12x24x1 Furnace Filters: What to Know About the Odd Size
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A 12x24x1 filter has an actual measured size of 11.68 by 23.68 by 0.81 inches, which is normal: nominal size names the slot and actual size is what the filter measures. The practical issue with less common sizes is choice. Our catalog carries 12x24x1 at MERV 11 only, while the more common 20x25x1 is available at MERV 13 from two brands.
It measures 11.68 by 23.68 by 0.81 inches. That gap is normal and intended: the nominal size names the slot, the actual size is the filter. The thing worth planning around with a less common size is not the dimensions but the choice — our catalog carries 12x24x1 at MERV 11 only.
Availability figures here describe our own catalog, not the whole market.
Common filter sizes get treated as a commodity. Less common ones behave differently, and the difference shows up not at the moment of measuring but at the moment of choosing a rating.
The dimensions, and why they look wrong
| Nominal width | 12 in |
|---|---|
| Actual width | 11.68 in |
| Nominal height | 24 in |
| Actual height | 23.68 in |
| Nominal depth | 1 in |
| Actual depth | 0.81 in |
Both the width and the height come in about a third of an inch under nominal, and the depth is nearly a fifth of an inch under. A filter cut to exactly 12 by 24 could not be slid into a 12 by 24 slot, so it is not.
The 12x24x1 we carry
Filtrete Filtrete 12x24x1 Air Filter, MERV 11, MPR 1000
MERV 11, MPR 1000, four to a pack with a stated three-month interval — about a year per pack. It is the only filter in this nominal size in our catalog.
- Nominal Size
- 12x24x1
- Merv Rating
- 11
- Mpr
- 1000
- Replace Months
- 3
- Pack Count
- 4
Get this if
- Systems with a 12x24x1 slot
- Homes that need a year of filters bought in one go
- Anyone who would rather buy a year at a time than reorder quarterly
Look elsewhere if
- Anyone specifically requiring MERV 13, which we do not carry in this size
- Systems with a different nominal slot, where this will not seal properly
The real cost of an uncommon size
Compare what is available in this size against a common one and the pattern is clear. In 20x25x1 our catalog carries MERV 13 from two different brands, which means a genuine like-for-like choice on pack size and price. In 12x24x1 it carries one filter at MERV 11.
| Product | Nominal size | MERV | Pack count | Link |
|---|---|---|---|---|
| Filtrete Filtrete 12x24x1 Air Filter, MERV 11, MPR 1000 | 12x24x1 | 11 | 4 | Check price |
| Filtrete Filtrete 20x25x1 Air Filter, MERV 13, MPR 1900 | 20x25x1 | 13 | 2 | Check price |
| Aerostar Aerostar 20x25x1 Air Filter, MERV 13 | 20x25x1 | 13 | 6 | Check price |
That is worth knowing before you assume you can follow the standard advice. The EPA's guidance is MERV 13 or as high as the system will accommodate — and in a less common size, availability may reach its limit before your blower does.
We do not hold pressure-drop figures for any filter in our catalog, so we cannot tell you how much more restrictive a MERV 13 would be than this MERV 11 in the same size. We also do not track market-wide availability, so the absence of a 12x24x1 MERV 13 here reflects our catalog rather than the market.
Ordering without getting it wrong
Four checks, in order.
Read the nominal size off the old frame
It is the only figure that reliably identifies what fits, and it is printed on every filter.
What to look for: Three numbers on the cardboard edge, usually in the form 12x24x1.
Confirm the depth as well as the face
A one-inch filter and a four-inch filter can share face dimensions and are not interchangeable.
What to look for: The third number. Everything on this page is a one-inch filter.
Take the best MERV available in your size
Higher capture is worth having, but only among filters that physically fit.
What to look for: The highest MERV offered in your nominal size that your system tolerates without airflow symptoms.
Buy the pack that matches your real interval
An uncommon size is harder to replace in a hurry, which makes having a spare on hand worth more than usual.
What to look for: Pack count times the stated interval. The four-pack here covers about twelve months at three months per filter.
For the general version of the nominal-versus-actual problem, including how the gap differs between two filters of the same nominal size, see nominal versus actual furnace filter sizes.
The short version
- 12x24x1 is a nominal name; the filter measures 11.68 by 23.68 by 0.81 inches.
- Never order by tape measure — order by the nominal size printed on the old filter.
- Less common sizes offer fewer MERV options, which is the real constraint.
- The EPA's guidance is MERV 13 or as high as the system will accommodate; availability may set the ceiling before your blower does.
Sources
- Dehumidifiers — ENERGY STAR
That dehumidifier capacity is measured in pints per 24 hours, and that the capacity needed depends on the size of the space and the conditions in it.
- Energy Conservation Program: Test Procedure for Dehumidifiers — US Department of Energy (Federal Register)
That a dehumidifier's capacity is defined by federal test procedure as the volume of water in pints removed from ambient air per day, normalized to a standard ambient temperature and relative humidity.
- A Brief Guide to Mold, Moisture and Your Home — US Environmental Protection Agency
That indoor humidity should be kept below 60 percent, ideally between 30 and 50 percent relative humidity.
- Guide to Air Cleaners in the Home — US Environmental Protection Agency
That a portable air cleaner should have a clean air delivery rate (CADR) large enough for the size of the room, that a higher CADR filters more particles and serves a larger area, and that most portable air cleaners carry a CADR rating on an AHAM label.
- Guide to Air Cleaners in the Home (room sizing) — US Environmental Protection Agency
That the room area should be compared against the maximum recommended room size on the packaging; that an open floor plan means considering the entire space the cleaner would serve; and that ceilings above 8 feet mean sizing up.
- Guide to Air Cleaners in the Home (furnace filters) — US Environmental Protection Agency
That a furnace or HVAC filter should be rated at least MERV 13, or as high as the system will accommodate, and that a dirty or overloaded filter does not work well.
- Air Changes per Hour FAQ — AHAM Verifide
That the AHAM Verifide seal gives a suggested room size based on 4.8 air changes per hour, alongside reduction rates for tobacco smoke, dust and pollen known as clean air delivery rates (CADR), and that certified units are independently tested by a third-party laboratory.
- Air Filtration Standards — AHAM Verifide
That AHAM's standard for measuring an air cleaner's efficacy is the Clean Air Delivery Rate (CADR), which indicates the volume of filtered air delivered with separate scores for tobacco smoke, pollen and dust, and that a higher CADR means faster filtering.