Sizing
Humidifier sizing chart by square footage
One table, in the only unit that can be checked. Find your floor area down the side, your ceiling height across the top, and read off the daily output the room needs.
By Scooter M. · Published · Updated
Every figure in this table is (floor area x ceiling height / 8) / 296, rounded to two decimal places. The 296 is derived on gallons per day from Aprilaire's own published capacity-and-coverage pairs. Nothing here is copied from another chart, which matters, because most of the sizing charts on the internet are copies of each other and none of them show a source.
| Floor area | 7 ft | 8 ft | 9 ft | 10 ft | 12 ft |
|---|---|---|---|---|---|
| 150 sq ft | 0.44 | 0.51 | 0.57 | 0.63 | 0.76 |
| 200 sq ft | 0.59 | 0.68 | 0.76 | 0.84 | 1.01 |
| 250 sq ft | 0.74 | 0.84 | 0.95 | 1.06 | 1.27 |
| 300 sq ft | 0.89 | 1.01 | 1.14 | 1.27 | 1.52 |
| 400 sq ft | 1.18 | 1.35 | 1.52 | 1.69 | 2.03 |
| 500 sq ft | 1.48 | 1.69 | 1.90 | 2.11 | 2.53 |
| 600 sq ft | 1.77 | 2.03 | 2.28 | 2.53 | 3.04 |
| 800 sq ft | 2.36 | 2.70 | 3.04 | 3.38 | 4.05 |
| 1,000 sq ft | 2.96 | 3.38 | 3.80 | 4.22 | 5.07 |
| 1,200 sq ft | 3.55 | 4.05 | 4.56 | 5.07 | 6.08 |
| 1,500 sq ft | 4.43 | 5.07 | 5.70 | 6.33 | 7.60 |
| 2,000 sq ft | 5.91 | 6.76 | 7.60 | 8.45 | 10.14 |
| 2,500 sq ft | 7.39 | 8.45 | 9.50 | 10.56 | 12.67 |
| 3,000 sq ft | 8.87 | 10.14 | 11.40 | 12.67 | 15.20 |
What the bands mean in practice
A number on its own is not advice. This second table translates the first into what you would actually buy, and which of our pages covers it.
| Gal/day | Typical space | What that means for the machine |
|---|---|---|
| Under 0.7 | A small bedroom, nursery or study | Almost any 2 to 4 liter portable, on a low setting. Refill every day or two. |
| 0.7 to 1.5 | A standard bedroom or a large one with high ceilings | A 4 to 6 liter portable. This is the band most 'bedroom humidifier' units genuinely serve. |
| 1.5 to 3.0 | A living room, or a bedroom plus landing | A large-tank portable or an evaporative unit. Expect daily refilling. |
| 3.0 to 6.0 | An open-plan floor | An evaporative unit with a published output, or two portables. See best for large rooms. |
| Over 6.0 | A whole house | A ducted whole-house unit, or a console. See whole house vs room. |
Three things to correct before you use it
The chart gives you a starting figure for a tightly constructed home in average winter conditions. Three adjustments are worth making before you treat it as final.
- Only count the space the air can reach. A humidifier in a bedroom with the door closed is humidifying that bedroom. If you are sizing for an open-plan floor, count the whole floor; if you are sizing for a room you sleep in with the door shut, count the room.
- Add for a drafty house. The rate is derived from figures for tightly constructed homes. Older housing exchanges much more air with outdoors and therefore loses much more moisture. No published multiplier exists that we could verify, so treat the chart as a floor rather than an answer.
- Subtract nothing for a humid climate in winter. It is tempting to assume a mild region needs less. In heating season the relevant number is not the outdoor humidity, it is how much you are heating the air, and heating cold air is what drives indoor relative humidity down regardless of where you live.
How to check a machine against this chart
Find your number, then look at any humidifier you are considering and ask a single question: does the manufacturer publish a daily output at least that large? Not a tank size, not a square-footage claim, an output. If it does, you can size it. If it does not, you can compute an implied figure by dividing the tank capacity by the published run time and multiplying by 24 - and you should assume that figure describes the lowest setting, because that is the setting run times are always quoted for.
The units in our set that pass this test cleanly are listed on best for large rooms and best filter-free humidifiers, with the arithmetic shown per unit. If your figure came out under a gallon a day, best for bedrooms is the more useful page.
Questions people actually ask
What size humidifier for 1,000 square feet?
About 3.4 gallons a day at an 8 ft ceiling, 3.8 at 9 ft and 4.2 at 10 ft. That is beyond what a typical 6-liter bedroom unit can move; you are looking at an evaporative unit with a published output, a console, or more than one machine. Full page: sizing for 1,000 sq ft.
What size humidifier for 800 square feet?
About 2.7 gallons a day at an 8 ft ceiling and 3.0 at 9 ft. Most large-tank portables sold for this size move well under that figure at the setting whose run time they publish, so check the output rather than the tank.
Is a humidifier sizing chart in liters any use?
Only for planning refills. A chart in liters is describing tank capacity, which is a reservoir, not a rate. Two units with identical 6-liter tanks can have daily outputs that differ by a factor of five depending on the setting you run them at.
Should I size up or down if I fall between rows?
Up, mildly, and only if the unit has a humidistat that will genuinely switch it off. A slightly larger machine runs at a lower setting, which is quieter and refills less often. A grossly larger one pushes the room past 50 percent relative humidity, which the EPA warns against and which produces condensation on cold windows.
Sources
Every figure on this page came from one of the documents below, read on the date shown. We do not cite other review sites, forum posts or video reviews as sources of fact.
- Aprilaire Model 400 - manufacturer spec page (retrieved September 1, 2026)
- Aprilaire Model 500 - manufacturer spec page (retrieved September 1, 2026)
- Aprilaire Model 700 - manufacturer spec page (retrieved September 1, 2026)
- AHRI Guideline F - Selection, Installation and Servicing of Residential Humidifiers (the residential sizing standard; ahrinet.org returned HTTP 403 to this build, so we cite it as the governing document without paraphrasing text we did not read)
- US Environmental Protection Agency - Use and Care of Home Humidifiers (relative humidity ceiling, three-day cleaning interval, low-mineral water, microorganism warning)
Read next
We earn a commission if you buy through the links on this page, at no extra cost to you. It does not change what we recommend or the order the picks appear in. Full disclosure. We show a live price when we have one and the words “Check price” when we do not. We never print a number we cannot currently verify.
Gallons per day: the derivation
The full derivation of the sizing rate, from three published manufacturer capacity-and-coverage pairs, including a plain statement of its limits.
Ceiling height, and the other corrections
Why a 10 ft ceiling needs 25% more water than an 8 ft one, with the multiplier table and the three other corrections worth making.
Best for large rooms
Best humidifiers for large rooms
Large-room picks, chosen on whether the published output reaches the requirement rather than on what the box claims.
Top pick: Vornado Evap40 · 4.0 gal/day
Found a number that is wrong? Tell us and we will fix it and say so. How we pick shows every formula used on this page.