The short answer
An average 1,800 sq ft house with 8 ft ceilings needs a humidifier that puts out about 6.4 gallons of water a day in Minneapolis. Its hardest days are around 20°F, when the house can hold 40% without fogging windows; on the coldest days the safe humidity is lower, so it needs less. In Houston (37°F) it needs about 4.2 gallons a day. Air leaks matter more than size: a drafty house in Minneapolis needs 10.
The same house in every state
An 1,800 sq ft house, 8 ft ceilings, average air tightness, sized for each state's hardest winter day at the highest indoor humidity its windows can take.
| Alabama Huntsville | 3A | 23 | 40% | 6.1 |
|---|---|---|---|---|
| Alaska Anchorage | 7 | -4 | 40% | 6.4 |
| Arizona Phoenix | 2B | 43 | 40% | 3 |
| Arkansas Little Rock | 3A | 25 | 40% | 5.9 |
| California Los Angeles | 3B | 47 | 40% | 2 |
| Colorado Denver | 5B | 7 | 40% | 6.4 |
| Connecticut Bridgeport | 5A | 16 | 40% | 6.4 |
| Delaware Wilmington | 4A | 18 | 40% | 6.4 |
| District of Columbia Washington | 4A | 23 | 40% | 6.1 |
| Florida Jacksonville | 2A | 42 | 40% | 3.2 |
| Georgia Atlanta | 3A | 27 | 40% | 5.7 |
| Hawaii Honolulu | 1A | 66 | 40% | 0 |
| Idaho Boise | 5B | 16 | 40% | 6.4 |
| Illinois Chicago | 5A | 6 | 40% | 6.4 |
| Indiana Indianapolis | 4A | 9 | 40% | 6.4 |
| Iowa Des Moines | 5A | 0 | 40% | 6.4 |
| Kansas Wichita | 4A | 13 | 40% | 6.4 |
| Kentucky Louisville | 4A | 16 | 40% | 6.4 |
| Louisiana New Orleans | 2A | 40 | 40% | 3.6 |
| Maine Portland | 6A | 7 | 40% | 6.4 |
| Maryland Baltimore | 4A | 19 | 40% | 6.4 |
| Massachusetts Boston | 5A | 14 | 40% | 6.4 |
| Michigan Detroit | 5A | 9 | 40% | 6.4 |
| Minnesota Minneapolis | 6A | -6 | 40% | 6.4 |
| Mississippi Jackson | 3A | 29 | 40% | 5.4 |
| Missouri Kansas City | 4A | 9 | 40% | 6.4 |
| Montana Billings | 6B | -4 | 40% | 6.4 |
| Nebraska Omaha | 5A | 4 | 40% | 6.4 |
| Nevada Las Vegas | 3B | 37 | 40% | 4.2 |
| New Hampshire Manchester | 5A | 9 | 40% | 6.4 |
| New Jersey Newark | 4A | 17 | 40% | 6.4 |
| New Mexico Albuquerque | 4B | 23 | 40% | 6.1 |
| New York New York | 4A | 18 | 40% | 6.4 |
| North Carolina Charlotte | 3A | 27 | 40% | 5.7 |
| North Dakota Fargo | 6A | -13 | 40% | 6.4 |
| Ohio Columbus | 4A | 11 | 40% | 6.4 |
| Oklahoma Oklahoma City | 3A | 19 | 40% | 6.4 |
| Oregon Portland | 4C | 30 | 40% | 5.3 |
| Pennsylvania Philadelphia | 4A | 18 | 40% | 6.4 |
| Rhode Island Providence | 5A | 11 | 40% | 6.4 |
| South Carolina Charleston | 3A | 32 | 40% | 5 |
| South Dakota Sioux Falls | 6A | -4 | 40% | 6.4 |
| Tennessee Nashville | 3A | 21 | 40% | 6.3 |
| Texas Houston | 2A | 37 | 40% | 4.2 |
| Utah Salt Lake City | 5B | 20 | 40% | 6.4 |
| Vermont Burlington | 6A | 0 | 40% | 6.4 |
| Virginia Virginia Beach | 3A | 27 | 40% | 5.7 |
| Washington Seattle | 4C | 30 | 40% | 5.3 |
| West Virginia Charleston | 4A | 16 | 40% | 6.4 |
| Wisconsin Milwaukee | 5A | 4 | 40% | 6.4 |
| Wyoming Cheyenne | 5B | 3 | 40% | 6.4 |
How it works
The formula
A humidifier has to replace the water carried out by the air that leaks out of the house, because the cold outdoor air that replaces it is nearly dry. So the need is the air leaking in, times the difference between the water indoor and outdoor air hold.
