Calculate the area in square feet of the location you are heating. If the area is rectangular, measure its length and width, and multiply them together. If the area is irregular, divide it into smaller rectangles and add their areas together.
Multiply the area by 12 to calculate your heating requirements, in watts, if this is an uninsulated area. Multiply the area by 9.3 if only the ceiling is insulated. Multiply the area by 7.4 if the ceiling and walls are insulated. Multiply the area by 5.6 if you have a specialized energy efficient home.
For instance, if the area measures 200 square feet, and the ceiling is insulated: 200 x 9.3 = 1,860 watts.
Multiply your answer by 3.41 to convert it to British Thermal Units per hour: 1,860 x 3.41 = 6,342 BTUs per hour.
Divide your answer by 0.85, which is the average efficiency of a propane heater. 6,342 / 0.85 = approximately 7,500 BTUs per hour. Buy a heater with at least that much heating capacity.