Humidifier Size Calculator
Find what size humidifier you need, in gallons per day, for one room or the whole house, from its size, how drafty it is and your winter.
A humidifier has to replace the moisture that leaks out with the air, so the size depends on how big the space is, how drafty it is, and how cold and dry the outside air gets. This runs the engineers' sizing method and names the gallons per day to look for on the box.
What are you humidifying?
A whole-house unit mounts on the furnace and only adds moisture while the blower runs; a portable sits in one room and runs on its own.
The space
The heated floor area for a whole house (a typical home is 1,500–2,500 sq ft), or the one room for a portable (a bedroom is 150–300, an open living area 500–1,000).
Ceiling height
An average across the space. Most homes are 8; newer builds and great rooms run 9–10.
How tight is the house?
Tight means newer or sealed: weatherstripped windows, storm doors, a dampered fireplace. Loose is a drafty older house with little insulation. Air leaking in is what carries the moisture out, so this input matters more than any other.
What does a cold winter month look like?
Look for a whole-house unit rated at least
17 gallons a day
Your 2,000 sq ft house loses about 11.8 gallons of moisture a day at 20°F
Why the target is 35 percent, and why your windows set the ceiling
The sizing assumes you hold 35 percent indoors, which is where dry-air symptoms stop; more than that and the moisture finds the coldest surface. At 20°F outside, the engineering tables put the limit before windows sweat at 43 percent with double-pane glass and 21 percent with single-pane, so single-pane windows will fog and drip before you reach the target and hidden condensation inside the walls starts sooner still. Set the humidistat by the windows, not the number: when the bottom inch of the glass beads up, back off five points and let it clear.
Load is the engineers' residential method: volume × air changes (tight 0.5, average 1, loose 1.5 an hour) × the moisture gap between 70°F at 35 percent and your outdoor card, less the family's own moisture. Ducted ratings assume a 70 percent blower run. Whole-house units run 12, 17, 18 and 21 gallons a day, steam to 34.6; portables 1, 2.4 and 4. 2026 figures.
💡About this calculator▼
A humidifier is sized by the moisture your house loses, not by the square footage on the box. Cold outdoor air holds almost no water, and every hour a drafty house swaps its air with the outside, carrying the humidity out with it. So the same 2,000 square feet needs a small bypass unit if it's tight and a steam unit if it's a drafty old farmhouse in a zero-degree winter.
This runs the engineers' method. Pick whole house or one room, enter the square feet and ceiling height, say how tight the house is and what a cold month looks like, and the calculator works out the gallons of moisture to replace each day, the rated output to look for, and which class of unit covers it.
The number on a furnace-mounted unit is bigger than the moisture you lose. A ducted humidifier only adds moisture while the blower runs, and the ratings assume it runs about 70 percent of the time, so a house losing 12 gallons a day needs a unit rated near 17. Portables run on their own, so their output is compared straight to the loss.
Start with the air that leaks in. The engineering handbooks size residential humidifiers on infiltration: a tight house swaps about half its air with the outside every hour, an average house once an hour, and a loose pre-1930s house one and a half times. The calculator multiplies your square feet by the ceiling height for the volume, then by that air-change rate and the weight of air, to get the pounds of outdoor air coming in each hour.
Work out how much water that air is missing. Air at 70°F and 35 percent humidity carries about 0.0056 pounds of water per pound of air; outdoor air at 20°F and 70 percent humidity carries about 0.0015, and at 0°F almost nothing. The difference, times the pounds of air per hour, is the water the humidifier must add. A family of four adds about 0.7 pounds an hour from breathing, cooking and showers, which the whole-house calculation subtracts.
Convert to gallons and pick the class. Pounds an hour times 24, divided by 8.34 pounds per gallon, is the gallons a day. For a furnace-mounted unit the rated output needed is that divided by 0.70, because the blower runs about 70 percent of the time in cold weather; the calculator then names the smallest common size that covers it, 12, 17, 18 or 21 gallons a day, or a steam unit above that. For a room it compares the loss straight to portable outputs of 1, 2.4 and 4 gallons a day.
