Quick answer: Your air filter size is printed on the edge of the filter you already have, as three numbers like 20x25x1. If there’s no label, measure the old filter or the slot it sits in and round up to the nearest inch. Your furnace or heat pump model number usually can’t tell you, because the filter size is set by the return duct or grille your installer built, not by the equipment.
Air filter size, defined: Filters are sold by nominal size, a rounded length × width × depth in inches (for example 16x25x1). The actual size is about ¼” to ½” smaller so the filter slides in without forcing. Always shop by the nominal size.
Why your model and serial number won’t give you the filter size
We hear this on the phone almost every day: a homeowner reads us the model and serial number off the furnace, hoping it unlocks the filter size. It’s a smart guess, but in most Puget Sound homes it doesn’t work.
The filter usually sits in one of two places: a return-air grille in a hallway wall or ceiling, or a filter rack built into the return duct right beside the furnace. Both were sized by whoever installed the ductwork, based on how much air the system moves. Two houses with the identical furnace can take completely different filters.
The exception: some furnaces and air handlers have a filter slot or door built into the cabinet itself. In that case the owner’s manual does list the size. If you have the manual handy, it’s worth a look. If not, the steps below work for every setup.
How to find your air filter size in 6 steps
Step 1 Turn the system off
Set the thermostat to Off so the blower doesn’t pull dust into the system while the filter is out.
Step 2 Find the filter
Check these spots in order:
- Large return grilles in walls or ceilings (often a hallway). A filter grille has latches or thumb screws and swings open.
- A slot in the duct between the return and the furnace, usually covered by a thin metal or plastic flap.
- A door on the furnace or air handler cabinet.
- A thick box bolted beside the furnace (a media cabinet), which holds a 4″ or 5″ filter.
Step 3 Read the size on the frame
Pull the filter out and look at the cardboard edge. The large numbers are the nominal size, written length × width × depth. That’s what you buy.
Step 4 No label? Measure it
Measure length, width and depth with a tape measure, then round each number up to the nearest inch. A filter measuring 19½ × 24½ × ¾ is a 20x25x1. If there’s no old filter, measure the inside of the slot or grille opening and match it to the closest standard size.
Step 5 Count your filters and note the airflow arrow
Many homes have more than one filter grille, and every one needs changing. Each filter has an arrow on the frame: it points in the direction air flows, toward the furnace and away from the room.
Step 6 Write the size where you’ll see it
Use a permanent marker to write the size on the duct or the inside of the grille, and snap a photo on your phone. Next time, you can order without pulling anything out.
Nominal vs. actual size: why your filter “measures wrong”
Filters are made slightly undersized so they slide in and out of the rack. That’s normal, not a defect.
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| Printed (nominal) size | Typical actual size |
|---|---|
| 16x20x1 | about 15¾ × 19¾ × ¾ |
| 16x25x1 | about 15½ × 24½ × ¾ |
| 20x20x1 | about 19½ × 19½ × ¾ |
| 20x25x1 | about 19½ × 24½ × ¾ |
| 16x25x5 (media) | varies by brand, often 4¼ to 4⅞ deep |
Thick 4″ and 5″ media filters vary the most between brands. Match the cabinet’s brand when you can, or bring the old filter to the store.
Where is my filter? Common setups in Western Washington homes
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| Your home | Where the filter usually is | Typical filter |
|---|---|---|
| Furnace in garage or basement | Slot in the return duct beside the furnace, or a media cabinet | 1″ in a slot, or 4 to 5″ in a cabinet |
| Furnace or air handler in attic or crawlspace | Return grille in a hallway ceiling or wall | 1″ behind the grille |
| Furnace in a closet | Return grille on the closet door or wall, or a slot under the furnace | 1″ |
| Heat pump with air handler | Same as a furnace: grille, duct slot or cabinet door | 1″ or 4 to 5″ |
| Ductless mini-split | Washable mesh screens behind the front panel of each indoor unit | Nothing to buy; rinse every 1 to 2 months |
What MERV rating should I buy?
