How to size an air purifier for an HDB bedroom in Singapore

Key takeaways

• Required CADR = floor area (m²) × ceiling height (m) × target ACH; homes should target 3 to 4 ACH.
• Most HDB bedrooms don't need a hospital-spec unit; oversizing wastes cost and adds unnecessary noise and footprint.
• Most HDB flats share a broadly similar ceiling height, which keeps that part of the calculation simple.
• One correctly sized unit can sometimes cover a bedroom plus a small connected walk-in wardrobe or study nook.
• Sizing the whole flat means repeating the calculation room by room, not multiplying one bedroom's figure by the number of rooms.

Why an oversized unit wastes money in a small bedroom

It's tempting to buy the biggest unit in a range on the logic that more coverage can't hurt. In a small bedroom, it usually just means a larger footprint taking up floor or desk space, a higher noise floor at any given fan speed, and paying for coverage capacity you'll never use in that room.

A unit sized correctly for the room reaches its target air changes per hour comfortably without running flat out, which also tends to mean quieter operation overnight, since it isn't working near its maximum fan speed just to keep up. Sizing accurately isn't about spending less for its own sake; it's about matching the product to the actual job the room needs done, night after night.

Applying the CADR formula to a typical HDB bedroom size

The formula is straightforward: required CADR (m³/h) = floor area (m²) × ceiling height (m) × target ACH. Measure your room's floor area directly rather than assuming a figure, since HDB bedrooms vary noticeably between flat types and even between units of the same flat type built in different decades. Most HDB flats use a standard ceiling height, which simplifies the height variable in the calculation and leaves floor area as the main number you need to get right.

Once you have the floor area, the calculation itself takes seconds. Our free Air Purifier Size Calculator does this automatically and maps the result to a specific AIRE model, so you don't need to do the arithmetic by hand or guess between two similarly sized options.

Recommended ACH for a home bedroom

Homes generally target 3 to 4 air changes per hour, lower than the 4 to 5 ACH recommended for offices or the 5-plus ACH used for clinics and dental settings. A bedroom is a good candidate for the upper end of that home range, 4 ACH, since it's the room where you spend the most continuous hours breathing the same air overnight, often with the door closed and the aircon recirculating.

This is also why a unit built for a clinic waiting room, sized around 5-plus ACH targets, is usually more than a bedroom needs. It's not wrong to use one, but it's rarely the most efficient or the quietest choice for a home-sized space where overnight noise matters.

When one unit can cover the bedroom and a connected space

If your bedroom opens directly onto a small walk-in wardrobe or a study nook with no door separating the two, you can often treat the combined floor area as one zone and size a single unit for the total. This only works cleanly when air can circulate freely between the spaces; a closed door effectively creates two separate rooms for sizing purposes, each needing its own calculation.

AIRE's Reno 500S, built for home use with coverage of 20 to 40 m² and CADR of 210 m³/h, is sized well for exactly this kind of combined bedroom zone in most HDB layouts, and its whisper-quiet sleep mode suits overnight use in a room people are actually sleeping in.

Scaling up if you're sizing the whole flat

Sizing a whole flat isn't a matter of multiplying one bedroom's CADR requirement by the number of rooms; each room needs its own calculation based on its own floor area, since rooms in the same flat are rarely the same size or shape. A living and dining area combined into one open zone, for instance, usually needs a separate, larger calculation from any single bedroom, and a kitchen or bathroom typically isn't sized the same way as a bedroom at all.

Work through each enclosed zone individually using the same formula, then compare the results against the coverage ranges for AIRE's home and compact models on our Singapore buying guide to see which size fits which room before deciding what to buy for the whole flat.

Next step

Run your bedroom's dimensions through our free Air Purifier Size Calculator to get a specific CADR target and a matching AIRE model, rather than guessing from your flat type alone.

Frequently asked

Does an oversized unit actually help in a small HDB bedroom?

Not meaningfully. A unit sized well above what the room needs mostly adds unnecessary footprint, cost and potential noise at higher fan speeds without a proportional benefit in a small enclosed space. Sizing correctly using the CADR formula usually points to a compact or mid-range unit for a typical bedroom.

What ACH is recommended for a home bedroom?

Homes generally target 3 to 4 air changes per hour, and a bedroom is a reasonable candidate for the upper end of that range since you spend many continuous hours there overnight. This is lower than the 4 to 5 ACH used for offices or the 5-plus ACH recommended for clinics.

Can one unit cover a bedroom and an adjoining space?

Yes, if the bedroom and the adjoining space, such as a walk-in wardrobe or study nook, share open air with no door separating them. Treat the combined floor area as one zone in the CADR calculation; a closed door effectively splits it into two separate rooms for sizing purposes.

How do I size air purifiers for an entire HDB flat, not just one bedroom?

Apply the CADR formula to each enclosed zone separately, using that zone's own floor area rather than reusing one bedroom's figure. A combined living and dining area, for example, usually needs its own larger calculation distinct from any individual bedroom in the same flat.

Related reading

• The Best Air Purifier in Singapore: An Honest Market Guide