Advanced options
A rule-of-thumb BTU estimate; north-facing rooms, large windows and open stairs all add demand, and heat pump systems size differently (lower flow temperatures need larger emitters). For a new heating system, get a room-by-room heat loss calculation.
What The Radiator Size Calculator Works Out
- Room volume: length x width x ceiling height in cubic metres. A 4m x 4m room with standard 2.4m ceilings is 38.4 m3.
- Heat output needed: volume multiplied by a watts-per-cubic-metre factor that varies with room use (living rooms 46, bathrooms 43, bedrooms 42, kitchens 40), plus 20% for poorly insulated or single-glazed rooms. The 4m x 4m living room comes out at roughly 1,750 W, about 6,000 BTU in the units radiator listings still favour.
- Suggestion: up to about 1,200 W one standard panel will do; up to 2,200 W a large double or two mediums; above that, split the output across two or more radiators for even heat. For a single replacement, matching or slightly beating the old one's output is usually right.
- Watts per m3 override and uplifts (advanced): replace the room-type factor with your own, change the poor-insulation uplift, and add a percentage for north-facing rooms, big glazed areas or rooms above passageways, which the volume rule cannot see.
- Catalogue output to buy (advanced): manufacturers quote outputs at Delta T50 (water 50 degrees above the room). Output falls with the 1.3 power of the temperature difference, so a heat pump running at Delta T30 gets only about 51% of the catalogue figure and needs panels roughly twice the size; old Delta T60 tables inflate outputs by about a fifth.
- Per radiator (advanced): the catalogue output split across the number of radiators in the room, in watts and BTU, for shortlisting panels.
- Heat loss per m2 of floor (advanced): the output divided by floor area, as a sanity check against the 50 to 100 W/m2 that room-by-room heat loss surveys typically find. Much higher suggests the factors are too generous for a modern insulated room.
- Gas cost on a cold day (advanced): the output running flat out for your heating hours, divided by boiler efficiency and priced at the gas unit rate. Real days average well below full output, so treat this as the ceiling for a design-temperature day.
Further reading: Energy Saving Trust.