Furnace filter size finder

What size furnace filter do I need?

Start with the cardboard edge of the filter that is in the unit right now. Nearly every filter has its size printed there, something like 16x25x1, which reads as the two face dimensions and the thickness in inches. If you can read a printed size, that is your answer: buy that exact printed size, and ignore what a tape measure says about the filter, the box, or the slot. The printed size is the ordering size, and every brand sells by it.

The finder above earns its keep in the messier cases: the printing has faded, the old filter went out with the trash, you just moved in and the slot is empty, or the filter in there looks cut down or jammed and you suspect it was never right. For those, measure and let the finder do the conversion, because the conversion is exactly where people get burned.

Why the size on the box doesn't match your tape measure

Measure a brand new 20x25x1 filter and you will get roughly 19.5 by 24.5 by 0.75 inches. Nothing is wrong with it. The printed number is the nominal size, a rounded label, and the filter is built smaller so it slides into the slot without binding. The gap between label and reality is usually a quarter to a half inch per dimension, and it varies by brand: a Filtrete 16x25x1 measures 15.69 by 24.69, while many house brands cut the same label size to 15.5 by 24.5. No industry standard pins this down, so two filters with identical labels are not always identical filters.

This is why the direction of the conversion matters. Going from a measurement to a size to buy, you round up. Going from a printed size to an expectation of fit, you expect it to measure small. People who do not know the convention measure 19.5, search for a 19.5 inch filter, find nothing, and conclude their size is discontinued. It was a 20 all along.

Furnace filter sizes chart: nominal vs actual

These sizes cover most US homes. The actual column is the typical aftermarket dimension; individual brands drift by a fraction of an inch:

If your slot takes 16x20x1, 16x25x1, 20x20x1, or 20x25x1, any hardware store has it on the shelf. The rest of the chart is easy to order online. A size that is not on the chart at all is not a dead end either; it is just a size the big-box shelf skips, which the last section covers.

How to measure the filter slot when nothing is printed

Two rules, depending on what you are measuring. Measuring the old filter itself: measure each face dimension and the thickness to the nearest eighth of an inch, then round each one up to the whole inch. A filter measuring 19.5 by 24.5 by 0.75 is a 20x25x1. Round up, never to the nearest, because rounding down buys a filter with a gap around it. Measuring an empty slot or return grille: measure the opening frame edge to frame edge and round to the nearest whole inch instead, because the slot itself is built to roughly the nominal size. The filter you order will measure smaller than the opening, and that clearance is correct.

One warning either way: if the fit is only achievable by bending or forcing the filter, the size is wrong even if the math said it was right. A filter should slide in with light resistance and sit flat. And once the new one is in, the arrow on the frame has to point the right way; our furnace filter direction guide covers which way in ten seconds.

1-inch, 2-inch, 4-inch, or 5-inch: which thickness fits your slot

Thickness is set by the hardware, not by preference. A wall or ceiling return grille and a standard furnace slot take 1-inch filters, some slots take 2-inch, and the wide cabinet mounted next to the furnace (a media cabinet) takes 4-inch or 5-inch media filters. You cannot trade across tiers: a thicker filter physically will not slide into a shallower track, and a 1-inch filter dropped into a 4-inch cabinet leaves a canyon on either side that air happily flows around, carrying dust straight to the blower and coil.

Media cabinets are also where label sizes stop being trustworthy. A Honeywell filter labeled 16x25x4 actually measures about 4 and 3/8 deep, one brand's 20x25x4 matches another brand's 20x25x5, and some brands run wider than the label. For a 4 or 5 inch cabinet, match the part number printed on the cabinet or the old filter rather than trusting the nominal size, or measure the old filter exactly and order custom. Thicker media filters cost around $20 to $30 each against $7 to $15 for a good pleated 1-inch, but they also go many months between changes; our guide on how often to change a furnace filter breaks the schedule down by thickness.

Can't find your size in stores? Odd and custom sizes

Plenty of systems take sizes the shelf never carries, and older homes, Rheem and American Standard air handlers, and custom return grilles are the usual suspects. Online filter suppliers stock hundreds of sizes and build the rest to order in eighth-inch increments, so an odd size costs a little more and ships slower but is never unavailable. The one rule that flips: custom filters are ordered by the exact measured dimensions, not by a rounded nominal size. Give the maker the tape-measure numbers, and measure carefully, because custom orders are usually non-returnable.

What you should not do is trim a rigid pleated filter to fit. Cutting one exposes the media, collapses the frame under airflow, and opens bypass gaps. Cut-to-fit media rolls exist for washable frames and window units, but a furnace slot deserves a filter built to its size.

Is it OK to use a filter that's a little smaller?

No, and the reason is the same physics that makes the right size matter at all. Air takes the easiest path, and a quarter-inch gap around a too-small filter is a highway compared to the filter media next to it. Dust rides that gap past the filter and settles on the blower wheel and the evaporator coil, where it costs real money: a professional coil cleaning runs $100 to $400, and a badly fouled coil can push into repair territory. A too-small filter also announces itself: it rattles, whistles, or gets sucked out of its track into the blower. If your correctly sized filter still gets pulled in or collapses, the return side may be starved and pulling too hard through too little filter area; our return air sizing calculator shows what the return should measure for your system's airflow. And once the size is settled, the rating printed next to it is a separate decision with its own tradeoffs; our MERV filter calculator matches the rating to your system and household.