🌫️What size humidifier does this space need?

✓ Tested & verified Updated: Aug 9, 2026 How we calculate this

Humidifier size calculator

Floor area (sq ft)

2 gallons/day

Chart figure for 500 sq ft.

Floor area: 500 sq ft

In a cold, dry climate take the top of the band: the drier the outside air, the more of it leaks in and the harder the machine works to hold the same number.

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What size humidifier does a room need?

Match output in gallons per day to the floor area. Up to 300 sq ft needs about 1 gallon a day, up to 500 sq ft 2 gallons, up to 1,000 sq ft 3 gallons, up to 1,500 sq ft 4 gallons, and a whole house of 2,500 sq ft or more needs 6 to 9 — which is where a portable stops making sense and a unit plumbed into the ducts starts.

Output and tank size are different numbers and only one of them is on the box. A 2-gallon output with a 1-gallon tank is a machine you refill twice a day; the same output with a 4-gallon tank is one you refill every other day. Manufacturers print the tank because it is larger, so read the gallons per day figure — it is the one the room feels.

The target is 30 to 50% relative humidity. Below 30% you get static, cracked lips and split woodwork; above 50% you get condensation on the coldest window in the house and, eventually, mould behind it. A humidifier without a humidistat will happily run past both.

Output is what the room feels; tank size is what your evenings feel like. Both are below.

Floor area	Output needed	Typical machine	Tank you want
up to 300 sq ft	1 gal/day	tabletop	1 gal — daily refill
up to 500 sq ft	2 gal/day	tabletop or small console	1.5–2 gal
up to 1,000 sq ft	3 gal/day	console	3 gal+
up to 1,500 sq ft	4 gal/day	large console	4 gal, or plumbed in
up to 2,500 sq ft	6 gal/day	whole-house bypass	plumbed in
over 2,500 sq ft	9 gal/day	whole-house steam	plumbed in

The last two rows are the honest limit of a portable. A machine that evaporates six gallons a day has to be fed six gallons a day, and carrying that up the stairs in jugs stops being charming in the first week of January.

Frequently asked questions

What size humidifier do I need for a 1,000 square foot house?

About 3 gallons a day. That is a large console portable or a small bypass unit on the furnace — a tabletop model rated at 1 gallon will run continuously and still not lift the humidity, because it cannot put moisture into the air faster than the house loses it in cold weather.

Is a bigger humidifier always better?

No. An oversized humidifier without a humidistat overshoots into condensation: water beads on the coldest surface in the house, which is usually the inside of a window or an outside-wall corner, and that is where mould starts. Size to the space and let a humidistat hold 30 to 50%.

What is the difference between output and tank size?

Output is gallons per day — how much water the machine can put into the air. Tank size is how much it holds before you refill it. A 3-gallon-a-day machine with a 1.5-gallon tank needs filling twice a day. Size on output; then check the tank so the refilling is bearable.

Do I need a whole-house humidifier or will a portable do?

Up to about 1,500 sq ft a large portable keeps up if the doors stay open. Beyond that, or if you want one machine instead of three, a bypass or steam unit on the furnace is the practical answer — it uses the ducts you already have and there is no tank to fill.

What humidity should I aim for in winter?

30 to 40% when it is very cold outside, 40 to 50% the rest of the time. The colder the windows, the lower you have to sit to avoid condensation on them: at −20 °C outside, even 35% will fog a single-glazed window. If the glass runs wet, drop the setting rather than wiping it.

Where should the humidifier stand?

Central, off the floor if possible, and not against a wall or in a corner — it needs air moving past it. Keep it away from wooden furniture and books, which take up moisture unevenly, and away from the thermostat, which does not measure humidity but does react to the cool plume from an evaporative unit.

How we calculate this

Output is taken from the standard sizing bands published by humidifier manufacturers and reflected in the AHAM sizing guidance: roughly 1 gallon a day up to 300 sq ft, 2 up to 500, 3 up to 1,000, 4 up to 1,500, 6 up to 2,500 and 9 above that. Because it is a table lookup rather than a rate per square foot, the tool reports the band value directly rather than interpolating — the bands exist because output is sold in steps, not in a continuum. The 30–50% relative humidity target is the range published by the EPA and used in ASHRAE guidance on indoor humidity; the lower end applies in very cold weather, when the limiting factor is condensation on cold glass rather than comfort.

Worked example

With Floor area (sq ft) 500, this page works out 2 gallons/day. Chart figure for 500 sq ft.

Floor area500 sq ft

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