A simple Example for a small room.
measure the width, length and height of your room
multiply the three measurements together.
width 4m x length 5m x height 2m = 40 cubic metres.
If your room or house is new build and fitted with very good insulation then divide the room volume by 25 = 1.6 kW nominal required
If the room has average to good insulation then divide the volume by 15 = 2.66 kW (this is the normally given figure, as in the calculator).
If the insulation is poor, then divide the rooms volume by 10 = 4 kW
If you have two rooms with an arch in between, normally it is advised to treat as one room, but the arch will stop the heat from travelling - it will convect accross the ceiling, and quite a high proportion will be stopped by the top of the arch.
For open staircases, add 10-20%.
Some manufacturers give maximum outputs or an output range such as “3-11kw”.
*A respectable manufacturer of quality stoves will give an accurate figure of how much extra heat could be gained from stove; but these figures are never tested and are never proven, and should not be used when choosing a stove.
Moreover, you would not want to run your stove at full tilt, with all vents fully open, refuelling every 5 minutes, with small pieces of bone dry fast burning pine, to achieve the mythical maximum figure; any more than you would drive your car with the accellerator always pressed to the floor at maximum revs in first gear.
Sometimes nominal outputs are rounded down to 4 or 5 kW in order to avoid the air brick regulation, and these stoves will often be capable of 6 kW or slightly more.
Sometimes maximum outputs are just utter nonsense and wildly exaggerated - e.g. the Chinese cast iron stoves that you'll see on ebay with small fireboxes and fake CE plates. A stove's maximum output is pretty much proportional to the size of the combustion chamber - the more wood you can get in, the higher the kW.
However it is also important to note that manufacturers will restrict the size of the air inlets to gain higher efficiency against lower nominal output and a longer re-fuelling period, which is a requirement of the new CE test.
So sometimes you will get a large stove with a large firebox with an incongruously low kW rating. In this case the stove will have restricted air inlets which prohibits fast burning and associated low efficiency. These will have a longer re-fuelling period. - see intermittent burning vs continuous burning stoves.
Do bear in mind also that if you choose a stove with a kW rating more than 50% higher than the output given in the calculation, then the stove could be unbearably hot in that room.