What Size Air Filter Do I Need?
The size is almost always printed on the filter already in your system. Here is where to look, how to measure when the print has worn off, and the rounding rule that trips most people up.
Your filter size is printed on the cardboard edge of the filter already in your system — a three-number code like 16x25x1. Read that number and order the same one. Only reach for a tape measure if the print has worn off.
Nominal sizes are an industry-standard rounded label. The actual filter measures roughly half an inch smaller in length and width.
Almost every "what size air filter do I need" search ends the same way: the answer was already written on the filter sitting in the return duct. The problem is that most people never think to look there, and the ones who do often find a faded, greyed-out edge with nothing legible on it.
This guide covers both cases. It takes about sixty seconds if the label is readable, and about three minutes if it is not.
Where the size is printed
On a standard pleated filter, the nominal size is screen-printed along the long side of the cardboard frame, usually in the largest text on the filter. It sits near the airflow arrow, which is the other marking you need to pay attention to.
To find your filter in the first place, look in one of three places:
- The air handler or furnace cabinet. There is usually a slot beside or below the blower compartment, closed with a removable metal cover or a plastic clip.
- A return air grille. In many homes the filter sits behind a large vent in a hallway wall or ceiling, held by two thumb latches. If the grille is bigger than a sheet of paper, that is your filter location.
- An external filter rack. A separate metal box bolted to the return duct, often added during a retrofit. This is the one that catches people out, because the rack size frequently differs from the size named in the equipment manual.
If your system has an external rack or a media cabinet, trust the rack — not the manufacturer's manual. A manual describes the factory-standard installation. A retrofitted rack overrides it, and we flag models prone to this on their individual pages.
Nominal versus actual: the rounding rule
This is the single most common ordering mistake, and it is worth understanding rather than memorising.
Filters are labelled with a nominal size — a rounded figure used across the industry for ordering. The filter is manufactured to an actual size that is roughly half an inch smaller in length and width, so it slides into the slot without binding or buckling.
Here is how the common sizes translate. This is an industry convention, not a per-brand specification, so it holds across manufacturers:
| Nominal size (order this) | Actual size (what it measures) | Common in |
|---|---|---|
| 14x20x1 | 13.5 x 19.5 x 0.75 | Compact air handlers, closet units |
| 16x20x1 | 15.5 x 19.5 x 0.75 | Small furnaces, apartment systems |
| 16x25x1 | 15.5 x 24.5 x 0.75 | The most common residential size |
| 20x20x1 | 19.5 x 19.5 x 0.75 | Square return grilles |
| 20x25x1 | 19.5 x 24.5 x 0.75 | Larger furnaces, high-airflow systems |
| 25x25x1 | 24.5 x 24.5 x 0.75 | Large ceiling return grilles |
The practical takeaway is short: if your tape measure says 15.5 x 24.5, you need a filter labelled 16x25. Ordering "15.5x24.5" either returns nothing or gets you a filter an inch too small in both directions, which lets unfiltered air bypass the media entirely around the gap.
How to measure when the label is gone
If the print has worn off, or the filter is missing altogether, measure the filter or the slot directly.
- Measure length and width across the face. Use the cardboard frame, edge to edge, not the pleated media inside it.
- Measure the depth. This is the thickness of the frame. Most residential slots are 1 inch, but 2, 4 and 5-inch media cabinets are common on newer systems.
- Round each figure up to the nearest whole inch. 15.6 becomes 16. 24.4 becomes 25 if you measured the slot, or stays 24 if you measured a filter frame that reads 24.5 — when in doubt, measure the slot and subtract nothing.
- Write the three numbers in order: length x width x depth. That is your nominal size.
Measuring the slot rather than the filter is the more reliable method, because a warped or compressed old filter can read up to a quarter-inch small. If the slot measures 16 x 25 x 1, order 16x25x1.
