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20x25x1
Quick answer: A 20x25x1 filter measures about 19½ × 24½ × ¾ inches. With about 3.47 square feet of nominal face area, it is the largest common 1-inch size and suits roughly 1,000–1,200 CFM — typical of 2.5- to 3-ton systems. Look for it in large return grilles, furnace side slots and filter racks.
The big one-inch filter
If you have a single large return grille in a house of moderate size, chances are it holds a 20x25x1. The size gives builders the most face area that still fits between ceiling joists or within a typical wall, and it matches common return plenum openings on mid-size furnaces. It is widely stocked in every efficiency level.
Dimensions
| Measure | Nominal | Typical actual |
|---|---|---|
| Width | 20 in | about 19½ in |
| Length | 25 in | about 24½ in |
| Depth | 1 in | about ¾ in |
| Face area | 3.47 ft² | about 3.32 ft² actual |
Airflow capacity
At 300 fpm, one 20x25 filter passes about 1,040 CFM. At 350 fpm, about 1,215 CFM. That places it at the edge for a 3-ton system on its own: workable, but resistance will be higher than at the rating point, especially with MERV 13 in 1-inch. Systems of 3.5 tons and up usually need more than one return or a 4- or 5-inch media filter. See airflow and pressure drop.
Where it lives
- A large ceiling return in a central hallway.
- A big wall return near the stairs in two-storey homes.
- The side-entry or bottom-entry filter slot on many gas furnaces.
- A filter rack on some air handlers.
Picking MERV
Because this filter often serves the whole house at fairly high velocity, MERV 8 or 11 is a common choice in 1-inch format. MERV 13 can work where airflow allows; otherwise, the usual upgrade path is a 20x25x4 or 20x25x5 media cabinet at the furnace. Read 1-inch vs 4-inch before upgrading.
Worked example
A two-storey home has a 3-ton furnace and AC moving about 1,200 CFM through one 20x25x1 hallway grille. Face velocity: 1,200 ÷ 3.47 ≈ 346 fpm. The owners want higher capture during wildfire season. Their technician measures total static pressure near the blower's limit with the current MERV 8, so a 1-inch MERV 13 would likely reduce airflow. Instead, they price a media cabinet for a 20x25x4 MERV 13, which would run at the same face velocity but with several times the media and lower resistance.
Illustrative note (2026-10-29): This example was written on 2026-10-29 for illustration. The home, measurements and recommendation are hypothetical; consult a qualified HVAC technician for your system.
Price and annual cost
20x25x1 filters cost somewhat more than smaller sizes at the same MERV. With a hypothetical $13 filter changed every 90 days, that is about $53 a year; at every 60 days, about $79. Plug in your own numbers at cost per year.
Installation
In a furnace side slot, the arrow points into the furnace. In a ceiling grille, it points up. Large filters flex more, so handle them by the frame and make sure the corners sit fully in the track. If the filter bows inward when the blower runs, it may be overloaded or too restrictive.
Related sizes
- 20x25x4 and 20x25x5: deep media versions for cabinets.
- 20x24x1: an inch shorter, used by some grilles — measure.
- 16x25x1: four inches narrower, common in furnace slots.
When a single return is not enough
A whistle at the grille, a filter that clogs fast, and rooms that never feel comfortable can all point to limited return capacity. Adding a second return is a design change for an HVAC contractor, but it often makes a bigger difference than any filter swap.
Seasonal checks
Because a 20x25 often carries the whole house's air, it collects dust fastest during peak heating and cooling months. Check it at the start of each season and halfway through. If it is fully grey or bowed, change it even if the calendar says it is early.
Storing spares
Large filters bend easily. Store them flat, in their wrap, away from moisture. A crushed corner can leave a gap once installed.
Fibreglass versus pleated at high airflow
A 20x25x1 often runs at higher velocity than smaller filters because it carries the whole house's return air. Flat fibreglass panels cope with that easily but capture little fine dust. Pleated MERV 8 adds a lot of capture for modest resistance. Above that, check airflow: the combination of high velocity and dense 1-inch media is where restriction problems usually show up. If registers weaken or the grille starts whistling after an upgrade, step back down one level and talk to a technician.
Furnace side slots
On many upflow gas furnaces, the 20x25x1 slides into a slot in the side of the return plenum or at the bottom of the cabinet. These slots often have a simple sheet-metal cover that falls off or gets lost. Without it, air pulls in around the filter and through the open slot, bypassing filtration and potentially drawing in air from the furnace room. If yours is missing, ask a technician or check with the furnace maker for a replacement; a proper cover is cheap.
Two-filter setups
Some homes with a 20x25x1 grille also have a filter at the furnace. Confirm which one the installer intended, and remove the other.
Measuring before you buy
Large grilles vary more than small ones. Open the grille door and measure the inside of the frame. If it measures about 20 by 25 inches, a nominal 20x25 fits; if it is closer to 20 by 24, buy that size instead.
Frequently asked questions
Is 20x25x1 enough for a 4-ton system?
Usually not by itself at 1-inch depth; larger systems typically need more return area or a deeper filter.
What does 19.5x24.5 mean?
It is the actual size of a 20x25 nominal filter.
Can I cut a 20x25 to fit a 20x24 slot?
No. Trimming breaks the frame seal. Buy the correct size.