HVAC filter sizing guide
Measure your filter.
Choose the right replacement.
Read the size. Check the frame. Match the cabinet.
Start with the dimensions on the old filter, then measure its outer frame. For a dedicated filter cabinet or rail system, start with the cabinet model and replacement filter number.
Choose ordinary framed panels by dimensions and equipment requirements. Choose dedicated filters by cabinet model and filter number.
Find your starting pointMeasure the whole frame
- Read the nominal sizeUse the label to find candidates.
- Compare the actual sizeMeasure both sides and the full depth.
- Match your equipmentCheck the cabinet and filtration requirements.
For replaceable filters in home central HVAC systems, including furnaces and central air conditioners. For washable mini-split screens or portable air purifier filters, use the appliance-specific instructions.
Reviewed .
1. Start with your filter’s construction
| What you have | What to record | How to choose |
|---|---|---|
| A panel filter with a complete rigid frame and three printed dimensions | Nominal size, actual size and filtration rating | Search the nominal size, then compare the actual dimensions and equipment requirements. |
| A filter in a dedicated cabinet, on rails, or expanded into a separate frame | Cabinet brand/model and the old filter number | Use the manufacturer’s replacement list for that cabinet, including any frame or upgrade-kit requirements. |
Filters may sit behind a return grille, in a rack beside the furnace or in a separate cabinet. Check the manual for the filter location; the installer may also determine where it goes. Carrier: filter locations
AprilAire’s official shop lets you browse by filter model, rating, nominal size and air cleaner model. Compatible filters may also be listed inside the cabinet door. That is a direct starting point for a dedicated system. AprilAire: find a replacement filter
2. Use nominal size to search and actual size to check fit
Nominal size is the product’s size name. Actual size is the physical measurement of the filter. A product page may show both. AprilAire: size definitions
| Official example | Nominal size | Published actual size | How to use it |
|---|---|---|---|
| Nordic Pure measuring-guide example | 16 × 25 × 1 | 15.5 × 24.5 × 0.75 | Search the nominal size, then compare actual dimensions. |
| AprilAire 213 dedicated filter | 20 H × 25 W × 4 D | 20.6 H × 26.63 W × 4 D | Match the cabinet and installation structure first, then check dimensions. |
These illustrate how to read dimensions. Check each product page for its compatible equipment. Copy the manufacturer’s actual measurements directly: subtracting half an inch from every nominal dimension does not work for every construction.
3. Have the old filter? Measure in four steps
- Shut down the system, then remove the filter. Follow the equipment’s shutdown/power-off instructions. Wait for the fan to stop before opening the user-serviceable filter cover. Photograph the label and airflow arrow before removal.
- Photograph the edge label. Record the brand, filter number, nominal size and MERV/MPR rating. Photograph a separate cabinet model label too.
- Measure both outer-frame sides and the depth. Measure end to end across the complete frame, including its thickness. Write down the original readings. If the frame is bent or fitted poorly, use the manual/cabinet route below.
- Open the candidate product’s specifications. Compare nominal size, Actual Size, identifiers and equipment requirements. Width/height or length/width order may vary; map each side and depth separately. Use the airflow arrow to determine installation direction.
Carrier’s replacement instructions cover shutting down, recording the old size, following airflow direction and securing the cover. Use your own equipment instructions for removal and installation. Carrier: filter replacement steps
| Record | Your details |
|---|---|
| Installation location | For example: hallway return grille or cabinet beside the furnace. |
| Cabinet model or filter number | Copy the label exactly. Leave blank if your ordinary framed panel has no part number. |
| Nominal size | ___ × ___ × ___ inches |
| Actual size | Side 1: ___; side 2: ___; depth: ___; unit: ___ |
| Equipment-approved filter specification | Record the manual’s requirement or the installer’s confirmation, including its source. |
Keep a separate record for each filter location in your home and count replacement quantities by location.
