●  ANY DIMENSION · MERV 1–13 · 1/2″–4″ DEPTH

Custom Size Air Filters — Built to Your Exact Dimensions

Furnace, AC, heat pump, and HVAC custom filters made to spec by AAF Flanders, Koch, and the PrimeShield manufacturer network. Enter your exact size below — we build to the fraction.

Not sure what to measure? Read the 60-second measuring guide →

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Custom Filter Builder

Pick your filter, depth, and size — we'll handle the rest.

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Choose Your Filter & Depth

Select a filter on the left, then choose depth on the right.

Filter Type
Depth

Select a filter first.

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Longer Lead TimeWashable aluminum filters are custom fabricated. Please allow extra production time.
Exact-size build

Built to the dimensions you enter. No rounding, no "close enough."

MERV 1 – MERV 13

Fiberglass airflow up through MERV 13 — full efficiency range in custom sizing.

Case of 12 (or 6 at 4″)

Case pricing keeps custom filters economical on a per-unit basis.

Trusted manufacturers

AAF Flanders, Koch, and the PrimeShield network. No overseas knockoffs.

18 products

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The full custom filter guide

Any dimension. Any MERV. Made to your exact spec.

If your furnace, AC, heat pump, or HVAC system takes a non-standard size, you have two real options: keep cutting down a standard filter (leaking bypass air around the frame) or order the exact dimension you actually need. Below: what counts as custom, how to choose depth and MERV, which of our custom lines fits your application, and everything else you'd want to know before ordering a case of 12.

What counts as custom

If it's not a standard nominal size, it's a custom filter.

Standard nominal sizes are the ones every retailer stocks — 16x20, 16x25, 20x25, and a few dozen others. If your system takes anything outside that list, you're in custom territory. That includes:

✓
Whole-inch sizes that nobody stocks — 13x21x1, 17x19x1, 22x24x1, and similar. Real openings on older furnaces and regional HVAC brands.
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Half-inch and quarter-inch actuals — 19½x21½x1, 15¾x24¾x1. Common on Trane, Carrier, and Lennox air handlers where the actual filter slot is tighter than the nominal label suggests.
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Heat pump air handlers — often take deeper 4″ media with non-standard face dimensions. Rheem, Goodman, and American Standard heat pumps all have their share.
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Commercial rooftop units (RTUs) — big face dimensions (24x24, 20x24) but often in uncommon depth combinations, or pre-filter + final-filter stacks.
✓
Media air cleaner cabinets — Honeywell, Aprilaire, Trion, Lennox PureAir. Some cabinets take standard OEM sizes; many take custom dimensions depending on install year and model.
Actual vs. nominal

A "20x25x1" standard filter isn't actually 20 by 25 by 1. Nominal sizes are rounded. Actuals are typically ⅜″ to ½″ smaller per dimension so the filter slides into the slot without binding.

For custom orders, tell us the exact size you want the filter built to. If your opening measures 19½ × 21¾, that's what we build. We don't add our own rounding.

Rule of thumb: measure the filter slot (not the old filter) and subtract ¼″ from each face dimension so the new filter slides freely without leaving a gap.

Don't cut a standard filter down.

It's the most common shortcut we see, and it defeats the filter. When you trim a standard filter to fit a smaller opening, you expose the media edge, weaken the frame, and — most importantly — create bypass air gaps around whatever you couldn't trim cleanly. That bypass air carries unfiltered dust, pet dander, and pollen straight into the blower and evaporator coil.

A $20 custom-sized filter we build for you and ship free beats a $15 standard filter that's been hacked into the slot. Every time.

Choosing depth

1″, 2″, or 4″ — what your system actually fits.

Depth is set by your filter slot, not preference. Measure it. Putting a 2″ filter in a 1″ slot doesn't work — and a 1″ in a 2″ slot lets air slip around the frame.

Slim
1/2″ – 1″

Most residential furnaces and air handlers. The default slot depth on gas furnaces from the '90s through today.

Replace: every 1–3 months.

Standard-deep
2″

Common on heat pump air handlers and light commercial. More media surface area than a 1″, so it loads more dust before restricting airflow.

Replace: every 3–6 months.

Media cabinet
4″

Dedicated media air cleaners — Honeywell, Aprilaire, Trion, Lennox PureAir. Deep media, high capacity, low pressure drop. Case of 6.

Replace: every 6–12 months.

Don't guess
Measure it

Pull the existing filter and measure the slot top-to-bottom with a tape. The number printed on the old filter is a starting point, not proof.

Measuring guide →
Choosing MERV

MERV 1, 8, 10, 11, or 13 — pick by equipment, not marketing.

Higher MERV captures smaller particles, but it also resists airflow more. Lower MERV protects equipment with minimal airflow restriction. Match efficiency to what your blower is rated for and what your household actually needs — going higher than necessary strains older motors without delivering benefit.

MERV 1–4
Max airflow

Fiberglass media. Equipment-first filtration — captures lint, large dust, and debris that would foul the blower or evaporator coil.

Best for: older systems sensitive to airflow restriction, equipment-protection use, replaceable-media inserts, commercial prefilters.

