Chapter 01Records · Know-how
Nominal vs actual HVAC filter sizes, explained
The size printed on a filter is its nominal size, rounded to whole inches. The filter itself is a little smaller, so it slides into the slot. Filterbuy's size chart lists the actual length and width as 0.5 inch under the nominal size and a 1-inch filter as 0.75 inch deep, so a 16x25x1 filter measures about 15.5 x 24.5 x 0.75 inches. Shop by the nominal size.
Source: Filterbuy air filter size chart, checked October 9, 2026.
- Under nominal, length and width0.5 in Filterbuy air filter size chart, October 9, 2026
- Actual depth of a 1-inch filter0.75 in Filterbuy air filter size chart, October 9, 2026
- Replacement interval manufacturers typically recommend60 to 90 days EPA, Guide to Air Cleaners in the Home, October 9, 2026
Two sizes on one filter
Every filter has two sizes. The nominal size is the rounded one printed on the frame and the box, the one you type into a store's search. The actual size is what the tape measure shows. Filterbuy's chart puts the actual length and width half an inch under the nominal size.
Source: Filterbuy air filter size chart, checked October 9, 2026.
Depth works differently. By the same chart, a 1-inch filter is 0.75 inch deep, a 2-inch filter is 1.75 inches and a 4-inch filter is 3.63 inches.
Source: Filterbuy air filter size chart, checked October 9, 2026.
Why filters are sized this way
The filter has to go into its slot without a fight. The EPA says a filter should fit snugly so air does not leak around it, but without being bent or crushed. A filter cut a little under the round number slides in and still fills the slot.
Source: EPA, Guide to Air Cleaners in the Home, checked October 9, 2026.
One maker's chart
The half-inch pattern comes from one manufacturer's chart, and Filterbuy itself says other brands can differ slightly. Check the printed size before you buy a whole case.
Source: Filterbuy air filter size chart, checked October 9, 2026.
How to find your size
Read the frame
Pull out the old filter and look along the cardboard edge. The nominal size is usually printed there as three numbers.
No print? Measure it
Measure the old filter's length, width and depth in inches if it is still flat. If it has bowed, measure the slot instead and round each side to the nearest inch.
Add half an inch and round
Add 0.5 inch to the length and the width, then round to the nearest whole inch. A filter that measures 19.5 x 24.5 inches is a 20x25.
Match the depth
Match the measured depth to the nearest one in the chart: 0.75 in is a 1-inch filter, 1.75 in a 2-inch, 3.63 in a 4-inch.
Write it where you'll see it
Put the nominal size and the date on the printable sheet, and keep the sheet near the grille.
Source: Filterbuy air filter size chart, checked October 9, 2026; Filterbuy, How to Measure Your Air Filter, checked October 9, 2026.
Common mix-ups
- Stores list filters by nominal size, so a search for 15.5 x 24.5 finds little. Search for 16x25x1 instead.
- A bowed old filter measures short. If yours has bent, measure the slot opening and round each side to the nearest inch.
- Length and width can come in either order on a box, but the depth has to match: a 1-inch filter will not fill a 4-inch slot.
- A home with more than one return grille can need more than one size, so check each one.
Source: Filterbuy air filter size chart, checked October 9, 2026; Filterbuy, How to Measure Your Air Filter, checked October 9, 2026.
Checklist to print
Keep this by the return grille:
- Nominal size read from the old filter's frame
- Actual size measured, if the frame has no print
- Depth matched to the 1, 2 or 4 inch size
- Every return grille checked for its own filter
- Install date written on the new filter's frame
- A monthly check on the calendar
- Change window noted: 60 to 90 days from the install date
- Size taped up near the grille for next time
Tips and tricks
- Tip 01
Shop by the printed size
Search and buy by the nominal size on the old filter's edge, not by your tape measure reading.
- Tip 02
Depth is the number to guard
Length and width can swap places on a box. The depth cannot. A thicker filter will not fit a thin slot.
