What CADR Do I Need? Start With the Room

The method, in order#

1. 1Measure the room. Floor area is length times width, in feet. One room at a time, the minimums are published per room, not per home.
2. 2Find the published minimum, rounding up. The rows are 100 sq ft to 65 cfm, 200 to 130, 300 to 195, 400 to 260, 500 to 325 and 600 to 390. A room between two rows takes the larger row: a 150 square foot bedroom takes the 200 square foot row, 130 cfm. A room below 100 square feet takes the smallest row anyway, 65 cfm.
3. 3Check the tobacco-smoke CADR. EPA advises a high tobacco-smoke CADR for small particles, because smoke represents the smallest particles tested. Dust and pollen figures flatter weak units; the smoke figure tells the truth.
4. 4Account for the ceiling. The published rows assume an eight-foot ceiling. If yours is well above that, EPA's advice is a higher CADR, no conversion is published, so the defensible move is the next row up.
5. 5Place it properly. Clearance on all sides, intake unobstructed, near the source or the seat, door closed. Placement is free delivery; a starved intake wastes the CADR you paid for.

Worked examples#

A 120 square foot bedroom sits between the 100 and 200 rows, so it takes the larger: at least 130 cfm. A 340 square foot living room takes the 400 row: at least 260 cfm. A 650 square foot open-plan space runs past the end of the table, 390 cfm, the largest published minimum, is a floor rather than an answer, and no published figure exists above it.

Notice that none of these examples interpolated. The table's steps are coarse on purpose: CADR is a minimum, so rounding up is always safe and rounding down is always wrong.

What the answer does not cover#

The published minimum is for particles. There is no published CADR for gases, odours or VOCs, so the method cannot size a unit against a smell, the honest answer to "what CADR for odours" is that no such number exists.

Nor does MERV answer it. MERV rates HVAC filter capture, not portable-unit room size, and no published table converts between them.

If two units both meet the minimum#

Then you are choosing on everything the table does not measure: the speed at which each meets the minimum, the noise at that speed, the filter cost the manufacturer sets, and the placement the room allows. The sizing question ends once the minimum is met; the shopping question is what meets it most quietly.

A unit that exceeds the minimum is not wasteful. It reaches the same delivery at a lower fan speed, which is the one good reason to buy above the row. compare HEPA air purifiers on Amazon with your row's figure written down first.