What size humidifier do you need?

What the number means

A humidifier has to put back the water that leaves the house. In winter the outside air is dry, the house leaks, and the indoor air loses moisture; the humidity load is the amount of water vapor, in gallons per day, that has to be added to hold a target relative humidity. A humidifier's rated output is in the same unit, so the match is simple: the rating on the unit should be at least the load. This calculator gives the load for a whole house or a large open area from the floor area, the ceiling height and how tight the construction is. It uses two published charts, not a formula of its own, and shows both because they do not agree.

Where the numbers come from

The first chart is in Honeywell's TrueSTEAM Humidification Reference Guide. It is the humidity load from AHRI Guideline F, the industry guideline for humidifier capacity, for maintaining 35% relative humidity in a tight four-person home with 8-foot ceilings when it is 20°F and 70% relative humidity outdoors. Honeywell prints the loads for tight, average and loose homes from 500 to 3,000 square feet. The second chart is Aprilaire's sizing chart in its Humidifier Regional Applications sheet (Form 288, 2016), for 8-foot ceilings from 1,000 to 5,000 square feet. It also cites AHRI Guideline F but names no outdoor conditions or humidity target. Aprilaire's chart notes that a family of four adds about 2 gallons of humidity per day through breathing, cooking, bathing and washing. At 1,000 square feet the Aprilaire figure for a tight home is 2 times Honeywell's.

Tight, average and loose

Honeywell passes on AHRI's definitions. A tight home is well insulated with vapor retarders, has tight storm doors, weather-stripped windows and a dampered fireplace, and exchanges about half of its air each hour. An average home is insulated with vapor retarders but has loose storm doors and windows and a dampered fireplace, and exchanges its air about once an hour. A loose home is generally built before 1930 with little or no insulation, no storm doors, no insulated windows, no weather stripping, no vapor retarders and an undampered fireplace, with about one and a half air changes an hour. Choose the description that fits your house.

How the calculator finds the load

Honeywell's guide says that when the ceiling is higher than 8 feet the volume matters, and it restates its chart in cubic feet: 4,000 cubic feet is the 500 square feet of the 8-foot chart, and a 10-foot ceiling means 25% more air. So the calculator multiplies the floor area by the ceiling height and divides by 8 to get an equivalent floor area at 8 feet, then reads the charts. Between two printed sizes it interpolates in a straight line, and at a printed size it shows the printed number. Outside the charts it shows nothing: Honeywell's starts at 500 square feet and ends at 3,000, Aprilaire's runs from 1,000 to 5,000, and extending them would be guessing. Gallons are multiplied by 3.785 to get liters. If you enter the output of the humidifier you are considering, hours per day = load ÷ rated output × 24, which is how Honeywell's own run-time chart works: a 6.5 gallon-per-day unit on a 4.4 gallon load runs 16.2 hours a day, and a 9 gallon unit 11.7.

Worked example

A house of 1,800 square feet with 8-foot ceilings and average construction lies between the 1,500 and 2,000 square foot rows. Honeywell's average column reads 7.8 and 11.1 gallons a day there, which interpolates to 9.8. Aprilaire's reads 12.8 and 17, or 15.3. So the load is 9.8 to 15.3 gallons a day (37 to 58 liters). A unit rated 9 gallons a day could not keep up with either: Honeywell's load alone would take it 26.1 hours a day, more than a day has. For a tight house of 1,600 square feet with 10-foot ceilings the volume is 16,000 cubic feet, equal to 2,000 square feet at 8 feet. Honeywell's tight column gives 6.5 gallons and Aprilaire 8.5; with 8-foot ceilings, the same floor area would need 4.8 and 6.8. The 2,000 square foot average house takes 11.1 and 17.

What humidity to aim for

The EPA's mold guidance is to keep indoor humidity below 60 percent, and ideally between 30 and 50 percent relative humidity, and it notes that a humidity meter costs roughly $10 to $50 at a hardware store. The charts here use 35% (Honeywell's stated basis), which sits inside the EPA's range. Relative humidity also depends on the air temperature, so the same amount of vapor reads lower in a warm room: the dew point calculator converts between the two. These are guidance figures, not legal limits.

