedroom at night. The full plan mostly adds clean air to rooms at times when nobody is in them, which barely changes what people breathe.
"Less haze breathed" is our estimate of the drop in fine particles you'd breathe while at home, compared with no purifier and windows shut. It is an estimate of exposure, not a health outcome. How we calculate it.
What changes the answer
- Doors left open at night: bedrooms can share the living-area purifiers. In some layouts that costs less; in this one it made almost no difference (S$1,346 against S$1,340 over two years).
- Windows: with windows often open, far more outdoor air comes in, and the same purifiers achieve much less.
- Someone home all day: the living area matters more, so a budget plan shifts towards it.
- A higher target: 7 air changes an hour needs bigger units or more of them.
- Your actual layout: 4-room flats vary by block and year. Measure your rooms, or start from this layout in the planner and adjust it.
Try it with your own flat. Pick "HDB 4-room", adjust the rooms, and choose how many purifiers to buy. The map shows where the clean air reaches.
Plan my homeSources and method
- All results from the purifierplan planner, run on 15 October 2026 with its default settings (target 5 air changes an hour, sized for medium speed, bedroom doors shut, windows shut, two-year cost). Method and ranking: How we rank.
- Electricity: SP Group regulated household tariff for 1 October to 31 December 2026, 28.59¢/kWh before GST (31.16¢ with 9% GST). SP Group tariff information.
- Product specifications from brand and retailer listings, checked 10 October 2026. "Tested" means a smoke CADR verified under the AHAM programme.
Questions people ask
How many air purifiers does a 4-room HDB flat need?
In our typical layout, covering every room to 5 air changes an hour at medium speed takes 6 units: 3 in the open living, dining and kitchen area and one in each bedroom. Three well-placed purifiers, one carried between the living room and the main bedroom, get close to the same result.
Can one air purifier clean a whole 4-room flat?
Not to the same level in every room. Bedroom doors are usually shut at night, so a purifier in the living room doesn't reach them. Carrying one purifier into the bedroom at night cut the estimated haze breathed at home by about half in our example.
How much does it cost to run air purifiers in a 4-room flat?
In our example, six purifiers cost S$1,054 to buy and about S$1,340 over two years, including replacement filters and electricity at the Q4 2026 SP Group tariff. Electricity was a small part of that; filters were the main running cost.