Worked example. A 2,000 sq ft average house with 8 ft ceilings in a 20°F winter: 16,000 cubic feet × 0.074 × 1 air change × 0.00406 = 4.8 pounds of water an hour, less 0.7 from the family = 4.1; × 24 ÷ 8.34 = 11.8 gallons a day; ÷ 0.70 = 16.9, so a 17-gallon-a-day large bypass unit.
📐How it's calculated▼
Moisture loss (lb/h) = 0.074 × volume (cu ft) × air changes per hour × (indoor moisture − outdoor moisture) − family moisture. Air changes: tight 0.5, average 1, loose 1.5. Indoor moisture at 70°F and 35% is 0.0056 lb per lb of air; outdoor at 70% humidity is 0.00245 at 30°F, 0.00154 at 20°F and 0.00056 at 0°F. Family moisture is 0.7 lb/h for the whole house and 0 for a room.
Gallons a day = lb/h × 24 ÷ 8.34.
Rated output needed (whole house) = gallons a day ÷ 0.70. A room compares the loss directly.
Default: 2,000 × 8 = 16,000 cu ft; 0.074 × 16,000 × 1 × (0.0056 − 0.00154) = 4.81 lb/h; − 0.7 = 4.11; × 24 ÷ 8.34 = 11.8 gallons a day; ÷ 0.70 = 16.9, covered by a 17-gallon-a-day unit.
Tight 3,000 sq ft at 20°F: 24,000 × 0.074 × 0.5 × 0.00406 = 3.6 lb/h; − 0.7 = 2.9; = 8.4 gallons a day; ÷ 0.70 = 12, the manufacturers' own coverage for their 12-gallon unit.
300 sq ft bedroom, average, 20°F: 2,400 × 0.074 × 1 × 0.00406 = 0.72 lb/h = 2.1 gallons a day, so a 2.4-gallon-a-day console.
Safe indoor humidity comes from the handbook table for no window condensation: at 30°F outside, 50% with double-pane windows and 29% single; at 20°F, 43% and 21%; at 0°F, 30% and 10%.
📎Sources:ASHRAE Handbook — HVAC Systems and Equipment, Chapter 22: Humidifiers (load equation, infiltration by construction, window condensation limits),AHRI Guideline F (SI/I-P)-2015 — Selection, Installation and Servicing of Residential Humidifiers,AprilAire — Model 700 whole-house humidifier (18 gallons per day; AHRI Guideline F rating basis),Vornado — Evap40 evaporative humidifier (4 gallons per 24 hours, up to 1,000 sq ft)
🔍Finding your inputs▼
What are you humidifying: Whole house means a unit mounted on the furnace or air handler that treats every room the ducts reach; one room means a portable you plug in. The math is the same, but the whole-house version adds the family's own moisture and the furnace blower factor.
Square feet: For a whole house, the heated floor area the ducts serve; a typical home is 1,500 to 2,500. For a room, just that room: a bedroom is 150 to 300, an open kitchen and living area 500 to 1,000. Don't count an unheated basement or garage.
Ceiling height: An average across the space. Most homes are 8 feet; newer builds and great rooms run 9 to 10. Volume is what leaks, so a 10-foot ceiling needs a quarter more than an 8.
How tight is the house: The input that matters most, because a loose house needs three times the moisture of a tight one. Tight is newer construction or an older house that's been sealed, with weatherstripped windows, storm doors and a fireplace damper. Loose is a drafty pre-1930s house with little insulation. If you're unsure, pick average; if the windows rattle and the curtains move on a windy day, pick loose.
What a cold winter month looks like: The outdoor temperature the humidifier has to keep up with on an ordinary cold day, not the record low. Around 30°F is the mid-South and coastal Northwest; around 20°F is the Midwest, Northeast and mountain West, and the industry's standard rating point; around 0°F is the upper Midwest, northern New England and the northern Plains.
⚠️Special situations▼
The manufacturer says its 12-gallon unit covers 3,600 square feet, but you say 3,000. Who is right?
Both, at different assumptions. The sizing method reproduces the maker's own older spec sheet almost exactly: 12 gallons a day for a 3,000 sq ft tight house at 20°F. The current marketing coverage runs about 20 percent more generous. In a tight house in a mild climate the bigger claim will hold; in an average house in a real winter it won't, and an undersized humidifier is the most common reason a whole-house unit "doesn't work." Size to the method and treat the marketing number as the best case.
