Humidifier Sizing Calculator

Written byOleh Soloninupdated 25 September 2026

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 matters
In 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.

How to use

Enter the floor area and the ceiling height.
Enter the room temperature and the humidity your hygrometer shows.
Set the humidity you want — the Russian standard GOST 30494 puts the winter optimum for a living room at 30 to 45 percent.
The air change rate defaults to 0.5 an hour. A window left ajar or a strong extractor raises it, and the water use rises with it.

Output by room size

The one-off fill is a twelfth of a day's use: 210 grams against 2.5 litres in a 20 m² room. Everything else the humidifier spends on replacing the moisture the ventilation carries away.

So compare the output in g/h from the humidifier's specification with this table, not only the room size printed on the box.

A 2.7 m ceiling, 22 °C, raising the humidity from 25 to 45 percent, 0.5 air changes an hour. Every row comes from the calculator on this page.
Floor area, m²	Volume, m³	Output, g/h	Water a day, l	One-off fill, g
10	27	52	1.26	105
15	40.5	79	1.89	157
20	54	105	2.51	210
30	81	157	3.77	314
40	108	210	5.03	419

What moves the water use most

The air change rate scales the load directly: double it and the output doubles. At two air changes an hour a 20 m² room needs 419 g/h and 10 litres a day.

Temperature acts more gently: warm air holds more moisture, so at 24 °C the room needs 40 percent more than at 18. Every five points of target humidity add 26 g/h.

A 20 m² room with a 2.7 m ceiling; whatever a row does not change is as in the first table. Every row comes from the calculator on this page.
What changes	Value	Output, g/h	Water a day, l
Air changes an hour	0.3	63	1.51
Air changes an hour	0.5	105	2.51
Air changes an hour	1	210	5.03
Air changes an hour	2	419	10.06
Temperature	18 °C	83	1.99
Temperature	24 °C	117	2.82
Target humidity	40%	79	1.89
Target humidity	50%	131	3.14

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?

The Russian standard GOST 30494-2011 sets 30 to 45 percent as the winter optimum for a living room and 60 percent as the upper limit. Below thirty the air dries out skin and airways. Condensation forms on any surface colder than the dew point — which can happen below 60 percent too — and the risk of it and of mould rises with humidity.

What humidifier does a 20 m² room need?

About 105 g/h to take it from 25 to 45 percent at 22 °C with 0.5 air changes an hour. It will get through 2.5 litres a day, so a smaller tank means refilling during the day.

Why does the humidifier never keep up?

Check the air change rate. An open window or a strong extractor multiplies it, and the load rises in step: at two air changes an hour a 20 m² room needs 419 g/h, not 105.

How much water does a humidifier use a day?

As much as the ventilation takes away: 1.26 litres for a 10 m² room, 2.51 for 20 m² and 5 litres for 40 m², going from 25 to 45 percent at 22 °C. The one-off fill is a twelfth of that.

Why does a warm room need more water?

Because warm air can hold more moisture. The same forty-five percent at 24 °C is more grams per cubic metre than at 18: a 20 m² room needs 117 g/h instead of 83.

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