18x20x4 Koch MicroMAX PL4S Header Style MERV 14 120-625-1820 (3 Filters)

The size is printed on the edge of your current filter. Read the nominal size — for example, 16x25x1 — and order that size. The actual filter will measure about 1/2″ less than the nominal size on each dimension. That's industry standard and is how the filter slides into the slot without binding.

If the printed size is worn off, measure the filter slot itself — not the old filter — and round up to the nearest standard size. Our How to Measure guide walks through it step by step.

You're covered by our Filter Fit Guarantee. If the filter doesn't fit your system as expected, contact us within 30 days and we'll make it right — including custom-sized replacements for non-standard slots. One flat $30 fee covers exchanges under the guarantee, and we'll walk through sizing with you so the replacement fits the first time.

Custom filters are made to specification down to 1/8″ precision — useful for older homes, clinical spaces, schools, and commercial AHUs with non-standard slot dimensions. Available in every MERV rating from 1–4 through 13, plus washable and extended-surface options.

If you don't see your size listed in our standard lineup, contact us or request a quote and we'll confirm what's available in the media and efficiency you need.

MERV (Minimum Efficiency Reporting Value) is the industry-standard scale for how effectively a filter captures airborne particles. Higher numbers capture smaller particles. Most homes do well on MERV 8 to MERV 11. MERV 13 is the step up for pets, allergies, or wildfire smoke exposure — if your HVAC system supports it.

The MERV Rating Explained guide has the full breakdown, or take the 60-second MERV quiz for a personalized recommendation.

Not when it's matched properly. Higher MERV filters do add airflow resistance, but most modern furnaces and air handlers handle MERV 11 comfortably, and many support MERV 13 — especially systems with 4″ or deeper media cabinets. The filter slot depth matters as much as the MERV rating.

Older systems (pre-2000) or tight 1″ slots may struggle with MERV 13. If airflow feels weak from vents, the blower sounds strained, or the coil ices up, step back down to MERV 10 or MERV 11. When in doubt, upgrade in steps — MERV 8 → 11 → 13 — and monitor for a cycle before moving up again.

MERV is the universal industry standard, published through ASHRAE 52.2 testing and used by HVAC professionals worldwide. MPR (Micro-Particle Rating) is 3M's internal scale for their Filtrete line. FPR (Filter Performance Rating) is Home Depot's proprietary rating for filters sold in their stores.

MPR and FPR roughly correlate to MERV within their own brand lineups, but they aren't directly comparable across retailers. A Filtrete MPR 1900 is roughly MERV 13; an FPR 10 is also roughly MERV 13. Always check the MERV rating for an apples-to-apples comparison.

Standard stocked filters ship within 1–2 business days from the warehouse nearest to your shipping address. Most orders arrive in 2–5 business days via UPS or FedEx. Custom-sized filters are made to order and have their own production lead time listed in the product description.

Current production and shipping timelines are always live on our production schedule page.

We ship to Alaska and Hawaii — shipping is calculated at checkout based on the address. Free shipping on orders $50+ applies to the lower 48 contiguous states only; AK and HI orders ship at carrier rates.

PO boxes cannot be accepted. Filter boxes are physically oversized and our carriers (UPS and FedEx) won't deliver to PO boxes at this size. You'll need a street address, business address, or pickup location for delivery.

Unopened stocked filters can be returned within 30 days under our refund policy. Custom-sized filters are made to your specification and aren't returnable unless they arrived damaged or incorrectly sized from us.

If a filter arrives damaged or doesn't match what was ordered, submit a claim and we'll replace it — no back-and-forth.

Most filters ship on recurring delivery at a 10% discount — cadences from every month through every 6 months, plus an annual option at 5% off. Pick the interval that matches your change-out schedule on the product page, and we'll auto-ship on time with no reordering required.

You're only charged when an order ships, and the discount applies to every delivery — not just the first one.

Yes, any time. Pause, skip a delivery, change the cadence, swap the filter, update the shipping address, or cancel entirely — all from your account dashboard. No emails required, no hold times, no cancellation fees. See our subscription policy for the full details.

Yes — bulk and volume pricing is available for facilities, multifamily properties, fleets, and any organization ordering in standing rotation. We work with schools, clinics, property management companies, government agencies, and industrial facilities on recurring orders with negotiated pricing.

Request a quote with your filter specs and estimated annual volume, or visit our Commercial Services page to see the full commercial lineup.

Yes. We work with schools, government agencies, non-profits, and resale-exempt buyers regularly. Submit your certificate through our tax-exempt purchases page and we'll set up your account so future orders ship tax-free with no repeated paperwork. Purchase orders and government procurement are supported for established accounts.

Yes. Every filter we stock comes from a manufacturer we've vetted — AAF Flanders, Koch, NaturalAire, PrimeShield, and a short list of others. We don't operate as a marketplace, so there's no third-party seller swapping inventory behind the listing.

If you've had issues with counterfeit or low-quality filters from other sellers, that's one of the reasons customers switch to us. Real brands, same specs every time, same source every time.

Call 866-469-8556 Monday–Friday, 6 AM – 5 PM PST. Text 425-515-2727 anytime — we reply next business day. Email support@rememberthefilter.com for written support, including tax-exempt setup, bulk orders, and order status.

Full options — including claims, price match, and quote requests — live on our contact page.