Yes there is a calculation, and under the new building regulations extraction for all rooms should be calculated. The problem we have is that all
cooker hoods are not for room extraction, only cooking fumes, moisture and such. The air movement for any room is given as a standard and are about 15 for a domestic kitchen, so if we then take the dimentions of the room say 15x15 with a ceiling height of 2.5 we have a room volume of 562.5 m3, and need 15 air changes per hour, this would require a fan capable of 8348m3 per hour performance or 146.6 m3 per minute. The performance of the fan will be printed in the litrature supplied with the fan. A typical 4" fan for instance is around 101m3 per hour. Sidewinder has just done the course that allows him to test the extraction systems in rooms, and I guess he would be able to tell you 90% of fitted extraction units are far below the required levels.