MERV 8
Everyday baseline

Captures dust, pollen, lint, and pet dander. The equipment-friendly default for most residential systems with no special air-quality concerns.

MERV 10
Step up

Adds capture of finer dust and some mold spores. A reasonable bump if mild allergies are a factor but MERV 13 feels like overkill.

MERV 11
Allergy homes

Significantly better fine-particle capture. Good balance for households with allergies, asthma, or pets where MERV 13 would restrict airflow too much.

MERV 13
Max residential

Captures bacteria-size particles, smoke, and the smallest airborne allergens. The ceiling we recommend for residential. Confirm your equipment is rated for it.

Confirm your blower can handle your chosen MERV — check the unit's install manual or ask the installer. Most modern variable-speed systems are fine up to MERV 13 at 1″ or 2″; older PSC motors may struggle with higher MERV.

Which line is right for your system

Matching the custom line to the application.

Quick guide to when each of our custom lines makes sense. If you're still on the fence, call 866-469-8556 — we'll walk through your equipment and pick with you.

AAF Flanders

Deep bench, engineering pedigree

One of the oldest filtration manufacturers in the country. Strong choice when you want a well-known brand name on a custom build and the confidence of a full commercial catalog behind it.

Best for: residential custom sizes, light commercial, media air cleaner cabinets.

Koch

Four U.S. plants, regional ship

Manufactured at four U.S. facilities. Good option when you want regional shipping routing and established commercial-grade build quality across the residential and commercial range.

Best for: custom residential, commercial rooftop units, anywhere regional shipping speed matters.

NaturalAire

Individually sealed

Each filter is individually sealed before shipping. Helpful for customers who store inventory for months at a time or for commercial properties where dust exposure during storage is a concern.

Best for: property managers, long-term inventory, clean-storage environments.

PrimeShield

Built to RTF specs

Produced by the same partner network that builds for national brands, to our specifications. Full MERV ladder (MAX Airflow, Essential, Enhanced, Advanced, Elite) in custom sizing.

Best for: customers who want name-brand build quality without name-brand markup.

Exact Filter

True-to-dimension build

Our in-house line built to the exact measurement you enter — no nominal rounding. Best when you want the filter to match your measured opening to the fraction.

Best for: odd-fraction sizes, old equipment with tight slots, situations where even ⅛″ matters.

Common applications

Where custom-size filters actually show up.

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Heat pumps & air handlers

Heat pump systems often pair with vertical or horizontal air handlers that take 2″ or 4″ depth in dimensions you won't find stocked anywhere. Rheem, American Standard, Goodman, and regional brands all have their share of oddball sizes.

Common pick: MERV 8 or MERV 11 at 2″ for balance of filtration and airflow; MERV 13 at 4″ if you have a media cabinet.

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Older residential furnaces

Furnaces from the '70s, '80s, and early '90s frequently take sizes that modern manufacturers don't bother stocking — 13x21, 17x19, 22x24, and others. The equipment still works fine; you just need filters that match.

Common pick: MERV 1–4 for equipment-sensitive blowers, or MERV 8 at 1″ if airflow allows. Case of 12 for the year.

3

Commercial RTUs & light industrial

Commercial rooftop units on small office buildings, restaurants, and retail often take face dimensions like 20x24 or 24x24 in 2″ or 4″ depth. Multi-filter stacks are common. We build these as cases on a regular replacement cadence for property managers.

Common pick: MERV 8 pre-filter paired with MERV 11 or 13 final, both custom sized.

4

Media air cleaner cabinets

Honeywell, Aprilaire, Trion, Lennox PureAir, and Space-Gard cabinets take 4″ or 5″ deep media. Some match OEM nominals (20x25x5, 16x25x4); others are genuinely custom depending on install year. Replacement is typically 6–12 months.

Common pick: MERV 11 or MERV 13 deep media at 4″ depth, case of 6.

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Return-air grilles (wall & ceiling)

Homes with the filter mounted at the return grille (rather than at the furnace) often have larger face dimensions — 20x30, 25x25, 16x30 — in a slim 1″ profile. Older construction tends toward oddball dimensions here too.

Common pick: MERV 8 or MERV 11 at 1″, higher MERV if residents have allergies.

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Commercial HVAC in mixed-size buildings

Office buildings with multiple HVAC vintages or additions often have every size under the sun across units. One custom order, routed by suite or rooftop, replaces the "hunt for each size" ritual.

Common pick: Consolidate SKUs where possible; build custom for the rest.

Replacement cadence

Custom filters follow the same replacement rules as standard sizes.

Custom construction doesn't extend filter life. What extends filter life is depth — deeper media (2″ or 4″) holds more dust before it restricts airflow, which is why a 4″ media filter runs 6–12 months while a 1″ runs 1–3 months.

Buying a case of 12 means you're set for roughly a year of 1-month changes, or three years of 3-month changes. That's the math.

