Radiator BTU Output Calculator (EDR to BTU/hr)

EDR (Equivalent Direct Radiation) is the historic cast-iron radiator sizing standard: 1 sq ft EDR liberates 240 BTU/hr at ~215°F/~1 psi steam and 70°F room air (EDR's own defining condition, confirmed consistently across every source checked). For hot water, output per sq ft EDR follows a power-law curve — BTU/hr = C × (average water temperature − 70°F)^1.3 — using the EN 442 "characteristic curve" exponent industry-standard for panel/cast-iron radiator output (typical range 1.2-1.35), with the constant C calibrated so the curve passes exactly through the universally- agreed 240 BTU/hr steam condition. This average water temperature is the mean of supply and return, not the supply temperature alone. When EDR is determined by tube count and sections, the EDR-per-section figures are typical planning values at a common ~26 in height — real EDR varies meaningfully by manufacturer and casting design (confirmed by professional discussion of this exact issue, since the old IBR independent-testing program was discontinued and manufacturer ratings diverged afterward) — height scaling from the 26 in reference is an approximation, not a verified per-height table. A known actual EDR (nameplate, catalog, or documentation) is always more accurate than these planning defaults. Required EDR and required sections (when a target heat output is entered) are rounded UP, never down, so the displayed figure is never below the true requirement. This is a planning estimate, not a substitute for a licensed heating contractor's design or a specific manufacturer's published rating.