Is Your Filter the Correct Size for Your A/C? Steps to finding the correct size filter for your system. Size your A/C unit   Go to the unit outside your house and look at the MODEL number. DO NOT worry about the serial number. The manufacturers all put a designation in the model number to determine the tonnage. (See the label below) For the model number, you will see a series of letters and numbers mixed together. Somewhere in that group of numbers you will see a 24, 30, 36, 42, 48 or 60. (in our example it is 60) Divide this number by 12 and this will tell you how many tons of a/c that you have.    e.g. 60 divided by 12 = 5 tons of a/c. (see chart for common tonages)                    Determine the size filter needed in square inches    The standard in the HVAC industry is to have a minimum144 square inches of filter space per ton of air conditioning. Our extensive testing and surveying has shown that this is the case in the majority of homes. So, for a 5 ton unit you will need 5 X 144 = 720 square inches of filter for this unit. Measure your filter in square inches    Scenario 1 You have a single 20 X 30 filter for the unit, so 20 X 30 = 600 square inches. You are starving your A/C for air and need a bigger filter. Scenario 2 You have two 20 X 25 filters on that unit, so there are 20 X 25= 500 square inches each, totaling 1000 square inches. Your filters are big enough for this system. Scenario 3 You have one 25 X 32 filter, so 25 X 32 = 800 square inches. You have enough surface area for your air conditioner.