Start with the filter already in the system
Most disposable HVAC filters have three numbers printed on the cardboard frame, such as 16 × 20 × 1. Those numbers are the nominal width, height, and thickness used when shopping for a replacement.
Before removing a filter from a furnace or air handler, turn the system off at the thermostat or service switch. If you are unsure which panel is safe to open, stop and ask an HVAC technician.
Where the filter may be located
Depending on the home and equipment, the filter may be:
- Behind a large return-air grille in a wall, ceiling, or hallway.
- In a narrow slot beside the furnace or air handler.
- Inside a larger media cabinet attached to the HVAC equipment.
- In more than one return location if the home uses multiple filters.
Do not assume there is only one filter. Some systems use multiple returns or multiple HVAC systems.
Read the nominal size on the frame
Look along the edge of the filter for a size such as 16x25x1 or 20x25x4. Write down all three numbers, including the thickness.
If the frame also shows actual dimensions, do not confuse those with the nominal replacement size. A filter labeled 16 × 20 × 1 may physically measure slightly less than those dimensions.
What if there is no printed size?
If the filter has no readable label, measure the outside cardboard frame. If the slot is empty, measure the opening carefully.
An empty-slot measurement does not always translate cleanly into a nominal filter size, so use the equipment manual or ask an HVAC technician if the correct replacement is uncertain. HomeOS is intentionally designed not to guess this number.
Save the size once you know it
Filter size is exactly the kind of home detail that is easy to forget between replacements. Once you have confirmed it, keep it with your equipment records so the next replacement starts with a known fact instead of another trip to the furnace.