FilterSizer: free filtersizer planning calculators
Quick answer: To find your furnace or AC filter size, read the nominal size printed on the old filter's frame, or measure the slot and round each side up to the next whole inch. A filter labelled 16x25x1 actually measures about 15½ × 24½ × ¾ inches. Then choose a MERV range your system can handle and a change schedule.
Filter sizing without the guesswork
FilterSizer is a set of free planning tools for the air filter in your furnace, air handler, heat pump or central air system. The filter is one of the cheapest parts of an HVAC system and one of the easiest to get wrong: the label shows one size, the filter measures another, and the slot it slides into might be a third. The filter size finder converts what your tape measure says into the nominal size printed on store shelves, and tells you the actual dimensions to expect. From there the MERV picker, replacement schedule and cost per year tools help you choose a filter you can live with.
This site is about air filters for heating and cooling equipment only. It does not cover vehicle cabin filters, vacuum bags or other non-HVAC filter types. When we say "filter" we mean the pleated or fibreglass panel that sits in the return air path of a forced-air system.
Nominal, actual and slot sizes
Air filters are sold by nominal size — rounded whole numbers such as 16x25x1. The real, or actual, size is usually about half an inch smaller on length and width, and a 1-inch filter is often about ¾ inch thick. The undersizing lets the filter slide into a slot built to the nominal size. If you measure an old filter at 15¾ × 24¾ inches, you round up to 16 × 25. Our nominal vs actual guide explains why, and how to measure an air filter covers slots with no old filter to copy.
Choosing efficiency without over-filtering
Filters are rated on the MERV scale, defined by ASHRAE Standard 52.2 and explained by the U.S. EPA. Higher numbers capture a larger share of small particles. Higher MERV filters can also add resistance to airflow, especially in 1-inch format, so the right choice balances capture with what your blower and ductwork can handle. Read MERV ratings explained and MERV 8 vs 11 vs 13, and see airflow and pressure drop for the technical side. We do not make health claims for any filter; for indoor air quality guidance, the EPA and Health Canada publish material for homeowners.
Worked example
A homeowner pulls out a dusty filter whose label has faded. They measure it at 19⅝ × 24½ × ¾ inches. Rounding each side up gives a nominal 20x25x1. The return grille's slot measures 20 × 25 inches, confirming the size. They have a dog and want better dust capture than the basic fibreglass panel, but their system is an older furnace with a 1-inch slot, so they pick a MERV 8 pleated filter as a reasonable first step and plan to watch airflow and the filter's appearance over the first two months. Using the cost tool with a hypothetical $12 filter changed every 60 days, they estimate about $73 a year.
Illustrative note (2026-10-29): This example was prepared on 2026-10-29 for illustration. The filter price, change interval and system details are hypothetical and do not refer to any specific product or brand.
Common sizes and where they show up
Some sizes appear in a huge share of homes: 16x25x1, 16x20x1, 20x20x1 and 20x25x1 for 1-inch return grilles and furnace slots, and 16x25x4 for media cabinets. Each has its own guide covering actual dimensions, common locations, face area and airflow. The furnace filter size chart lists many more, and Canada filter sizes explains what changes north of the border — mostly retail availability, since sizes are still sold in inches.
Special cases
Apartments often use filters in ceiling or wall return grilles; mobile and manufactured homes may have filters behind a grille in the furnace closet door; and some people use HVAC filters on box fans to build portable air cleaners. Guides cover apartment filters, mobile home filters and box fan filters. If you are not sure what your equipment needs, your owner's manual or an HVAC technician is the best source.
How often to change it
There is no single right interval. Manufacturers often suggest one to three months for 1-inch filters and six to twelve months for 4-inch media filters, with shorter intervals for pets, smoking, renovation dust or heavy runtime. The schedule tool gives a labelled guideline based on depth and household factors. Checking the filter visually each month is still the best habit.
What we will not tell you
We will not tell you that a filter will cure allergies, stop illness or remove viruses from your home. Filter performance in a real house depends on how much air passes through it, how well it seals in its slot and how many hours the blower runs. We will not rank brands, and nothing on this preview site is sponsored. For anything beyond sizing — unusual equipment, airflow problems, frozen coils, short cycling — talk to a qualified HVAC technician.
Frequently asked questions
Where is my furnace filter?
Usually in a slot between the return duct and the furnace or air handler, or behind a return grille in a wall or ceiling. Some homes have both.
Can I use a filter that is slightly smaller?
A gap lets unfiltered air bypass the filter. Use the correct nominal size and make sure it seals.
Is a 4-inch filter better than a 1-inch?
Deeper filters usually have more media and lower resistance at the same MERV, but only fit systems with a matching cabinet. See 1-inch vs 4-inch.
Do the calculators save my numbers?
No. They run in your browser and nothing is sent to us.