4. Changing brands or missing a label? Follow your situation
| Your situation | Next step | Ready when |
|---|---|---|
| The old filter fits and the label is clear; you want a different brand | Keep the nominal size and compare the new brand’s actual dimensions, filtration specification and compatibility notes. | The candidate meets equipment requirements and its actual dimensions are within the manufacturer’s accepted range. |
| The label has faded, but the frame is intact and fits properly | Measure all three dimensions. Check the cabinet label, manual or previous order for the nominal size or part number. | The product specifications can be matched to your equipment requirements. |
| The old filter is missing | Start with the cabinet/equipment model. If measurement is needed, measure the internal filter seat and supporting rails; send photos and all three measurements to the manufacturer or installer. | You have a corresponding replacement number or explicit actual-size requirements. |
| The new filter jams, bends, sits loosely or leaves obvious edge gaps | Remove it and recheck all three dimensions, the support position and cabinet model. Choose the correct specification or arrange an exchange. | The frame sits properly, its edges fit snugly and the cover closes normally. |
| The listing says 4-inch or 5-inch, unlike the old filter’s label | Compare actual depth and check the specified cabinet model, rails or upgrade kit. | The manufacturer’s instructions cover your installation combination. |
If you can only measure the slot, use those readings to ask the manufacturer for the correct specification. The filter seat, supports and installation clearance affect the required frame size. Ask for an explicit recommended size rather than applying one universal clearance.
Read thick-filter listings closely. Nordic Pure’s 16×25 category includes actual depths of 3⅝, 4⅜, 4⅞ and 5¼ inches. Open the individual product for its dimensions. Nordic Pure: thick-filter size options
5. Check the filtration rating as well as the size
MERV and MPR describe filtration performance; dimensions describe installation space. Check these separately when ordering.
For routine replacement, start with the filter specification confirmed by the equipment manual or installer. For improved fine-particle filtration, EPA recommends MERV 13 or the highest rating the system fan and filter slot can accommodate. Confirm the appropriate product against equipment requirements and any needed professional assessment. EPA: guide to home air cleaners
If the manual does not specify an allowed rating, send the manufacturer or installer your equipment model, cabinet model and candidate product page. Ask whether that product’s airflow and pressure-drop requirements suit the system.
6. Check your order, then check the installation
| Check | Ready to buy when |
|---|---|
| Construction | You have identified a complete framed panel, an expandable cabinet filter or an insert for a reusable frame. |
| Dimensions and identifiers | Both sides, depth, units and any required filter/cabinet numbers are checked. |
| Installation accessories | Any required rails, outer frame or upgrade kit are already present or included in the order. |
| Filtration specification | It meets the equipment manual or professional confirmation for this product. |
| Quantity and delivered price | Count each installation location by size and check how many filters come in the pack. |
| Return conditions | For a new brand or construction, check the rules for opening, trial fitting and returns. |
On arrival, check the label and condition. Remove the packaging as instructed and keep the supporting frame. Point the Airflow arrow along the airflow path toward the blower, seat the frame naturally and close the cover. Restart once everything is fitted. Filtrete: installation steps
Your final check: an intact frame, properly seated supports, snug edges and a closed cover. EPA also advises a snug fit without crushing or bending the frame. EPA: filter installation advice
Common questions
My label says 16×25×1 but the filter measures smaller. Is it wrong?
Check the product’s Actual Size. Nominal and actual dimensions often differ on ordinary framed filters. Compare the actual measurements and equipment requirements before deciding.
Are 20×25 and 25×20 the same size?
If depth is identical, the first two numbers may simply list the sides in a different order. Check both actual sides, construction and installation direction. Follow the model-specific instructions for rails and seals.
Can I replace a 4-inch filter with a 5-inch filter?
Compare actual depth and the compatible cabinet models. Brand size names can differ; choose a product that matches your cabinet and installation structure.
Without an old filter, can I measure the outside of the return grille?
Measure the internal seat that actually supports the filter and record the equipment/cabinet model. The decorative grille’s outside edge describes the grille, while filter selection needs the internal seat information.
What if the cover will not close?
Keep the system off, remove the filter and check depth, rails and airflow direction. Fit the correct filter and close the cover normally before restarting.