How the math works
ClariSelect mathAirflow a filter passes = face area (width × height ÷ 144) × face velocity. Pleated 1-inch filters are commonly sized at about 300 ft/min, 2-inch at about 400 and deep media at about 500. Cooling systems move about 400 CFM per ton.
LimitsYour tonnage is on the outdoor unit's model number (for example, a 36 in the model often means 36,000 BTU, or 3 tons). These are rules of thumb; a technician can measure your system's actual static pressure.
Questions
What happens if my filter is too small for my system?
Air moves through it faster, which raises resistance, can make the filter whistle or bow, and can reduce airflow. A second return or a deeper filter cabinet fixes it.
Can I use MERV 13 in a 1-inch slot?
Often yes in a well-sized system, but 1-inch MERV 13 filters add the most resistance; check this calculator, and watch for whistling or a bowing filter.