How Many Air Purifiers Does Your Office Need?

One purifier per enclosed zone — not one per office

The number of air purifiers your office needs is set by your floor plan, not your headcount. Clean air stops at a closed door, so every enclosed space counts as its own zone: the open floor, each meeting room, reception, and any pantry or quiet room people actually spend time in.

This is the step most offices get wrong. A single large machine parked in the open area does nothing for the meeting room next door — usually the smallest, most crowded and worst-ventilated space in the office, where eight people share air for an hour at a time.

So the honest answer to "how many?" is: count your enclosed, occupied zones, then size one unit for each. The maths below tells you which unit each zone needs.

The sizing maths: CADR, room volume and air changes

Every serious purifier publishes a CADR — clean air delivery rate — in cubic metres of clean air per hour. Divide that by your room's air volume and you get its air changes per hour (ACH): how many times the unit cleans the room's entire volume of air each hour.

Required CADR (m³/h) = floor area (m²) × ceiling height (m) × target ACH

For offices, we recommend targeting 4–5 ACH. Public-health guidance since the pandemic (including the US CDC) recommends targeting at least 5 air changes per hour of clean air in occupied spaces — so treat 4 as your floor and 5 as the standard for densely occupied rooms.

Most Singapore offices sit under a false ceiling of around 2.6 m, which keeps the arithmetic simple. If you would rather not do it by hand, our air purifier size calculator runs the same working for you, and our CADR explainer covers what the rating does and does not measure.

Three worked examples

Here is the method applied to three common Singapore office layouts, assuming a 2.6 m ceiling and a 4–5 ACH target.

Two things are worth noticing. First, one Pro 800S cannot carry a 100 m² floor on its own — 800 m³/h into 260 m³ of air is barely 3 ACH, so a second unit is the correct fix, not a bigger fan speed.

Second, the meeting rooms land well above 5 ACH, and that is deliberate. Occupant density spikes when six people fill a 15 m² room with the door shut, so the extra headroom is exactly where you want it. In shared rooms like these, active sterilisation also earns its keep: our UV-C plus copper-silver ion stage eliminated more than 99.99% of tested airborne pathogens within 60 minutes in chamber testing, rather than leaving them sitting on a passive filter.

Where to put each unit

Placement decides whether your calculated ACH actually reaches people's lungs. A few practitioner rules:

• Put units in the breathing zone — near desk clusters and seating, not hidden in a corner behind a cabinet where the intake starves.
• Leave clearance around the intake and outlet so air can circulate freely through the unit.
• On open floors with two units, place them diagonally at opposite ends so their airflow patterns cover the whole floor rather than overlapping.
• Keep units away from constantly open doors, where they end up cleaning corridor air instead of your office's.
• Meeting rooms: run the unit continuously with the door closed. The Fillo Plus stays under 53 dB, so it will not fight the conference call.

The Pro 800S sits on castors, which matters more than it sounds: you can roll it to the townhall corner on all-hands day and back to the desk rows afterwards.

Three mistakes that waste money

1. Buying one oversized unit for the whole floor. Doors and partitions defeat it. Several right-sized units in the right zones always beat one hero machine in the wrong place.
2. Sizing by floor area alone. The formula uses volume. An industrial-style office with a 3.5 m exposed slab holds about a third more air than the same footprint under a 2.6 m ceiling — recalculate with your real height.
3. Sizing for top speed, then running on whisper mode. A unit only delivers its headline CADR at higher fan speeds, so check the noise range before you buy. The Classic 400S runs 30–55 dB across 8 speeds on a DC motor, which is why it can hold a useful clean-air rate at volumes an office will actually tolerate.

Rent, buy — or pilot the whole layout first

Multi-unit deployments are exactly where flexibility matters. Buying makes sense for a stable long-term office; rental avoids the upfront outlay and lets you add or return units as headcount and floor plans change. We offer both, and our rental vs buying guide walks through the trade-offs.

Better still, you do not have to decide on paper. Our free 30-day pilot lets you trial the full layout — say, one Pro 800S plus two Fillo Plus — in your own office at zero upfront cost, watch the built-in air-quality sensors, then buy, rent or return. WhatsApp us at +65 8082 3699 and tell us your floor plan; we will do the sizing maths with you.

Frequently asked

Can one large air purifier cover my whole office?

Only if your office is genuinely one open room within the unit's rated coverage. Closed doors and partitions stop airflow, so each enclosed meeting room, reception or private office needs its own correctly sized unit. Count zones first, then size each one by CADR.

What if my office ceiling is higher than 2.6 m?

Use your actual ceiling height in the formula, because it works on air volume, not floor area. A 100 m² office under a 3.5 m exposed slab holds 350 m³ of air instead of 260 m³, so it needs proportionally more CADR to hit the same air changes per hour.

Should office air purifiers run all day?

Yes — continuously through occupied hours, because people constantly re-introduce particles and airborne microbes as they work, talk and move. Timers and WiFi scheduling on units like the Classic 400S and Fillo Plus make this automatic, and switching off overnight when the office is empty is perfectly fine.

Is it cheaper to rent or buy air purifiers for an office?

It depends on your horizon. Buying usually wins for a stable office you will occupy for years, while rental avoids capital outlay and stays flexible when headcount or floor plans may change. We offer both routes, so you can start on rental and switch to purchase later if the layout settles.

Related reading

• Best Office Air Purifiers in Singapore: A Sizing Guide
• Can Air Purifiers Reduce Sick Days at the Office?
• What Is CADR? Air Purifier Ratings Explained