We have a heat pump and the blower runs a lot but the air is cool. Does a bypass unit still work?
Poorly. Bypass and fan-powered units evaporate water off a pad using the heat in the supply air, and a heat pump's supply air at 90 to 100°F evaporates far less than a furnace's at 130. Some makers don't rate bypass units for heat pumps at all. A fan-powered unit does better and a steam unit doesn't care, because it boils its own water and can run whether or not the blower is heating. If you have a heat pump or a furnace that rarely runs, take the steam route even if the gallons say bypass.
Our old farmhouse comes out at 30 gallons a day. That seems like a lot of water.
It is, and it's the house talking. A loose pre-1930s house swaps its air one and a half times an hour, three times what a sealed house does, and at 0°F every one of those air changes brings in almost bone-dry air. You can buy a steam unit that size, but the cheaper fix comes first: weatherstripping, storm windows, a fireplace damper and sealing the attic hatch will move the house toward average and cut the need by a third, and the same work cuts the heating bill.
The bedroom is 300 square feet but the calculator says 2.4 gallons a day. The 1-gallon unit says it covers 300.
The 1-gallon claim is for a tight room in a typical winter with the door shut; the calculator's 2.4 is for an average room. Portable coverage claims vary a lot between makers and most assume the best case. If the room is newer and sealed, the small unit will hold it; if it's an older house or the door stays open, the console will, and it will need refilling once a day instead of twice. When in doubt, buy the output, not the tank.
❓Common questions▼
What size humidifier do I need for my house?
Size by the moisture the house loses, which depends on its volume, how drafty it is and how cold the outside air is, not on square footage alone. A typical 2,000 sq ft average house in a 20°F winter loses about 12 gallons of moisture a day and needs a furnace-mounted unit rated around 17 gallons a day, because the blower only runs part of the time. A tight house of the same size needs the 12-gallon unit; a drafty old one needs a steam unit rated over 20.
What size humidifier do I need for a bedroom?
A 150 to 300 sq ft bedroom in a typical winter loses between half a gallon and two gallons of moisture a day depending on how tight the room is, so a 1-gallon-a-day tabletop unit covers a newer sealed bedroom and a 2.4-gallon console covers an older one. Buy by the output in gallons per 24 hours, not the tank size, and keep the door closed; a 1-gallon tank on a 1-gallon-a-day unit means a refill every day.
Why is the rated output higher than the moisture I lose?
A furnace-mounted humidifier only adds moisture while the blower is moving air past it, and the industry rates coverage assuming the blower runs about 70 percent of the time in cold weather. So a house that loses 12 gallons a day needs a unit rated near 17 to actually deliver 12. Steam units and portables run on their own schedule, so their output compares straight to the loss.
What humidity should I keep in winter?
The sizing assumes 35 percent, which is enough to stop dry skin, static and cracking wood. The ceiling is set by your windows: the handbook table for no condensation allows 50 percent with double-pane glass when it's 30°F outside, 43 percent at 20°F and 30 percent at 0°F, and roughly half those with single-pane windows. When the bottom of the glass beads up, moisture is also condensing inside the walls where you can't see it, so back the humidistat off five points.
Bypass, fan-powered or steam: which one do I need?
Bypass units are the cheapest and quietest and cover most tight and average homes up to about 17 gallons a day, but they need a furnace that runs and a bypass duct. Fan-powered units add their own fan, reach 18 to 21 gallons a day and work where there is no room for a bypass. Steam units boil their own water, run whether or not the furnace is heating, and go to 34.6 gallons a day; they are the answer for loose houses, cold or arid climates, heat pumps, and any load above about 20 gallons a day.
Should I buy a portable humidifier by tank size or by output?
By output, the gallons it puts into the air in 24 hours; that is the number this calculator gives you. A big tank on a small wick still can't keep up, and a small tank on a strong unit means refilling twice a day. Evaporative units are self-limiting and the safest to leave running; ultrasonic units mist whatever is in the water, so hard water leaves white dust on furniture. Run it continuously on low with the door closed rather than blasting it for a few hours.
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