Humidity describes the proportion of water vapor in the air, liquid water (e.g. rain, dew) does not count. The relative humidity indicates the proportion of the highest possible saturation. 100% means that no more water vapor in the air can be absorbed. The absolute humidity indicates the mass of water vapor per cubic meter of air. The higher the temperature, the more water vapor the air can hold.
This calculator uses an approximate formula1 that is accurate to within 0.1% at temperatures between -30°C and 35°C and normal atmospheric pressure. The more the temperature deviates from this range, the more inaccurate the result becomes.
This result applies to the theoretical case that the room is hermetically sealed and the water completely dissolves in the air. In practice, more water than the calculated amount will be necessary. On the other hand, most rooms are not empty, which reduces the space for air and correspondingly reduces the amount of water required. An example of a sealed space where humidity should be controlled is a terrarium.
Surprisingly, moist air is slightly lighter than dry air. This is counter-intuitive at first, but for a simple reason. The density of a gas depends not only on pressure and temperature, but also on the mass of the gas molecules. Dry air consists mainly of nitrogen (N2), oxygen (O2) and argon (Ar). These have molecular weights of 28, 32, and 40, respectively. Water (H2O) has a molecular weight of 18, which is significantly less than the previous three. This lowers the density, see the air density calculator.1 Mander P (2012): How to convert relative humidity to absolute humidity