Gallons per day: the derivation

Every sizing figure on this site traces back to one constant: 296 square feet of floor area per gallon per day, at an 8 ft ceiling, in a tightly constructed home. This page is where that number is derived, in full, so you can decide whether to trust it rather than taking our word.

Why we could not just cite the standard

Residential humidifier sizing in the United States is governed by AHRI Guideline F. It is the right document and we tried to read it. On 1 September 2026 the PDF on ahrinet.org returned HTTP 403 to our request, and we do not have a copy through any other route.

So we did not cite it as though we had. Paraphrasing a standard you have not read is how a wrong number gets laundered into a hundred other pages, and this category has plenty of examples. What follows is a derivation from primary data we could actually obtain, with the standard named as the thing it ultimately rests on.

The three pairs

Aprilaire is the only manufacturer in our fetched set that publishes both a capacity in gallons per day and a coverage in square feet for the same unit, and states that the pairing follows AHRI Guideline F for tightly constructed homes. Three of its whole-house models, read from shop.aprilaire.com on 1 September 2026:

Inverted, that is roughly 0.0034 gallons per day per square foot, or a gallon a day for every 296 square feet. Applied to a room:

What this number cannot tell you

Naming the limits of a method is the strongest signal that the rest of it is real, so here they are in plain terms. Four things this constant does not know:

1. How leaky your house is. Aprilaire's figures describe tightly constructed homes. A drafty 1920s house exchanges far more air with the outdoors and therefore loses far more moisture. Aprilaire does not publish a multiplier for average or loose construction and we have not invented one. For an older house, treat the output as a floor rather than a target.
2. How cold it is outside. Humidification load rises as outdoor air gets colder and drier, because the air coming in has less moisture in it to begin with. A single annual constant averages that away. On the coldest week of the year you will need more than it says.
3. What ceiling height the source figures assume. Aprilaire publishes square feet, not cubic feet, and does not state the ceiling height behind the number. We assume the near-universal 8 ft and scale from there. If their internal assumption were 9 ft, every figure on this site would be about 12% conservative.
4. Whether the air will actually reach the space. A portable unit in a bedroom with the door shut is humidifying that bedroom. The same unit is not humidifying the rest of the floor, regardless of what the arithmetic says about square footage. Size the space that air can circulate through, not the space on the floor plan.

Using it to check a manufacturer's claim

The most useful thing you can do with the constant is run it backwards. Take any humidifier's coverage claim, divide by 296, and you have the daily output that claim requires. Then look for that output on the product page. Here is what happens when you do it across our set:

Read that table carefully, because the honest interpretation is narrower than it first looks. The implied figures are computed from the run time each maker publishes, and that run time is always for the quietest, lowest setting - the flattering one. The coverage claims describe the loudest setting. So most of these gaps are not lies; they are two true statements about opposite ends of the same machine, with the number that would connect them left out.

That is still a problem, and it is the problem this site exists to fix. If a maker will not publish output at the setting its coverage claim describes, a buyer has no way to check the claim, and the practical result is that people buy for the number on the front of the box and get the number on the back.

Which is why three makers deserve credit

Vornado publishes up to a 4 gallon water output when operated continuously on high, and claims 1,000 square feet. Levoit publishes 550 mL per hour maximum warm mist for the LV600S, which is 3.49 gallons a day, against a 753 square foot claim. Aprilaire publishes gallons per day for its entire whole-house line. In all three cases the claim survives being checked, which is the entire test.

Publishing a checkable number is a small thing that almost nobody in this category does, and it is worth saying so out loud. When you are choosing between two units with similar prices and similar tanks, the one whose maker told you the output is a meaningfully safer purchase, because it is the one whose claim you were allowed to verify.