Walls that face outside — not shared with another room or property.
Solid floor or floor above heated space.
This is a guide based on industry-standard heat loss figures. Room shape, window area, and local climate all affect real-world performance. For a precise specification, consult a heating engineer.
This calculator uses a volume-based heat loss method, multiplying room volume by a W/m³ figure that reflects how quickly the space loses heat. Well-insulated modern rooms lose heat slowly and need less output; draughty or exposed rooms need significantly more. External walls and exposed floors increase heat loss further, so the calculator applies standard adjustments for each. The result is converted to BTU/h — the unit radiators are most commonly rated in — alongside the kW equivalent used in modern heating specifications.