Before you begin. This procedure is general reference information, not professional advice. Equipment varies by manufacturer, model, and year. Always follow the documentation supplied with your equipment — where it differs from this page, follow the manufacturer. Verify that you have the tools, the working conditions, and the ability to do this task safely. If anything is unclear, if the equipment is damaged, or if the task exceeds your experience, stop and use a qualified professional. You are responsible for your own safety and for the outcome. This site accepts no liability for any loss, damage, or injury arising from the use of this information.
Specifications marked unverified could not be traced to a manufacturer source. Do not rely on them — obtain the value from your equipment documentation.
Summary
Two things identify a filter: the part number printed on its own frame or housing, and its own physical dimensions. Between them, they let you test any “compatible,” “generic,” or “will fit” claim before you spend money on a filter that turns out to be the wrong size or the wrong shape. This page covers HVAC and furnace filters, refrigerator and other water filters, cabin air filters, engine air filters, vacuum filters, and pool or spa filters — anywhere a reader stands in front of a wall of near-identical boxes trying to work out which one is actually theirs.
What this page will not do is hand you a cross-reference chart mapping one brand's part number to another's. A chart like that is either the manufacturer's own parts list — which you can get directly, for your own equipment, and which this page shows you how to use — or it is a third party's compilation, which is unverified by definition and which we will not publish as fact. What you get instead is the method: read the marking, take the measurement, and compare both against any candidate before you buy or install it. The filtration rating and the replacement interval for your specific equipment stay on that equipment's own page, not here.
Necessary equipment
| Item | Note |
|---|---|
| Tape measure or ruler | Measure the length and the width of a panel filter with this tool. |
| Caliper | A caliper gives you the depth and the diameter of the filter. |
| Camera or phone | Use the camera to photograph the part number on the filter. |
| Notebook or equipment record | Record the part number and your own measurement here. |
| Soft cloth | Wipe the dust from a used filter with this cloth. |
| Work gloves | Wear the gloves before you handle a metal filter frame. |
| Dust mask optional | Wear the mask before you handle a used filter. |
| Magnifying glass optional | Use this tool to read small or worn print on the filter. |
| Candidate filter material | This is the filter that you want to compare with your own filter. |
Read the marking on the filter
Estimated 6 min to 16 min
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
WEAR GLOVES BEFORE YOU HANDLE A METAL FILTER FRAME.
A sharp edge on the frame can cut your hand.
- Find the filter that you want to identify.<1mReport a problem with step 1, Read the marking on the filter
- See your own equipment's procedure if the filter is not out yet.
WEAR A DUST MASK BEFORE YOU WIPE A USED FILTER.
Dust and trapped allergens can irritate your eyes and your airway.
- Wipe the dust from a used filter with a dry cloth.2mReport a problem with step 2, Read the marking on the filter
- Look at the frame or the housing of the filter for printed text.<1mReport a problem with step 3, Read the marking on the filter
- Find a string of letters and numbers near the manufacturer's name or logo.<1mReport a problem with step 4, Read the marking on the filter
- Record every character of the part number exactly.<1mReport a problem with step 5, Read the marking on the filter
- A letter after a number is part of the part number.
- Photograph the part number for your own record.<1mReport a problem with step 6, Read the marking on the filter
- If no part number is visible, find the model number of your equipment instead.2mReport a problem with step 7, Read the marking on the filter
- The model number identifies the correct filter through the manufacturer's own parts list.
Measure the filter
Estimated 9 min to 18 min
Source: Filtrete™ Brand Products — FAQs · tier 1 · accessed 2026-08-04
- Measure the length and the width of a panel filter with a tape measure.2mReport a problem with step 1, Measure the filter
- Measure the depth of a panel filter with a caliper.2mReport a problem with step 2, Measure the filter
- For a cartridge filter, measure the outer diameter and the height with a caliper.2mReport a problem with step 3, Measure the filter
- Measure the diameter of a connector or the width of a thread, if present.1mReport a problem with step 4, Measure the filter
- Record each measurement to the nearest one-eighth of an inch.1mReport a problem with step 5, Measure the filter
- Compare your own measurement with the printed size on the frame.1mReport a problem with step 6, Measure the filter
- The printed size is often a rounded size, not the actual size.