- Tip 03
Bowed filter? Measure the slot
An old filter that has sat in place all season often bows and reads short. Measure the slot opening and round each side to the nearest inch.
Source: Filterbuy, How to Measure Your Air Filter, checked October 9, 2026.
- Tip 04
No standard size matches?
Look for a custom-size filter made to your actual measurements instead of forcing the nearest standard one.
What we would do
Read the edge, then use the finder
Pull the old filter and read the size on its cardboard edge. If the print is gone, measure the filter itself and let the size finder round it to the nominal size. Buy one to test the fit before you buy a pack.
Questions
What does nominal size mean on an air filter?
It is the rounded size printed on the frame and the box, the one you shop by. The filter itself is a little smaller.
Is a 16x25x1 filter really 16 by 25 inches?
No. By Filterbuy's chart it measures about 15.5 x 24.5 x 0.75 inches.
Source: Filterbuy air filter size chart, checked October 9, 2026.
Can I use a filter a little smaller than the slot?
The EPA says a filter should fit snugly so air does not leak around it, without being bent or crushed. Buy the nominal size the slot was made for.
Source: EPA, Guide to Air Cleaners in the Home, checked October 9, 2026.
Does the order of the numbers matter?
Length and width can be listed either way round. The depth is the one to get right, because it decides whether the filter fills the slot.
How often should I check the filter?
ENERGY STAR's checklist says to inspect, clean or change it once a month, and the EPA says manufacturers typically recommend a new one every 60 to 90 days.
Source: ENERGY STAR maintenance checklist, checked October 15, 2026; EPA, Guide to Air Cleaners in the Home, checked October 9, 2026.
Part of a system
This page is one part of a bigger job. The system page puts the parts in order.
SystemFall home checklist and winter-prep recordWhat to book, check and write down before winter: the heating check-up, the air filter, smoke alarm tests, hoses and drafts, on one printable page.8 steps · 4 parts · 1 page to printOpen the system
SystemFurnace filter record: every slot, its size and the change datesFind every filter slot, write down its size and arrow, buy a year of spares, then check monthly and log each change on one printable record.8 steps · 4 parts · 1 page to printOpen the system
SystemHome records binder: your house manualSix tabs for the facts a house keeps asking for: appliances and warranties, filter sizes, paint colors, measurements, manuals and service visits.8 steps · 6 parts · 1 page to printOpen the system
SystemMove-in first month: the new home checklistFour weeks, one short job list each: smoke alarms, the air filter, appliance labels, a room-by-room inventory and the monthly reminders, on one printable page.9 steps · 6 parts · 1 page to printOpen the system
The tool and more guides
Chapter 01HVAC filter size finder and replacement plannerTurn a nominal size like 16x25x1 into the filter's actual dimensions, or measure an old filter to find the size to buy, then plan the next change.Open the tool
ProblemNo size printed on my air filter: how to find itThe print on the frame is gone or the slot is empty. Measure the opening, round to the nearest inch, and know when you need a custom size.Read the guide
Practical noteWhat to change on move-in day: locks, garage opener and codesNew house, old keys. Here's the order to work in on move-in week: count what you got, change the locks, clear the garage opener, reset codes, then cut copies.Read the guide- Chapter 01All records toolsPrintable home records with a calculator behind each one: appliance and warranty log, HVAC filter sizes and change dates.Open the chapter
Sources
- Air Filter Sizes: Nominal vs. Actual Dimensions, Filterbuy (US air filter manufacturer). Checked October 9, 2026.
- Guide to Air Cleaners in the Home, U.S. Environmental Protection Agency. Checked October 9, 2026.
- How to Measure Your Air Filter: Complete Step-by-Step Guide, Filterbuy (US air filter manufacturer). Checked October 9, 2026.
- Heating and Cooling Maintenance Checklist, ENERGY STAR (U.S. EPA and DOE). Checked October 15, 2026.
Every outside number on this page names its source and the date we checked it. See them all on the sources page. Spotted an error? Tell us.