water (lb an hour) = air × (indoor − outdoor humidity ratio)
gallons a day = water × 24 ÷ 8.34
Assumptions
- Indoors is 70°F. Air changes an hour: 0.7 drafty, 0.45 average, 0.25 tight, the same as our heating and cooling calculators.
- Outdoor air is at your winter design temperature and 70% humidity. Cold air holds so little water that its humidity barely changes the result.
- The indoor target is capped by outdoor temperature so windows don't fog or frost, from University of Minnesota laboratory studies (for 70°F indoors and double-pane windows). Triple-pane windows can take a little more.
- We check every outdoor temperature from your design temperature up to 40°F and size for the day that needs the most water. With the window limit that's usually a 10 to 20°F day, not the coldest one. If you type a fixed humidity, the coldest day needs the most.
- Water from people, cooking, showers and plants is ignored; it typically adds a gallon or two a day, so a household's real need is a little lower.
- Humidifier capacities: console units up to about 4 gallons a day, bypass whole-house 12 to 17, fan-powered about 18, steam up to about 35 (typical manufacturer ratings). Bypass and fan-powered units only work while the furnace blower runs, so we assume they deliver about half their rating on cold days; steam and console units deliver their full rating.
Worked example: 1,800 sq ft in Minneapolis
8 ft ceilings, average tightness. The hardest day is about 20°F outside, when the windows can take 40% indoors.
- Air: 1,800 × 8 × 0.45 = 6,480 cu ft an hour × 0.075 = 486 lb
- Water held: 43.4 grains per lb indoors, 11.2 outdoors (7,000 grains = 1 lb)
- 486 × (0.0062 − 0.0016) = 2.23 lb an hour × 24 ÷ 8.34 = 6.4 gallons a day
Too big or too small
- Too small: dry air on cold days. It runs nonstop and the house stays dry, with static shocks, dry skin and gaps in wood floors.
- Too high a setting: condensation. The bigger risk than the size. Water on windows means water inside the walls; turn it down as it gets colder.
- Seal before you size up. Sealing leaks cuts both the heating bill and the water a humidifier has to add.
- Steam for big needs, tight or heat pump homes. Bypass humidifiers only work while the furnace blows warm air, so their real output is below the rating; steam units work any time.
Questions people ask
What size humidifier do I need for my house?
It depends on how much outdoor air leaks in and how cold it is. An average 1,800 sq ft house with 8 ft ceilings needs about 6.4 gallons a day in Minneapolis: the hardest day is around 20°F outside, when the house can hold 40% without fogging the windows. That's a bypass or fan-powered whole-house humidifier. A drafty house there needs 10; a tight one 3.6.
What humidity should I keep my house at in winter?
Low enough that windows don't fog or frost. At 70°F indoors, University of Minnesota studies suggest at most 40% when it's 20 to 40°F outside, 35% at 10 to 20°F, 30% at 0 to 10°F, 25% at −10 to 0°F, 20% at −20 to −10°F and 15% below that. More than that and water collects on windows and inside walls.
Why does my house get so dry in winter?
Cold outdoor air holds very little water. When it leaks in and warms to 70°F, its relative humidity drops to 10% or less. The more air leaks in, the drier the house, and the more water a humidifier has to add.
Is a whole-house humidifier better than a portable one?
For a whole house in a cold climate, usually. A portable console unit puts out a few gallons a day and has to be filled by hand; a whole-house unit on the furnace runs on a water line. In Houston, where winter design is 37°F, the example house needs only 4.2 gallons a day, so a console or two can do.
Can a humidifier be too big?
A humidifier with a humidistat only adds water until it reaches the setting, so a bigger one isn't harmful by itself. The risk is the setting: too high a humidity in cold weather causes condensation, mold and rot. Set it by the outdoor temperature, or use a model with an outdoor sensor.