- Filtrete states that the actual size of a filter can differ from its printed size.
- Write the printed size and your own measurement side by side in your record.1mReport a problem with step 7, Measure the filter
Compare a candidate filter
Estimated 11 min to 26 min
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.
Source: How to Avoid Purchasing Counterfeit Filters · tier 1 · accessed 2026-08-04
DO NOT TREAT A CROSS REFERENCE CHART FROM A THIRD PARTY AS A MATCH.
Compare the part number and your own measurement yourself.
- Find the part number of a candidate filter on the page of the seller.3mReport a problem with step 1, Compare a candidate filter
- Compare each character of the candidate part number with your own part number.1mReport a problem with step 2, Compare a candidate filter
- A match of only the rounded size is not a proof of fit.<1mReport a problem with step 3, Compare a candidate filter
- Look for a statement from the candidate filter's own manufacturer, not only the seller.3mReport a problem with step 4, Compare a candidate filter
- A claim from a seller is not evidence by itself.
- The Federal Trade Commission stopped a brake-pad maker for a performance claim it did not prove.
DO NOT INSTALL A CANDIDATE FILTER THAT YOU HAVE NOT MEASURED YOURSELF.
A wrong connector or a wrong size can leak or can block the flow of air or water.
- Measure the candidate filter yourself after it arrives, before you install it.2mReport a problem with step 5, Compare a candidate filter
- Compare each of your own measurements with the measurement of the candidate filter.1mReport a problem with step 6, Compare a candidate filter
- Do not accept a claim of a universal fit in place of your own measurement.<1mReport a problem with step 7, Compare a candidate filter
Record the match
Estimated 2 min (partly estimated)
DO NOT DISCARD YOUR OLD FILTER UNTIL YOU INSTALL THE CANDIDATE FILTER.
A wrong filter can leave your equipment without a working filter.
- Keep your old filter next to the candidate filter until you install it.<1mReport a problem with step 1, Record the match
- Write the correct part number and your own measurement in your equipment record.1mReport a problem with step 2, Record the match
- Repeat this procedure for each filter in your home.Report a problem with step 3, Record the match
Terminology
This page uses one term for each item and holds it throughout, as ASD-STE100 requires. Equipment sellers and other guides use the names below for the same parts.
- part number
- Also called: model number, catalog number, cat number, SKU, OEM number, the number on the filter
- cross reference
- Also called: compatible filter, generic equivalent, aftermarket filter, will-fit filter, universal filter, interchange
- candidate filter
- Also called: replacement filter, generic filter, off-brand filter, cheaper filter
- nominal size
- Also called: printed size, trade size, rounded size, labeled size
- actual size
- Also called: measured size, true size, exact size, real size
When to use a professional
Stop if you cannot find a part number on the filter or a model number on your equipment. Contact the manufacturer's customer service with a photograph of the filter.
Stop if the part number of a candidate filter does not match your own part number. Stop if your own measurement does not match the measurement of the candidate filter either. Buy the manufacturer's own part instead.
Stop if the filter is part of a system with its own safety rating. A filter with a fire rating and a filter in a medical device are two examples. Contact the manufacturer of the system before you replace any filter with a candidate filter.
Do not install a candidate filter that you have not measured yourself. A wrong connector or a wrong size can leak or can block the flow of air or water.
Common questions
What size filter do I need?
Measure your own filter rather than trusting a size printed on a box or a website. Filter frames are often labeled with a rounded trade size — a 20x25x1 filter, for example — while the exact measured size can be smaller. If you cannot get to your own filter, the model number of the equipment and the manufacturer's own parts list are more reliable than a guess from the label.