1″
Every 1–3 months. Shorter intervals for pet households, allergy sufferers, recent construction, or wildfire regions.
2″
Every 3–6 months. More media surface, longer life. Default for most heat pump air handlers.
4″
Every 6–12 months. Media cabinet filters. Longest life per unit; check every 3 months in the first year to learn your system's pace.
Filter Fit Guarantee

$30 / 30 days. If it doesn't fit, we'll remake it or refund.

How it works →
Free shipping $50+

Ships from our distribution network. Lower 48 contiguous states.

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Real people

Mon–Fri, 6 AM – 5 PM PST. Call or text — we answer the phone.

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Commercial & Property Management

Managing dozens of custom sizes across buildings?

We build consolidated custom orders for commercial HVAC, rooftop units, and multi-tenant properties. Case pricing, scheduled delivery, and a single point of contact for every size you need.

Custom filter FAQ

Answers to what people actually ask.

If your question isn't here, call 866-469-8556 or text 425-515-2727.

What's the smallest and largest custom size you can build?

Our custom size range covers 6×6 minimum to 25×30 maximum on most product lines, with 1/2″, 1″, 2″, and 4″ depth options. If your dimension falls outside that range, contact us — we have commercial lines that build larger for RTU and commercial applications.

How long does a custom filter order take to ship?

Custom filters are built to order. Standard production runs 5–10 business days depending on the manufacturer and size, plus transit. Rush production is available on select lines — contact us for expedited timelines.

Why do custom filters ship as a case of 12 (or 6 at 4″ depth)?

Single-unit production on custom dimensions isn't economical — the per-filter cost would be dramatically higher because a production run has the same setup cost whether it produces one filter or twelve. Case pricing keeps custom-size filters competitive with standard-size filters on a per-unit basis. Case counts vary by depth: 1/2″, 1″, and 2″ ship as cases of 12; 4″ ships as a case of 6 due to volume.

Do you round the dimensions I enter, or build to the exact size?

We build to the size you enter. We don't add our own rounding or "adjust for fit." If you want the filter to be a little smaller than the opening so it slides easily, you tell us that size. Most customers measure the slot and subtract 1/4″ from each face dimension.

Which MERV should I choose for a custom filter?

Pick by equipment capability first, then air-quality needs. MERV 1–4 fiberglass is for equipment protection with maximum airflow — good for older blowers sensitive to static pressure. MERV 8 is the everyday default for most residential systems. MERV 10 or 11 is a reasonable step up for allergy households. MERV 13 is the max we recommend for residential, and you should confirm your blower is rated for it. Higher MERV = higher pressure drop, which strains older motors.

Can you make custom washable or electrostatic filters?

Yes. We have custom-size washable filters in addition to fiberglass and pleated media options. Washables last for years with regular cleaning, which makes them popular for hard-to-find sizes where ordering cases of disposables isn't practical. Note that washable filtration efficiency is lower than MERV-rated pleated filters — they're equipment protection first, fine-particle filtration second.

Where are these filters manufactured?

RTF sources through American business partnerships, including manufacturers like AAF Flanders and Koch that operate U.S. production facilities. Sourcing specifics vary by product line — check the individual product page for its manufacturing details. Because origin varies across the collection, we describe sourcing at the product level rather than making a blanket claim across every custom line.

Can I return a custom filter if it doesn't fit?

Yes, through our Filter Fit Guarantee: $30 flat fee, 30-day window. If the filter we built to your spec doesn't fit your equipment, we'll remake it or refund. This exists specifically because custom filters are built to order and the normal "return it unused" policy doesn't apply.

What's the difference between "custom" and "Exact Filter"?

Exact Filter is one of our custom-size product lines — specifically our in-house line that builds to the exact dimension you enter without nominal rounding. "Custom" is the broader category that includes Exact Filter plus our custom builds from AAF Flanders, Koch, NaturalAire, and PrimeShield. If you want the RTF in-house line, choose Exact. If you want a specific manufacturer's custom build, pick that line.

Will a custom filter work in my heat pump?

Yes — heat pump air handlers use standard HVAC filter slots. The only variables are dimension and depth. Measure the slot, pick the MERV that matches your blower capability, and order. Most heat pump applications run MERV 8 or 11 at 2″ depth; premium installations with media cabinets run MERV 13 at 4″.

How do I know which depth my system takes?

Pull the existing filter and measure the slot from front wall to back wall with a tape measure. If the slot is roughly 1″ deep, you need a 1″ filter. If it's 2″, you need a 2″. If you have a dedicated media cabinet bolted to the side of your furnace or air handler, you're typically looking at 4″ or 5″ depth — and those usually have the size and depth printed on the cabinet door.

Can I set up auto-delivery on custom filters?

Subscription and scheduled-reorder options are available on select custom product lines. Since a case of 12 covers most homes for a full year or longer, many customers prefer annual reorders over monthly subscriptions. Contact us if you want to set up a scheduled commercial cadence — we do this routinely for property managers.

Not sure what to order?

Start with our guides.

Three deeper reads that cover measuring, picking MERV, and understanding the size landscape before you commit to a case of 12.

Ready when you are

Build your custom filter in about 60 seconds.

Scroll back up to the custom filter builder, enter your exact dimensions, pick your MERV, and we'll produce your case. Any question, call or text — real people, same day.