Gallons per day of humidity to add, as printed in the two guides.

Common mistakes and limits

The first mistake is sizing by the “covers up to X square feet” line on a box. That is each maker's own claim; the load in gallons per day, compared with the unit's rated gallons per day, is the comparison that stands up. Second, using these charts for a bedroom. They start at 500 square feet (Honeywell) or 1,000 (Aprilaire), and a small room has no published value here, so use the output rating of the unit and the coverage the maker prints. Third, forgetting what the charts assume. Honeywell lists geography, ventilation, the number of people, ceiling height, window type, insulation and the type of equipment as factors that move the load away from the chart, and its numbers are for one outdoor condition, 20°F and 70% relative humidity. Fourth, ignoring how the unit is rated. Aprilaire's evaporative capacities assume the blower runs all the time and the plenum is at 120°F with cold water, and for its Model 400 on a heat pump, which supplies cooler air than a gas furnace, it says evaporation is about 60% of the rated capacity. Finally, this is about adding moisture. A damp basement needs the opposite, which the dehumidifier size calculator handles.

What size humidifier do I need for my house?

Find the daily load from the floor area, the ceiling height and the construction. For 1,800 square feet with 8-foot ceilings and average construction, the two charts give 9.8 and 15.3 gallons a day. Pick a humidifier whose rated output in gallons per day is at least that, preferably the one the maker's chart assigns to your house size.

How many gallons per day for a 2,000 square foot house?

With 8-foot ceilings, Honeywell's chart reads 6.5, 11.1 and 16.4 gallons a day for a tight, average and loose house, and Aprilaire's reads 8.5, 17 and 25.5.

What is the difference between a tight and a loose house?

It is the rate at which outdoor air replaces indoor air. The AHRI definitions, as Honeywell prints them, are about half an air change per hour for a tight home, one for an average home and one and a half for a loose one, together with the insulation, windows, doors and fireplace described above. A leakier house loses more moisture, so its load is larger.

Do higher ceilings need a bigger humidifier?

Yes, in proportion to the air volume. Honeywell's guide says a 10-foot ceiling means 25% more air than an 8-foot one and restates the chart per cubic foot. The calculator does the same: a 1,600 square foot house with 10-foot ceilings is treated as 2,000 square feet.

What humidity level should I keep?

The EPA recommends keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent. Honeywell's chart is for holding 35 percent. A humidity meter, which the EPA puts at about $10 to $50, tells you what the house actually reads.

How long will my humidifier run each day?

Enter its rated gallons per day in the last field. The run time is the load divided by the rating, times 24 hours. In Honeywell's run-time chart, a 6.5 gallon-per-day unit on a 4.4 gallon load runs 16.2 hours, and a 9 gallon unit 11.7. If the rating is below the load the unit would run all day and still fall short.

Why do the two charts give different numbers?

Honeywell states its basis: 35% relative humidity, a tight four-person home with 8-foot ceilings, and 20°F and 70% relative humidity outside. Aprilaire's sheet names a family of four adding 2 gallons a day and 8-foot ceilings but not the outdoor conditions or the target humidity, and its numbers for a tight home are about 2 times larger at 1,000 square feet. Without the same conditions, the figures cannot be reconciled, so the calculator shows the spread and leaves the choice to you and the unit's maker.

Sources: Honeywell, “TrueSTEAM Humidification Reference Guide,” Sizing Best Practices (Charts 1 to 5; AHRI Guideline F-2007); Aprilaire (Research Products Corporation), “Humidifier Regional Applications,” Form No. 288 (11.16); U.S. Environmental Protection Agency, “A Brief Guide to Mold, Moisture and Your Home” (page updated February 18, 2026), all read October 27, 2026. Interpolation and the ceiling-height conversion are ours. Related: dehumidifier size calculator, ventilation rate calculator, CFM calculator and dew point calculator. How we check our calculators.