You stand at the return grille holding a gray, dirty cartridge in your hand. You read the cardboard frame and see it is a merv 8 filter. The question is whether you reorder the same merv8 filter or step up to a higher rating. A pleated air filter traps particles as the furnace blower moves air through the house. You have to match the cartridge rating to the blower strength.

Water Filters FAST ships the MERV 8 rating by the box. It lists the 16x25x4 merv 8 filter at $59.99 for a box of three. It lists the 24x24x4 merv 8 filter at $69.99 for a box of three. It also stocks the 10x10x1 merv 8 filter at $49.99 for a box of six. All three boxes are in stock today.

Written by the Water Filters FAST filter experts. Reviewed by Phil McNamara, owner, Water Filters FAST. Updated September 2026.

What a merv 8 filter catches

The MERV rating measures how efficiently a cartridge traps particles in the air. A higher number means the pleated air filter catches smaller particles. An article from Trane states that a MERV 8 rating catches particles measuring 3 to 10 microns in size. This micron range covers pollen, larger dust particles, dust mites, mold spores, and lint.

A resource from Filterbuy provides the exact percentages for this capture rate. A MERV 8 captures at least 70 percent of particles in the 3 to 10 micron range. It also captures at least 20 percent of particles in the smaller 1 to 3 micron range. These numbers represent the starting capability of a brand new cartridge.

Water Filters FAST notes that the initial efficiency of its MERV 8 cartridges is 74.3 percent for particles 7.0 to 10.0 microns in size. After a few complete air changes in the house, the efficiency raises greatly. These replacement cartridges are electrostatically charged. This electrostatic charge means that the cartridge acts as a magnet for larger airborne particles. The magnetic action pulls dirt out of the air before it reaches the blower motor.

Compare ratings and pressure drop

You need to compare the cartridge capabilities to decide between a merv 8 vs merv 11. The MERV 11 catches smaller items like pet dander and fine dust. The tradeoff for better filtration is a higher pressure drop across the frame. The pressure drop measures airflow resistance through the pleated material.

Engineers measure pressure drop in inches of water gauge. This unit is a measure of how hard the blower has to pull to move air through the cartridge. A smaller number means an easier pull. A higher number means the blower works harder to maintain household airflow. The table below compares the ratings at an airflow rate of 1,000 cubic feet per minute.

Filterbuy lists these pressure numbers for a 20 by 25 inch frame. Notice how the depth changes the airflow resistance. A four inch MERV 11 cartridge creates 0.08 inches of water gauge of resistance. A one inch MERV 8 creates 0.10 inches of water gauge of resistance. A four inch MERV 11 filter is easier for your system to pull air through than a one inch MERV 8. The deeper pleats offer more surface area for the air to pass through.

The physical return check

Before you order a replacement box, you must perform a physical check at your furnace return. You need to know the depth of your housing. This physical check takes about two minutes to complete. Open the return grille on the wall or locate the filter housing at the furnace base.

Slide the old cartridge out of the housing and read the label. The label on the old part overrules any table. Read the size printed on the old cardboard frame. Measure the depth of the housing with a tape measure. You will find that the housing measures about one inch deep or about four inches deep.

Count the return grilles in your house. A home with an older furnace on a single small return requires a cartridge that allows easy airflow. A housing that fits a four inch cartridge changes the airflow math. You can step up to a higher MERV rating with a four inch cartridge because the deeper pleats lower the airflow resistance.

When to choose a MERV 8 and when to step up

Some heating and cooling units are unable to use higher MERV rated cartridges. Trane notes that not every system can handle a MERV 13, because it can restrict airflow. You want to choose the MERV 8 when an older system runs on a single small return.

Trane and Filterbuy both recommend the MERV 8 when nobody in the home has allergies or asthma. You also select this rating when there are no shedding pets in the house. The MERV 8 balances dust capture with low airflow resistance for standard houses. It provides plenty of filtration for an older furnace with basic airflow needs.

You step up to a MERV 11 when you have shedding pets in the house. You also step up to a MERV 11 if someone in the home has allergies or asthma. A four inch housing allows you to use a MERV 11 without straining the blower motor. You can shop the MERV 11 collection or read the Water Filters FAST MERV 11 filter buying guide to study the higher rating.

Nominal size and actual size differences

You will see two sets of numbers on a replacement box. The nominal size represents the rounded numbers that simplify your shopping trip. The actual size lists the exact physical measurements of the cardboard frame. The actual size is what physically fits inside the housing. A nominal size is always slightly larger than the actual physical frame. If the manufacturer made the actual frame equal to the nominal numbers, the cartridge would jam in the housing.

