The low efficiency of traditional water extinguishing systems is due to the formation of large droplets. Large droplets have a small external surface area, which causes poor heat removal from the ignition source. A large external surface area of droplets is provided by fire extinguishing systems with thinly sprayed water. They are effective in extinguishing fires at the initial stage of a fire in confined spaces and ensure the formation of droplets less than 150 microns. The analysis showed that water-gas nozzles with co-directional flows of water and gas are suitable for creating finely dispersed water with a droplet size of less than 100 microns. To ensure significant heat removal from the ignition source, the average diameter of the droplets of finely dispersed water should be less than 80 microns. A technique for determining the size of droplets when sprayed with water-gas nozzles is presented. According to the method, the calculation of the average diameter of droplets was carried out depending on the ratio of volume flow rates of gas and water. It is shown that fire extinguishing with finely sprayed water with the help of water-gas nozzles is effective at a gas pressure in front of the annular gap from 0,4 MPa to 0,6 MPa and the ratio of volumetric air flow to volumetric water flow is more than 2 000.