Sizing an air purifier: ACH, CADR and placement

Two numbers decide the outcome, and most rooms only get one of them right

Buyers usually arrive with a product they've seen elsewhere and a room they want it to work in, in that order. The better sequence runs the other way: measure the room, decide on a target air changes per hour (ACH), work out the clean-air delivery rate (CADR) that target requires, then pick the unit that meets it. Skip the first two steps and you end up with either an underpowered unit in a room too big for it, or an oversized one running loud and low most of the day because it was bought for a bigger room than the one it's actually in.

The formula, and what it looks like in three real rooms

Required CADR (m³/h) equals floor area (m²) multiplied by ceiling height (m) multiplied by target ACH. A small 15 m² consultation room with a 2.6 m ceiling, targeting 6 ACH for a room where masks come off, needs 15 × 2.6 × 6 = 234 m³/h. A mid-sized 35 m² meeting room or clinic waiting area at 2.7 m, targeting the US CDC's benchmark of 5 clean-air changes per hour for occupied spaces, needs 35 × 2.7 × 5 = 472.5 m³/h. A larger 70 m² lobby or open floor at 2.7 m and the same 5 ACH needs 70 × 2.7 × 5 = 945 m³/h — past the point where one unit is the right answer, and usually where two units, or a proper site visit, replaces a single-product guess. CADR and ACH: two different numbers that work together explains why the two figures aren't interchangeable even though they get used that way in casual conversation, and what is CADR covers where the rating comes from and how it's tested.

What ACH target to actually use

The US CDC treats 5 or more clean-air changes per hour as a useful benchmark for occupied spaces, but it isn't the only figure worth knowing. The table below shows how long it takes to clear 99% of an airborne contaminant at a given ACH, using the approximation minutes ≈ 276 ÷ ACH:

A home bedroom running at 2 ACH overnight is clearing air roughly six times slower than an office running at 6, and a procedure room pushed to 10 or 15 ACH is doing so because the time to clear matters more there than it does in a bedroom. Which end of that range applies to your room depends on use, not floor area alone — what ACH should you target for your room type sets out ranges for homes, offices, clinics and procedure rooms rather than one number for everything.

Matching the number to a unit, and placing it properly

Once you have a required CADR, the product range maps onto it fairly directly: the Fillo Plus for rooms to about 30 m² at CADR 300 m³/h, the Classic 400S for 31–54 m² at CADR 480, and the Pro 800S for 56–96 m² at CADR 800; the Reno 500S covers a 20–40 m² home room on plasma-ion technology with no filter changes, and the KJ100F is a desktop unit for a single workstation rather than a room figure at all. A unit rated for the right CADR can still underperform badly if it's pushed into a corner with its intake blocked by furniture — where to place an air purifier for the best airflow and placement mistakes that quietly reduce effectiveness cover clearance and the dead spots that cut delivered CADR well below the number on the spec sheet. It's also worth working out whether the room's existing ventilation already does part of the job before adding a standalone unit at all — see fresh-air ventilation vs air purifiers for what each actually addresses.

Buying against a measured number, not a guess

Units are quoted for purchase or as a serviced rental — filters, parts, preventive visits and two-way delivery included — once the room's ACH target and measured area are known, which is also why there is no flat price list against floor area alone: a room with a low ceiling and good existing ventilation needs a different spec from one without either. A one-month rental pilot is available for a specification to be tested in place before it's bought outright.

What we will not claim

A CADR figure describes clean-air delivery, not a reduction in infections or sick days, and we won't present it as one. Chamber testing shows the units eliminate over 99.99% of tested organisms — E. coli, Staphylococcus albus and Influenza A — within 60 minutes under chamber-test conditions, against roughly 4 hours 23 minutes of passive decay for a plain-HEPA comparison run, but a sealed chamber is not an occupied room with doors opening and people moving through it, and results there may differ. We also won't call a specification "compliant" or "MOH approved" on the strength of a CADR number alone; certification wording is limited to what's actually held: an FDA Class 2 medical device listing held by the manufacturer, H13 HEPA specification, CE, RoHS and ISO 9001/14001 management systems.

Getting the number measured rather than estimated

A floor plan gives an estimate; the Clinic Air Report gives a specification. It costs S$150 per site visit, covers a minimum of two rooms, and every room is measured on site rather than assumed from a drawing, with the report back by email within two working days. It carries a sized-right guarantee: if the specified unit doesn't reach the report's air-change figure once installed, we upgrade or add a unit at no extra cost. Sense-check your own numbers first with the sizing calculator, compare rental against buying, browse the full range on products, or read how the underlying technology works on technology. Contact us to book a visit.