Water Filters FAST lists both sizes for the three primary boxes it ships. The 16x25x4 nominal cartridge measures 15.5 by 24.5 by 3.625 inches actual size. A box of three costs $59.99, which breaks down to $20.00 a cartridge.

The 24x24x4 nominal cartridge measures 23.375 by 23.375 by 3.625 inches actual size. A box of three costs $69.99, which breaks down to $23.33 a cartridge.

The smaller 10x10x1 nominal cartridge measures 9.5 by 9.5 by 0.75 inches actual size. A box of six costs $49.99, which breaks down to $8.33 a cartridge.

You start your search by reading the nominal size on your old frame. Then you check the actual size on the product page to confirm the match. You can browse all available shapes on the general air filters collection page.

How to replace your pleated air filter

A new cartridge protects your furnace components and keeps the conditioned air moving smoothly. You perform this maintenance task entirely at the return grille or furnace housing.

1. Read the size printed on the old cardboard frame to get your starting measurement.
2. Measure the housing depth with a tape measure to confirm the space accommodates the physical frame.
3. Note the airflow arrow direction printed on the old cartridge before you pull it out.
4. Slide the new cartridge into the housing with the arrow pointing the same way.
5. Write the installation date on the outside frame of the new cartridge with a permanent marker.
6. Set a calendar reminder inside the 30 to 90 day window to check the cartridge status again.

Arithmetic for the replacement schedule

Trane states that homeowners generally need to replace these cartridges every 30 to 90 days. A clogged cartridge blocks air and forces the blower to work too hard to move air through the house. Buying by the box keeps replacements ready when the 90 days pass. You calculate the price per cartridge to see the long term savings.

Water Filters FAST sells the 16x25x4 size at $59.99 for a box of three. This arithmetic works out to $19.997 per unit, which rounds to $20.00 a cartridge. When you change the cartridge every 90 days, a box of three covers nine continuous months of furnace use. The 24x24x4 size sits at $69.99 for a box of three. That box price works out to $23.33 a cartridge for nine months of coverage.

The smaller 10x10x1 size comes in a larger pack. Water Filters FAST lists it at $49.99 for a box of six. That box divides out to $8.33 a cartridge. A box of six covers eighteen months of operation when you stick to the 90 day schedule. Stocking the correct box prevents you from running an exhausted cartridge too long.

Common questions about your furnace filter

Is a MERV 8 filter good enough for a home?

A MERV 8 rating works for a home with no shedding pets and no residents with allergies. It protects the blower and catches basic household dust. It also functions well for older furnaces with limited airflow capacity.

What does MERV 8 capture?

This rating captures pollen, larger dust particles, dust mites, mold spores, and lint. It handles particles measuring 3 to 10 microns with at least 70 percent efficiency. The electrostatic charge acts as a magnet to grab these larger airborne particles.

MERV 8 or MERV 11 for an older furnace?

An older furnace typically requires the lower airflow resistance of a MERV 8 filter. Older systems often run on a single small return. A MERV 11 can restrict airflow and strain an older blower motor pulling through a small duct.

How often do I change a MERV 8 filter?

You need to replace these cartridges every 30 to 90 days. A home with constant dust or open windows requires changes closer to the 30 day mark. A clean house with minimal foot traffic can wait the full 90 days.

Does a MERV 8 filter restrict airflow?

Every pleated air filter restricts airflow to some degree. The MERV 8 cartridge limits airflow less than higher ratings do. A one inch MERV 8 creates a pressure drop of 0.10 inches of water gauge at a flow of 1,000 cubic feet per minute.

What size do I order if the frame says 16x25x4?

You order the nominal size of 16x25x4 for your replacement part. The actual measurements of that cartridge will be slightly smaller at 15.5 by 24.5 by 3.625 inches. The actual size lets the frame slide easily into the narrow housing.

Order your replacement filters today

Your furnace blower needs a clean pleated air filter to push conditioned air through your home. Water Filters FAST stocks replacement boxes today. It lists the 16x25x4 merv 8 at $59.99 for a box of three. It lists the 24x24x4 merv 8 at $69.99 for a box of three. It also stocks the 10x10x1 merv 8 at $49.99 for a box of six.

Shipping is free over $75, and orders under that carry a $5 flat rate. A 30-day return policy and the Water Filters FAST satisfaction guarantee back every order. Call 855-789-FAST and the filter experts will confirm the match before you buy.