Know the size before you buy a filter.

How to find your filter size

The size is printed on the cardboard frame of the filter that is already in the system. It is usually three numbers, such as 16x20x1. That printed size is the one to write down.

Turn the system off at the thermostat or the service switch before you open a panel. If you are not sure which panel is safe to remove, stop and ask an HVAC technician.

1. Find the filter. It may sit in a return-air grille in a hallway, wall, or ceiling. It may also sit in a slot on the furnace or air handler. Some homes have more than one.
2. Slide the filter out and read the size on the frame. Write down all three numbers, including the thickness, before you throw the old filter away.
3. If the frame has no printing, measure the outer cardboard frame itself: the two face dimensions and the thickness, in inches. Measure the frame, not the pleated material inside it.
4. Check the edge of the frame for an airflow arrow before you put a filter back. The arrow points in the direction the air travels.

A washable filter, or a media cabinet that takes a non-standard cartridge, may not use one of the common one-inch sizes at all. If that is what you find, the equipment manual or a technician is the right next step. This page will not invent a size for that system.

Why the exact size matters

A filter that is even a little small leaves a gap. Air takes the easy path around the media, so dust goes downstream onto the coil and the blower, and the filter does less of the job you bought it for.

A filter that is too large for the slot can be forced in. The frame can bow, the media can tear, and the system can end up with less airflow than it was set up for. Thickness is part of the size. A 1-inch filter does not stand in for a 4-inch filter, and a 4-inch filter will not slide into a 1-inch slot.

The first two numbers are the face of the filter. A frame printed 16x20x1 and a frame printed 20x16x1 are the same nominal face; makers often list the smaller number first. The third number, the thickness, is not interchangeable.

You may also see a MERV number. MERV describes how fine the media is. It is not part of the size. A higher MERV rating is not automatically better for every system. A much finer filter can be harder for some blowers to pull air through. Use the range your equipment was built for, or ask a technician before you jump to a finer one. HomeOS will not guess a MERV rating either.

Why HomeOS does not guess

Filter size is a fact about the equipment in your house. It is not something that can be inferred safely from the age of the house, a photo, or a size that happens to be common.

Two houses built the same year can have different furnaces and different slots. The model number on an outdoor condenser does not reliably tell you the indoor filter size. These are poor substitutes for reading the frame:

• The year the house was built
• A neighbor’s filter
• The model number on the outdoor unit alone
• A bestseller in a size that looks familiar

HomeOS will treat a size as known only after you have entered it from the filter or the slot in that home. It will not fill one in for you. A wrong guess is worse than no suggestion, because it sends a filter that does not fit.

Nominal size and actual size

The size on the frame is the nominal size: the name of the filter, not always the measurement a ruler gives you. Nominal size and actual size can differ, and the difference varies by brand.

A filter labeled 16 × 20 × 1 often measures a little under 16 inches by a little under 20 inches, and close to 1 inch thick. You still ask for a 16 × 20 × 1. Rounding a measurement up to the next half inch will not lead you to a better fit. It usually leads you to a filter that does not exist, or one that will not go in.

When you are replacing a filter, match the nominal size printed on the old frame. When the slot is empty, measure the opening. That opening is often already the nominal size the filter is named for, while the filter itself is made slightly smaller to fit. Do not order a nominal size larger than the slot. If you are unsure which nominal size an empty opening calls for, check the equipment manual or ask a technician. HomeOS will not convert a ruler measurement into a nominal size for you.

What HomeOS will remember

Opening the furnace to read a frame is easy to put off, and the number is easy to lose once the filter is back in. When the iPhone app is available, you will be able to save the size that belongs to your system.

HomeOS will keep that saved size with the rest of the home record, so the next time a filter is due you can start from a number you entered. Until you have saved one, there is nothing for HomeOS to repeat back, and it will not supply a guess in the meantime.

The app is not available for download yet. This page is the guide. It is not a store.