Size the machine to the room

Work out what the room needs, then buy the machine

Required clean air delivery rate equals floor area times ceiling height times air changes an hour, divided by sixty. That one line decides almost every purifier question, and it is the reason a box that claims 1,500 square feet can be either adequate or hopeless in the same room. There are 223 sizing pages and 5 calculators here built on it.

Start with the room

If you know the square footage, the answer is already worked out.

All 30 room sizes

Start with the question

If you do not have a number yet, or you have one and do not know what to do with it.

Or start from what is in the air

A machine most rooms in this site are sized against

Genuine Winix Filter Q, 1712-0123-00 with Activated Carbon for Odor Control For Winix 5510, 5520 and 5530

A large room purifier with a published specification, which is what makes the arithmetic above applicable to it rather than to a coverage claim with no air change rate behind it.

$79.99Check pricePrices change often

Check the airflow figure against your own room rather than the coverage area printed on the box.

The filter is usually the larger half of the price

A machine bought once carries a filter bought repeatedly. Across the machines here with a matching replacement filter, filters account for as much as 62 percent of the five year cost at a twelve month replacement interval. The cheapest machine on the shelf is frequently the dearest one in the house by its second year. Cost any machine over five years, or read what each machine here costs to keep running.

Sizing by the room you actually have

A basement at a seven foot ceiling with a moisture problem and an open plan kitchen at nine feet with the largest particle source in the house need different arithmetic even at the same floor area. Each room type here carries its own ceiling assumption, its own air change target, and the caveat that matters for that space.

All 14 room types

Complete setups

Three builds at rising budgets. They are not one list at three prices: each adds a capability the one below it does not have, and each says plainly what to skip.

What this site will not do

It will not tell you that a machine will help your asthma or your allergies, because nobody can know that from a specification sheet. Filtration reduces the airborne concentration of particles at a rate that can be calculated, and that is the whole of what is claimed here.

It will not invent a number either. Every product on this site comes from a real listing with a real price and a real image, 351 of them, and a listing that does not return all three is left out rather than patched. Where a machine publishes a coverage area with no air change rate behind it, the pages say so instead of converting the claim into an airflow it never stated. And where no single machine can serve a room at the rate asked for, the calculator refuses and recommends two units rather than selling the largest thing on the page.

Purifiers with a published specification

Common questions

What does this site do?

It works out what a room actually needs before recommending anything. There are 5 calculators and 223 generated sizing pages covering room sizes, room types, MERV ratings, dehumidifier capacity and humidifier output, all built on one line of arithmetic: floor area times ceiling height times air changes an hour, divided by sixty.

What size air purifier do I need?

Multiply the floor area by the ceiling height, multiply by the air changes an hour you want, and divide by sixty. A 300 square foot room with an eight foot ceiling needs about 180 CFM for four and a half air changes an hour, which is the figure usually given for allergy households, and 80 CFM for general air quality at two.

Why does a coverage claim in square feet not tell me enough?

Because the number depends entirely on the air change rate the manufacturer chose, and most listings never state it. The same machine is honestly described as covering a large room at one change an hour and a much smaller one at five. Where a listing states both, the airflow follows by arithmetic and machines can be compared properly.

What is the two thirds rule?

A shopper convention published by AHAM: a smoke CADR of at least two thirds of the floor area. It is not separate advice, it is the standard sizing arithmetic with an eight foot ceiling and five air changes an hour already filled in, since eight times five divided by sixty is 0.667. It undersizes rooms with high ceilings.

Do air purifiers help with allergies or asthma?

What can be said honestly is that filtration reduces the airborne concentration of particles such as pollen and pet dander, at a rate you can calculate from the machine and the room. What happens to any individual person is a medical question and this site does not answer it. Talk to a doctor about symptoms.

Are ozone generators and ionizers safe?

Ozone generators are sold as air purifiers and produce ozone deliberately, which is harmful in occupied rooms. Some ionizing and plasma type devices produce it as a by-product. California CARB regulates ozone emissions from indoor air cleaning devices and certifies devices sold there. On a machine with a real filter, the filtration is doing the work and the ionizing mode can be left off.