Know the square footage of the space to know what kind of air handler to get, then multiply that figure by 25 to get the British thermal units (BTUs). Divide the total by 12,000, rounding to the nearest 0.5 to find the tonnage. For example, if you have a home that’s 1,000 square feet, do the following: 1,000 x 25 = 25,000 BTUs 25,000 / 12,000 = 2.08333 Tonnage required: 2