Once you know the size, pick the rating. MERV (Minimum Efficiency Reporting Value) measures how well a filter catches particles; higher numbers catch smaller ones.
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| MERV | Catches | Good for |
|---|---|---|
| 8 | Dust, pollen, lint | Basic protection for most homes |
| 11 | Plus pet dander and finer dust | Pets, mild allergies |
| 13 | Plus fine particles like wildfire smoke | Allergies, asthma, smoke season |
The EPA recommends MERV 13, or the highest rating your system can handle, especially during wildfire smoke. Higher-rated filters also restrict more air, and some systems with thin 1″ filters struggle with MERV 13. If your rooms get less airflow after an upgrade, ask us to check your static pressure at your next tune-up. More on smoke season in our wildfire smoke guide.
How often to change it
Check 1″ filters every month and change them every 1 to 3 months, sooner with pets or during smoke events. Thick 4″ and 5″ media filters typically last 6 to 12 months. Our guide on how often to change your furnace filter covers the signs it’s time.
When to call a pro
Call us if the filter slot is hard to reach, the filter is bent or sucked into the duct, you can’t find a filter at all, or you’d like to move from a 1″ filter to a media cabinet. Our technicians check and change your filter at every maintenance visit, including Comfort Club visits. We don’t carry filters on our trucks, so have your own filter ready, or ask us to bring one when you book your maintenance. The technician writes the date on the new filter, installs it and takes the old one away. Filters we bring are an added charge, and the price depends on the size and type. For example, a standard replacement filter is around $39 and a long-life MERV 16 media filter is around $229. You can also buy filters from our online shop. For whole-home filtration, purifiers and humidity control, see our indoor air quality services.
Air filter size FAQ
Can I find my air filter size by furnace model number?
Sometimes, but usually not. If the filter sits in a slot on the furnace cabinet, the manual lists its size. If it sits in a return grille or duct rack, which is common, the size was set by the ductwork and the model number won’t help. Check the old filter instead.
What do the three numbers on an air filter mean?
Length × width × depth in inches. A 16x25x1 is 16″ by 25″ and 1″ thick. The order of the first two numbers doesn’t matter when you shop.
Why is my filter smaller than the size printed on it?
The printed size is nominal, rounded up to the nearest inch. Filters are built about ¼” to ½” smaller so they slide into the slot. Buy by the printed size.
My filter has no size on it. What do I do?
Measure its length, width and depth and round each up to the nearest inch. If the old filter is missing, measure the inside of the slot or grille opening.
Can I use a slightly different size if it’s close?
No. A filter that’s too small lets air slip around the edges, carrying dust into the system. One that’s too big won’t seat and can bend. Use the exact nominal size.
Do I have more than one air filter?
Possibly. Homes with several return grilles often have a filter behind each one, and some have both a grille filter and one at the furnace. Check every large grille.
Which way does the arrow on the filter point?
Toward the furnace or air handler, in the direction air flows. At a wall or ceiling grille, the arrow points into the wall or ceiling.
Can I put a thicker filter in a 1-inch slot?
No. A thicker filter needs a slot or media cabinet built for it. A technician can add a media cabinet if you want longer-lasting, higher-capacity filtration.
Does a heat pump use the same filters as a furnace?
Yes. Ducted heat pumps use the same filter types and sizes, found in the same places. Ductless mini-splits use washable screens instead.
What happens if I use the wrong filter size?
Unfiltered air bypasses the gaps, dust builds up on the blower and coil, and efficiency drops. Over time that can lead to repairs.
Need help with your air filter?
Our technicians check filters, airflow and safety at every maintenance visit across the Puget Sound.
Get an estimate
A local comfort advisor will call you to set up your visit.
Sources: EPA, Guide to Air Cleaners in the Home; filter manufacturer nominal and actual size specifications. Checked October 26, 2026.