How many square feet will 120 BTU furnace heat?

The heating capacity of a furnace is often measured in BTUs (British Thermal Units), and a general rule of thumb is that one BTU can heat approximately 1 square foot of space. Therefore, a 120,000 BTU furnace can heat around 1,200 square feet, assuming standard insulation and ceiling height. However, factors such as climate, insulation quality, and layout can influence this estimate, so adjustments may be necessary based on specific circumstances.