Is a generic or cheaper filter the same as the OEM part?
Sometimes, and sometimes not, and the only way to know is to check the part number and the measurement yourself rather than trust the word “compatible” on a listing. The Federal Trade Commission has taken action against at least one aftermarket auto parts maker for a performance claim it could not back up with evidence — for brake pads, not filters, but the lesson generalizes: a marketing claim is not proof. Compare the numbers, and if you can, measure the candidate part physically before you rely on it.
How do I read a filter part number?
Look at the frame, the housing, or the cartridge of the filter for a printed string of letters and numbers, usually near the manufacturer's name or logo. Copy every character exactly — a letter after a number is often part of the number, and the digit 0 and the letter O are easy to confuse. Photograph the label too, since ink fades and cardboard frames tear.
Why won't this page just give me a cross-reference chart?
Because a chart that maps one company's part number to another company's part number is not something we can verify. The only genuinely reliable cross reference is the manufacturer's own parts list for your specific equipment, or a direct comparison of the part number and the physical dimensions you measure yourself. A chart copied from a retailer's website is a number that has traveled, and this site does not publish a value it cannot trace to its source.
My filter has no visible part number. What do I do?
Find the model number of the equipment instead — a rating plate, a data plate, or a sticker on the unit usually carries it — and use that with the manufacturer's own parts list or support line. A photograph of the equipment's own label, plus your own measurement of the filter, is normally enough for a manufacturer or a retailer to identify the correct filter.
Is a cheaper filter always a worse filter?
Not necessarily, but price by itself tells you nothing about fit or performance. A cheaper filter that carries an exactly matching part number, or that you have measured and found to be the same size, is a reasonable candidate. A cheaper filter you have not checked at all is simply an unknown, and installing an unknown into your equipment is what this page is trying to help you avoid.
Related procedures
- How to find and record the model and serial number of every appliance in the house
- How to measure a pipe or a hose so the fitting you buy actually fits
- Find the exploded parts diagram for a machine you own, and read a part number off it
- Identify a battery from the code printed on it, and find the right replacement
- How to change the cabin air filter in a car when the air conditioning smells or barely blows
- What size blade fits my mower, and how do I check the length, center hole and mounting pattern before I buy a replacement?
- How do I work out what size bolt this is, and what do the lines on the head mean?
- How do I decode the date code on my appliance and find out how old it is
Sources
Every specification on this page is traced to one of these sources. Tier 1 is a manufacturer document, a published standard, or a government safety authority. See Standards for the full tiering rule.
- Tier 1Filtrete™ Brand Products — FAQs
- Tier 1How to Avoid Purchasing Counterfeit Filters
- Tier 1FTC Acts to Stop False and Unsubstantiated Claims for Wagner OEX Brake Pads
Revision history
Current version 1 · updated 2026-08-04 · next review due 2027-08-04
- v1 · 2026-08-04First draft. Publishes no cross-reference chart and no OEM-to-generic part-number mapping: a chart like that is either the manufacturer's own parts list, which the record shows how to use directly, or a third-party compilation, unverified by definition under the independent-creation rule. The nominal-vs-actual size gap and the counterfeit-filter warning signs are sourced to the two manufacturers' own support pages. The FTC's Wagner OEX case is cited, transparently, as a brake-pad example of an unsubstantiated aftermarket equivalence claim, not as a filter-specific finding. Filter type, rating and replacement interval are left to each piece of equipment's own procedure page, per the backlog scope note.
Corrections to this page are listed in the public corrections log. Reports that are credible and not yet verified are listed in the public errata register, and an open one renders above the step it concerns.
Comments and corrections
Found an error? A correction with a manufacturer source is applied and logged publicly. You can also write to corrections@fixchecklist.com.
Every step above carries its own report control, which tags the report with that step so a correction reaches the right place. For anything about the procedure as a whole — the order of the sections, a step that is missing, something that did not match your equipment — use this: report something about this whole procedure.