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. The result is converted to BTU/h — the unit radiators are most commonly rated in — alongside the kW equivalent used in modern heating specifications.
Well-insulated modern rooms lose heat slowly and need less output — around 30 W per cubic metre. Draughty or uninsulated rooms can need more than double that, up to 70 W/m³. This single factor has the biggest effect on the result, which is why it's worth being honest about your room's actual insulation rather than assuming "average."
Every wall facing outside loses more heat than one shared with another heated room, so each additional external wall beyond the first adds to the total. Similarly, a floor above an unheated space (a garage, a crawl space, an exposed void) loses more heat than one above another heated room or on solid ground — the calculator applies a standard adjustment for both.
If a room has more than one radiator, the total heat requirement is divided evenly between them. For balanced heat distribution, fit radiators of equal output rather than one large and one small — an uneven split can leave one side of the room noticeably colder.
This is a guide based on industry-standard heat loss figures, not a substitute for a proper heat-loss survey. Room shape, window area and glazing quality, ceiling height variation, and local climate all affect real-world performance and aren't individually modelled here. For a precise specification — especially for a whole-house system or an unusual room — consult a heating engineer.