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Calculate the room's design heat loss using envelope conduction and infiltration at outdoor design temperature. The radiator must deliver at least this BTU/hr at operating water temperatures.
Required BTU = Room heat loss at design ΔTRadiator catalogs rate output at standard conditions (typically 180°F supply, 160°F return, 68°F room). Adjust rated output for actual system temperatures using the mean water temperature correction.
Output = Rated BTU × ((T_mean_actual − T_room) / (T_mean_std − T_room))^nCast iron radiators are sold by section. Divide required BTU by per-section output at your operating temperatures to determine section count.
Sections = Required BTU / BTU per section at ΔTUpdated: July 2026
A 300 sq ft living room losing 6,000 BTU/hr at design temp needs a radiator rated ~6,500 BTU at 180/160/68°F conditions. A 6-section cast iron column radiator at 1,100 BTU/section fits.
A bedroom with 3,500 BTU/hr loss on a condensing boiler at 140°F supply. Panel radiator rated 4,000 BTU at 180°F delivers ~3,200 BTU at 140°F — upsize to the next larger panel.
Small 80 sq ft kitchen losing 2,000 BTU/hr. A 24-inch towel radiator (1,800 BTU) plus under-cabinet panel (500 BTU) covers the load while fitting limited wall space.
Catalog ratings assume 180°F supply. A condensing boiler at 130°F supply delivers 60–70% of rated output. Upsize radiators or select low-temperature-rated models.
If you plan to insulate walls or replace windows, calculate future heat loss and size radiators accordingly — or install thermostatic valves to balance after upgrades reduce load.
Hydronic heating systems require each radiator to match the room's heat loss at design conditions. This calculator determines required BTU output from room dimensions, heat loss, supply and return water temperatures, and radiator type so every room reaches target temperature.