Radiator 

Cu. Ft. Per Sq Ft 

 Cast-iron wall

.028 

 Cast-iron column

.025

 Cast-iron tube

.013

Cast-iron thin tube (newer Style)

.009

 Cast-iron radiant radiator

.008

 Cast-iron baseboard (9" high)

.07

 Cast-iron cabinet convector

.0003

 Unit  heater      

.0002