How we size and pick air purifiers

Why the number on the box is usually wrong

A purifier's real job is to clean the air faster than dirty air leaks back in. How big a room it can handle depends on how many times an hour it can pass the room's whole volume through its filter: the air changes per hour.

Makers are free to pick the rate. Most pick a low one, because a lower rate makes a bigger room. Across the purifiers we list, the box claims work out to between 0.8 and 4.7 air changes an hour. At that rate a room is technically “covered”, but smoke and pollen keep coming in nearly as fast as the purifier removes them.

Why 4.8 air changes an hour

4.8 is the rate AHAM, the US appliance association that runs the independent CADR test programme, uses for the suggested room size on every purifier it verifies: room size in sq ft = smoke CADR in CFM × 1.55, for an 8 ft ceiling. AHAM chose it so a purifier can cut smoke particles in the room by about 80%, assuming the room's air is also replaced naturally about once an hour.

We use the same rate, in metric and with your ceiling height, so a figure on this site matches what AHAM would suggest. Its shorthand for shoppers, a smoke CADR of at least two thirds of the floor area in sq ft, gives almost the same answer.

We use the smoke CADR because smoke particles are the smallest and hardest to catch of the three AHAM tests (smoke, dust and pollen). A purifier that clears smoke clears the others too.

A worked example

Winix 5510 has a smoke CADR of 253 CFM. In metric that's 253 × 1.699 = 430 m³/h.

430 m³/h ÷ (4.8 × 2.5 m) = 35.8 m² = 385.6 sq ft, shown as 385 sq ft35 m² (35 m²385 sq ft)

The box says “Verified Room Size 392 sq.ft.”. To clean 392 sq ft36 m² it would only change the air 4.7 times an hour.

The box claims Verified Room Size 392 sq.ft., about the 385 sq ft35 m² it really cleans. A rare honest box.

With a 2.7 m ceiling the same purifier cleans 357 sq ft33 m². Higher ceilings mean more air per square metre, so each extra 10 cm above 2.5 m means about 4% less floor area. The calculator does this for your room.

Where each figure comes from

We don't test purifiers ourselves: no lab, no smoke chamber, no review units. Every figure on the site comes from a published source, and each one shows that source and the date we last checked it. Two kinds of source matter, and every figure tells you which one it comes from:

Certified: AHAM Verifide

An independent lab measures the smoke CADR for AHAM’s Verifide programme, and AHAM certifies and publishes the result. The only certified figures on the site. We use it for the smoke CADR, the figure every room size here is worked out from. 17 of the 26 purifiers we list have one.

Maker's claim

Product pages, manuals and spec sheets from the maker: the room size on the box, the filter and how often to change it, power, size and weight, and whether there's an ionizer or a sensor. Nobody checks these independently, so we label them as the maker's. AHAM doesn't list every model (Coway, Xiaomi, and Rabbit Air have models it doesn't), and for those the CADR is the maker's too: marked “Not verified”, shown with the maths and never a pick.

Other published sources

Used only where neither AHAM nor the maker states a fact. Not certified.

How we check. For each figure we open the source, copy the number as published and record the date. The site does the conversions (CFM to m³/h, inches to cm, sq ft to m²) so nothing is retyped by hand. When a maker changes a spec or you tell us a figure is wrong, we fix it, update the date and say what changed in our next video.

What we look at for every purifier:

Smoke CADR

From the AHAM Verifide directory wherever the model is listed. If it isn't, the maker's own figure, clearly marked “Not verified”. Unverified models are never picks.

Filter and replacement interval

The exact replacement filter and how often the maker says to change it. A cheap purifier with a filter you change every four months can cost more to keep than a dearer one with a yearly filter.

Ionizer

Whether it has one, and whether you can switch it off. Ionizers can produce ozone, so we flag any that are always on.

Power, size and weight

From the maker's spec sheet, so you can judge running cost and whether it fits where you want it.

