This online calculator answers question "how to choose humidifier", by calculating humidification requirements given initial and target relative humidity and building parameters.
The calculations were made possible after creation of Relative humidity to absolute humidity and vise versa calculators.
Now, to use the calculator, it is important to understand the logic behind calculating humidification loads.
For home humidifiers manufacturer usually specifies volume of air processed by humidifier per hour. Like, 180 cubic meters per hour.
Well, this is quite useless. The true specification of humidifier you should look for is the speed of humidification, i.e. how much water it can add to the air per hour. If you will be determine enough you will find it. At least you can measure it by yourself, if you already have humidifier.
Why is it per hour? Because your apartments isn't in vacuum, and you have intake and exhaust air. Otherwise you'd die without the oxygen, wouldn't you? The air flow is characterized by air changes per hour, ACH. For apartments this value is usually quite low, between 0.1 and 0.5. This calculator uses 0.5, which means that air completely renews for 2 hours. This is quite good value, actually.
So, because of air constantly renews, humidifier should constantly add the amount the water enough to close the gap between initial relative humidity and target relative humidity.
And here go the calculations.
Calculator below implements abovementioned calculations.