If the fire is a state of matter (plasma) and every matter has mass. My question is: how we can calculate the mass of a fire?
Fire is a series of actions and changes that produces a result — a process. In fact, it is an oxidation process (called combustion or burning) that gives out heat and light energy as well as glowing gas and a small amount of plasma.
So, talking about fire's mass is like talking about the mass of digestion, or boiling, or getting a driver's license — although it certainly makes us think about what fire is. But a process doesn't have mass.
Flames are another matter. They are burning gases and gasses certainly do have mass.so your question should be the mass of flames instead of fire .
For a given pressure the ideal gas law says that the density of a gas is inversely proportional to temperature, in Kelvin. You can use this fact, the temperature and density of air (300°K 1.3 kg/m3), and the temperature of your average run-of-the-mill open flame (about 1300°K) to find the density of fire.
For most “everyday” fires, the density of the gas in the flame will be about 1/4 the density of air. So, since air (at sea level) weighs about 1.3 kg per cubic meter (1.3 grams per liter), fire weighs about 0.3 kg per cubic meter.Hope this helps.