You open the media cabinet next to your furnace and slide the old pleated frame out toward you. It comes out heavy, because a full season of dark dust fills the media. You read the numbers printed on the cardboard edge, and you need a 25x29x4 air filter MERV 8 to get the system breathing again. We stock this exact size in three grades, and we ship it to your door. A fresh four inch filter restores the airflow to your house with one simple swap, and this guide covers the size match, the three grades and the change.
The frame shows 25x29x4 and the tape shows 24.375x28.375x3.625, and both are right
You lay a tape measure across the old frame and the numbers do not match the big print on the cardboard. The frame says 25x29x4. The tape shows 24.375 inches wide, 28.375 inches high and 3.625 inches deep. Both sets of numbers are correct.
The air filter industry uses two sizes for every filter. The nominal size prints on the cardboard frame edge, and it works as the name you order by. The actual size is what the tape measure gives you. The actual size of every 25x29x4 filter we sell is 24.375 x 28.375 x 3.625 inches. That small gap inside the metal track is what lets you pull the old filter out and slide the new one in. Ordering by the tape number gets the wrong filter. Read the printed nominal size on the old frame and order that number. A filter one inch off in either face dimension either refuses to seat or leaves a gap at the edge, and air takes the gap every time.
Three grades on one 3 pack table
We stock the 25x29x4 pleated air filter in three grades, and each grade comes as a box of 3. The table below puts the three side by side.
| Grade | Price per 3 pack | Price per filter | What it captures |
|---|---|---|---|
| MERV 8 | $84.99 | $28.33 | Dust, lint, pollen and pet hair |
| MERV 11 | $104.99 | $35.00 | Adds pet dander and finer dust |
| MERV 13 | $139.99 | $46.66 | Adds smoke and fine particles |
You pay $84.99 for the 25x29x4 MERV 8 Pleated Air Filter 3PK, which is $28.33 per filter. The step up to MERV 11 costs $20.00 more a pack, and the step to MERV 13 costs $55.00 more a pack. Per filter, MERV 11 is $35.00 and MERV 13 is $46.66.
The middle grade balances price against capture. MERV 8 handles dust, lint, pollen and pet hair. A 25x29x4 air filter MERV 11 adds pet dander and finer dust, and a 25x29x4 air filter MERV 13 adds smoke and fine particles. Every grade uses pleated, electrostatically charged media, and the charge helps the pleats hold airborne particles. You select the grade by what you want to pull out of the air, and all three grades share the same frame size and the same fit in the cabinet.
Four inches of pleat change the airflow math
A four inch pleat spreads the media across several times the surface of a one inch panel. The media folds back and forth in a deep zig zag, and that folded area is where the air passes through. More media surface means less resistance at the same grade, and more capacity before the pleats load up.
That depth is why a 25x29x4 loads slower. The extra surface gives a higher grade room to breathe. A higher grade costs less airflow in a four inch cabinet than it would in a one inch slot, and you get better capture without choking the blower.
MERV stands for Minimum Efficiency Reporting Value. The ASHRAE 52.2 test behind it measures what share of particles in each size range a filter captures. Our guide on MERV 11 vs MERV 13 reports that MERV 11 captures about 20 percent of particles between 0.3 and 1 micron, and that MERV 13 captures at least 50 percent of that range.
Your furnace or air handler manual lists the filter resistance the blower is rated for. That manual is the authority for your machine. Read it before you move up a grade, and confirm the maximum filter resistance it allows.
How to change a 25x29x4 filter
- Turn the system off at the thermostat.
- Note the direction of the airflow arrow on the old frame.
- Pull the old filter out of the return cabinet.
- Read the printed size on the edge of the cardboard.
- Slide the new filter in with the arrow pointing toward the blower.
- Write the date on the new frame with a marker.
Every pleated filter frame prints an arrow, and the arrow points toward the blower. The blower pulls the air through the media, and the arrow follows the air. The correct direction keeps the pleats braced against the pull of the blower, and a filter set backward can sag into the cabinet under load. You complete the whole change in a few minutes with clean hands. If you need another size for a second return, the MERV 8 Air Filters collection lists every size we stock in that grade.
How often a four inch pleat needs changing
We publish no replacement interval for air filters. The equipment manufacturer sets it, and the system documentation gives the number for your machine.
General practice puts a four inch pleated filter on a six to twelve month change. A deep cabinet filter holds far more dust than a one inch panel, and the deep pleats give you more time between visits to the furnace. Write the install date on the frame at each change. After two changes you know the interval your own house runs on.
Some conditions inside the house shorten that window. Pets add hair and dander to the circulating air. Pollen season loads the return grilles, smoke adds fine soot, and remodeling dust plugs the pores of the media fast. Check the filter more often during those stretches.
A 3 pack covers eighteen months to three years on the general practice interval. Keep the spare filters in a dry closet. For a different size, our Air Filters collection lists every size we stock, and our guide to MERV 13 in a 20x25x5 furnace cabinet covers the five inch deep media cabinets.
Set the reorder on Subscribe and Save
A replacement slips your mind when twelve months go by between changes. Our Subscribe and Save program fixes that. We apply a standing 5 percent discount to every recurring shipment, and you choose an interval of three months, six months or a year. The box arrives at your door when you plan to change the filter, and the spare filters go into the closet.
On the MERV 8 3 pack, 5 percent off $84.99 is $80.74. The discount applies to every scheduled shipment for as long as the plan runs. You skip the trip to the store and you always have a fresh filter ready to slide into the cabinet. You can cancel the schedule, pause a shipment or change the address at any time from your account.
Frequently Asked Questions
Is a 25x29x4 filter really 24.375x28.375x3.625?
Yes. The actual size is 24.375 x 28.375 x 3.625 inches, and 25x29x4 is the nominal size printed on the frame. Both describe the same filter. The small gap gives the frame clearance inside the metal track. Order by the printed nominal size.
Should I buy MERV 8 or MERV 11 for a 25x29x4 return?
MERV 8 handles household dust, lint, pollen and pet hair. MERV 11 adds pet dander and finer dust. Pick MERV 8 for basic system protection, and step up to MERV 11 when you have pets in the house. Your furnace or air handler manual lists the resistance the blower accepts. Read it before you switch to a tighter media, because the correct match protects the blower motor from strain.
How long does a 25x29x4 MERV 8 filter last?
We publish no replacement interval, because the equipment manufacturer sets it. General practice puts a four inch pleated filter on a six to twelve month change. Pets, heavy pollen, smoke and remodeling dust shorten that span. A dark gray media face means the time to swap has arrived.
How many filters are in the pack?
We sell this size as a box of three. On the general practice interval, a 3 pack covers eighteen months to three years. The shipping box works as storage for the spares. Seal the flaps and keep the box dry, and the loose dust stays off the fresh pleats until you need them.
Order the right filter for your media cabinet
We are Poseidon Filters. We sell the filters, and we do not service furnaces or air handlers. Buy the nominal size printed on your old frame. Order the 25x29x4 MERV 8 Pleated Air Filter 3PK from our store, or step up to MERV 11 or MERV 13 in the same size. We ship the box to your home. Call 855-789-3278 if you want us to confirm the match. We answer the phone Monday to Friday, 9 to 3 Central, and you can email info@poseidonfilters.com with the size off your frame.
Reviewed by the filtration specialists at Poseidon Filters, who match residential and office systems to the correct filter sizes every day.