Humidifier Sizing Calculator

Room area, m²

Ceiling height, m

Room temperature, °C

Humidity now, %

Humidity wanted, %

Air changes an hour

Result

Output, g/h105
Water a day, l2.51
Room volume, m³54
To reach the target once, g210
Tank for a day, l2.5
What mattersIn winter the incoming air is drier than this assumes, so the real draw runs higher: the colder it is outside, the less moisture the air brings in.

The room is easy; the ventilation is the work

Calculates the humidifier output a room needs in grams an hour and the water it will get through in a day.

Raising the humidity of the air in a room once is a small job: a twenty-square-metre room holds about fifty cubic metres of air, and moving it from twenty-five to forty-five percent takes a couple of hundred grams of water.

Keeping it there is the real load. Dry air keeps arriving through the vents and the windows, and every air change carries the moisture straight back out — which is why a humidifier runs all day rather than for twenty minutes.

What this calculator shows

The output needed in grams an hour
The water used in a day and the tank that covers it
The room volume and the one-off amount to reach the target

What to keep in mind

Saturated vapour density is calculated from the room temperature by the Magnus formula, so warm rooms need more water for the same relative humidity.
Incoming air is treated as having the same absolute humidity as the room started with. In hard winter frost the outside air is far drier and the real load is higher.

FAQs

What indoor humidity is right?

Forty to sixty percent. Below thirty the air dries out skin and airways; above sixty condensation and mould start to appear on cold surfaces.

Why does the humidifier never keep up?

Usually ventilation. An open window or a strong extractor can carry away more moisture an hour than a domestic humidifier produces.

Why does a warm room need more water?

Because warm air can hold more moisture. The same forty-five percent at twenty-four degrees is more grams per cubic metre than at eighteen.

How large a tank is needed?

At least a day's use, or you will refill it in the middle of the night. The daily figure here is what to compare tank sizes against.

Worked example

A twenty-metre room in winter

Input: 20 m², 2.7 m ceiling, 22 °C, from 25 to 45 percent, half an air change an hour

Output: about 105 g/h, 2.5 litres a day

Note: The one-off fill is only 210 grams. Everything after that is replacing what the ventilation carries away, which is why the tank empties overnight.

Useful links

All tools in Home & Utilities

Humidifier Sizing Calculator

Result

Related tools

Dew Point Calculator

Ventilation Calculator

Air Conditioner Sizing Calculator

Useful links