How room picks are chosen

Each room page works the same way, and you can check every step against the figures on the page:

1. Keep only current models whose verified smoke CADR reaches 4.8 air changes in that room with a 2.5 m ceiling.
2. Our pick: the smallest model with 25% more air than the room needs, so it needn't run flat out; if none has that much to spare, the smallest that covers the room. The card always says why.
3. Just enough: the smallest model that still covers the room. Usually the most compact.
4. Most headroom: the most spare air from a model no more than twice the size of the room, for faster cleaning or a room with an open door.
5. Up to three are shown; if one model wins two roles, the next role moves up. The rest that qualify are listed below them, up to twice the size of the room; bigger ones are counted, with a link to the full list.
6. If no single model covers a space (or only one does), we show how to split it: the fewest units and the one with the most spare air.
7. Models sold as big enough on their own but which aren't are listed separately, so you can see the trap.

Using the numbers in a real room

Room and model pages apply these to their own figures. The reasoning is the same everywhere, so it lives here.

High ceilings

The room sizes on this site assume a 2.5 m ceiling. A purifier cleans a volume of air, not a floor, so a higher ceiling shrinks the floor area it can cover: about 4% less for every 10 cm above 2.5 m, and a sixth less at 3 m. Each room page names the picks that still cover it at 2.7 m and 3 m; the calculator does any height.

Open-plan spaces

Air doesn't stop at an imaginary line. If your living room opens onto a kitchen or hall with no door between them, add the areas together and size for the total. With a door that stays shut, size for the room alone. With the door open, the purifier also has to clean the air that comes through it, so pick one with air to spare.

When one purifier isn't enough

The formula assumes the air in the room mixes well. In a long, L-shaped or divided space, the air near one purifier gets cleaned over and over while the far end waits. There, two smaller units at opposite ends do better than one big one, and each can run on a slower setting. Two units on the same setting sound about 3 dB louder than one. Closed doors split a home into separate rooms: one purifier can't clean the room next door.

Where to put it

Put it where you spend the time: near the bed in a bedroom, near the sofa or desk in a living room. Give the air inlet and outlet the clearance the maker's manual asks for, so not pushed into a corner, behind a sofa or under a desk. Keep windows shut while it runs if you're clearing smoke or pollen, or outdoor air replaces what it cleans.

Noise

We don't list noise figures. AHAM doesn't certify noise, and makers don't all measure it the same way, so their decibel figures don't compare well. What you can compare is spare air: a purifier with more air than your room needs reaches 4.8 air changes on a slower setting, while one that only just covers the room has to run near its top speed, usually its loudest. Room pages show how much air each pick has to spare.

Filters

Every CADR is for a new filter: AHAM measures it with one, and airflow falls as a filter fills. Change it when the maker says. On our list the intervals run from 3 to 24 months. We don't show prices, which change too often, so compare how often you'd buy a filter: every 3 months is 4 a year, every 12 months is one.

Limits

• 4.8 air changes an hour is a sizing reference, not a medical standard. For wildfire smoke, allergies or illness, more is better.
• The figures are only as good as their sources. AHAM's are certified; the makers' are not, and we label them so you can weigh them.
• The maths assumes the air in the room mixes well. In a long or L-shaped space, or with the door shut between two rooms, use one purifier per space.

What we never do

• No hands-on tests. We don't test purifiers ourselves, so nothing here depends on a sample a brand sent us. The CADR figures are AHAM's certified ones; the rest is the makers' word, labelled as such.
• No prices. They change daily and Amazon only allows prices pulled live from its own systems. Check the price on Amazon.
• No paid placements. Brands can't pay to be listed, ranked or picked. How the site makes money
• No cookies, analytics or tracking. Privacy

Sources

The rule and its reasoning come from AHAM: AHAM Verifide directory of air cleaners and its white paper on portable air cleaners and air changes per hour (PDF).

Every figure for every purifier links to one of these:

Found a mistake? Tell us. We fix it, update the “checked on” date and say what changed in the next video.