Depth is not optional
Length and width get all the attention, but depth is what determines whether the filter seals at all. A 1-inch filter dropped into a 4-inch media cabinet will sit loose, tilt under airflow, and let most of the air pass around it rather than through it.
| Depth | Typical application | Typical service interval |
|---|---|---|
| 1 inch | Standard furnace slot, return grilles | 60–90 days |
| 2 inch | Slightly upgraded slot, some air handlers | 3–4 months |
| 4 inch | Media cabinet, whole-house filtration | 6–12 months |
| 5 inch | Large media cabinet, high-capacity systems | 6–12 months |
You cannot substitute across depths without an adapter cabinet. If you have a 4-inch cabinet, buying three 1-inch filters and stacking them is not a solution — the frames will not seal against each other and the pressure drop will be wrong.
Which MERV rating to choose
Once you have the size, MERV is the second decision. MERV measures how effectively the media captures particles, on a scale that runs from roughly 1 to 20 for commercial equipment.
| MERV | What it captures | Suitable for |
|---|---|---|
| 5–8 | Dust, lint, pollen, carpet fibres | Standard homes with no allergy concerns |
| 9–11 | Above plus pet dander, fine dust, mould spores | Homes with pets or mild allergies |
| 12–13 | Above plus most bacteria-carrying particles, smoke | Allergy sufferers, systems designed for it |
| 14+ | Fine particulate, commercial-grade filtration | Not appropriate for most residential blowers |
Higher is not automatically better. A denser filter raises static pressure across the system. On a blower that was not sized for it, the result is reduced airflow, longer run times, and in bad cases a frozen evaporator coil. Stay inside the range in your equipment manual — every model page on this site lists it.
How often to change it
A 1-inch pleated filter in an average household runs about 90 days. That figure moves with conditions rather than with the calendar:
- Pets shedding indoors: inspect at 45 days, expect to change at 60.
- Ongoing construction or renovation: check monthly, sometimes sooner.
- Vacant or lightly used property: 120 days is reasonable.
- Heavy heating or cooling season: the filter loads faster simply because the blower runs more hours.
The reliable test is visual rather than temporal. Pull the filter and hold it up to a light. If you cannot see light through the media, it is finished, regardless of how long it has been in. Our guide on how often to change an air filter works through the intervals by depth and household in detail.
Four mistakes that cost money
- Ordering by the measured size. Covered above, and comfortably the most common. Order the rounded nominal figure.
- Installing the airflow arrow backwards. The arrow must point toward the blower, away from the return duct. Reversed, the filter collapses under pressure and sheds fibres into the system.
- Buying the highest MERV on the shelf. Restricting airflow to chase filtration you do not need is a net loss in both comfort and equipment life.
- Assuming all units of one brand share a size. They frequently do, but not reliably. Model-specific pages exist for exactly this reason.
Find the size for your specific model
The fastest route is to look up your equipment directly. Every model page on this site lists the nominal size, the actual size, the recommended MERV range, and the manual number and page the figure came from — so you can verify it yourself rather than taking our word for it.
Model not listed? Request it and we will look up the manual. We publish a size only once we can cite the document and page it came from — see our methodology for how that works.
Frequently asked questions
What size air filter do I need?
Read the nominal size printed on the cardboard edge of the filter currently in your system — a three-number code such as 16x25x1. Order that exact number. Only measure if the print is unreadable, and round each dimension up to the nearest whole inch.
Is the filter size the actual measurement?
No. The printed size is the nominal size, an industry-standard rounded label. A filter labelled 16x25x1 actually measures about 15.5 x 24.5 x 0.75 inches. Always order using the nominal figure, never the measured one.
Where is the filter size printed?
On the long cardboard edge of the filter frame, usually in the largest text, next to the airflow arrow. If the print has worn off, measure the filter slot and round each dimension up to the nearest whole inch.
Can I use a thicker filter than the one that came out?
Only if the slot or cabinet physically accepts that depth. A 1-inch filter will not seal inside a 4-inch media cabinet, and stacking thinner filters does not work because the frames cannot seal against each other.
What MERV rating should I use at home?
MERV 8 to 11 suits most homes. MERV 13 is reasonable for allergy sufferers if the system was designed for it. Above MERV 13 a residential blower usually cannot maintain airflow, which reduces efficiency and can freeze the evaporator coil.
How often should I change a 1-inch air filter?
About every 90 days in an average home, sooner with pets or during construction. The reliable test is visual: hold the filter up to a light, and if no light passes through the media it needs replacing regardless of the date.