To calculate the humidity you require 'wet bulb' temperature readings as well as the (dry bulb) temp given on METARs so there is no way of calculating the % humidity, save calling the tower after the readings have been taken and asking for the figures. A table is then needed to calculate humidity.
NB It is important not to confuse 'wet bulb temp' with dew point, the dew point is the lowest point at which saturated air can be cooled without condensation to water. It is a theoretical calculation based on 100% Relative Humidity.