BTU Heater Calculator

Calculate BTU units for a given room, choose surroundings, exposure, insulation, kitchen areasThe heating capacity of a furnace is measured in thousands of BTU (British Thermal Units). Furnaces are rated by the amount of fuel energy consumed when running, called input BTU. Different furnaces of the same input BTU will have different output BTU depending on the furnace efficiency. Select a furnace for your home according to the output BTU, as this is the actual heating capacity of the furnace. You can easily calculate the output BTU by multiplying the input BTU by the efficiency percentage. For example a 100,000 BTU furnace at 80% efficiency will produce 80,000 BTU of heat while a 100,000 BTU furnace at 95% efficiency will produce 95,000 BTU of heat output.