For example, a 24 ft. x 24 ft. garage with an eight-foot ceiling has a volume of 4608 ft.3 (24 * 24 * 8 = 4608) Final Formula. Once you have all the data assembled, multiply them together and divide by 1.6 to find the number of BTUs required to heat your garage. (Insulation * Volume * Temp. Rise)/1.6 = BTU. For example, if your garage was 4608 ft. 3 in volume, your temperature rise was 20°F, and your insulation rating was one, it would be: (1 * 4608 * 20)/1.6 = 57,600 BTU