How do you calculate BTU needed to heat a house?

To calculate the BTU needed to heat a house, first determine the square footage of the space to be heated. Next, multiply the square footage by a BTU factor that accounts for insulation quality, climate, and ceiling height, usually around 20 BTUs per square foot for average conditions. Finally, adjust for specific factors such as Windows, doors, and the number of occupants, using a more detailed heat loss calculation if necessary. This will give you an estimate of the total BTUs required for effective heating.