This involves the dimensions of the building or tent. If the length is 40 feet and width is 30 feet and the height is 10 feet, that means you have 12,000 square feet to heat up. 40 x 30 x 10 = 12,000 cubic feet. To best calculate the BTU’s needed for the event LB White has a BTU calculator that can be used to help or you can view the chart below.