How much will a 12000 btu heater heat?

A 12,000 BTU heater can effectively heat a space of approximately 400 to 600 square feet, depending on factors like insulation, ceiling height, and climate. In well-insulated areas, it may efficiently warm the larger end of that range, while in poorly insulated spaces, it might only cover the lower end. Always consider the specific conditions of the area to determine the heater's effectiveness.