The Stovesonline kilowatt calculator will give you a rough idea of the maximum heat output in kW needed for any room. If the room has stairs leading off it, is badly insulated, or has many windows/doors then you will need to adjust the heat output needed upwards. Enter the rooms dimensions below in mm or feet.
A note about the types of houses on here:
Uninsulated: is either a townhouse with double brick walls, single glazing, uninsulated roof, and poor draughtproofing, or a farmhouse with stone walls (and the rest of the details the same).
Uninsulated Townhouse with cavity: has cavity walls, single glazing, 50mm roof insulation, and poor draughtproofing.
Renovated Townhouse without cavity: has double brick walls, double glazing, 100mm roof insulation, draughtproofing and floor insulation.
Renovated Townhouse with cavity: has insulated cavity walls, double glazing, 100mm roof insulation, draughtproofing and floor insulation.
Newbuild: is a house built to current building regs.
Passive House: is a mega insulated house with heat recovery insulation and the works.