Boiler Radiator Sizing Calculator
Size a radiator to a room’s heat loss and convert the BTU/hr to square feet of equivalent direct radiation (EDR) for steam or hot-water systems.
How it works
Heat loss = area × loss factor × (ceiling ÷ 8) · Radiator EDR (ft²) = heat loss ÷ output per EDR (240 steam, 150 hot water)
Radiators are rated in square feet of Equivalent Direct Radiation (EDR), a historical unit tying surface area to output: 240 BTU/hr per ft² EDR for steam and about 150 BTU/hr per ft² EDR for hot water, which runs at a lower temperature. Sizing starts from the room’s heat loss, then divides by the per-EDR output to get the radiator area needed. Because steam radiators put out more heat per EDR, a steam room needs fewer square feet of radiator than the same room on hot water.
Worked example
A 200 ft² room with 8 ft ceilings and average insulation loses 200 × 35 = 7,000 BTU/hr. On hot water at 150 BTU/hr per EDR that is 7,000 ÷ 150 = 46.7 ft² EDR; on steam at 240 it would be only 29.2 ft² EDR.
Frequently asked questions
What is EDR?
Equivalent Direct Radiation is a rating of a radiator’s heat output as a surface area. One ft² EDR delivers 240 BTU/hr on steam or about 150 BTU/hr on hot water, so a radiator’s EDR rating times that figure gives its output.
Why do steam and hot-water radiators differ?
Steam radiators run near 215°F while residential hot water runs around 170–180°F. The hotter surface radiates more heat per square foot, so the same EDR delivers more BTU on steam than on hot water.
Can I reuse old cast-iron radiators?
Often yes — you can look up or measure each radiator’s EDR from its column count and height, then check that the total EDR meets each room’s heat loss. Cast iron’s large mass also gives a steady, comfortable heat.
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This calculator is for educational purposes. Double-check important results before acting on them.