20x20x4 Air Filter: Actual Size, MERV Guide & Replacement
Nominal vs actual size
| Nominal (order this) | 20 x 20 x 4 in |
|---|---|
| Actual | 19.5 x 19.5 x 3.63 in (Filterbuy spec) |
A common square 4-inch media-cabinet size for central forced-air furnaces, heat pumps and AC air handlers. One filter serves both heating and cooling. Honeywell and Lennox cabinets of this face size often take a deeper filter, so measure the depth before ordering. Filters are deliberately cut smaller than their nominal label so they slide into the slot; length and width typically run about 1/4 to 1/2 inch under nominal. Actual sizes vary slightly by brand, so compare against the small print on your old frame.
How to measure
- Turn the system off at the thermostat (and the service switch) before pulling the filter.
- Read the frame first: the large print is the nominal size (what you order); small print, if present, is the actual size.
- If there is no label, measure length, width and depth edge to edge with a tape measure.
- Round each measurement up to the nearest whole inch to get the nominal size (e.g. 19.5 x 24.5 x 0.75 → 20x25x1).
- No old filter? Measure the filter slot or cabinet opening the same way.
- Note the airflow arrow: it should point toward the furnace or blower.
MERV 8 vs 11 vs 13
| MERV | Minimum capture (EPA / ASHRAE 52.2) | Good for |
|---|---|---|
| 8 | ≥20% of 1–3 µm, ≥70% of 3–10 µm | Basic dust, pollen, lint; the minimum EPA Indoor airPLUS requires |
| 11 | ≥20% of 0.3–1 µm, ≥65% of 1–3 µm, ≥85% of 3–10 µm | A step up for pets and allergies |
| 13 | ≥50% of 0.3–1 µm, ≥85% of 1–3 µm, ≥90% of 3–10 µm | EPA's recommended level for fine particles (PM2.5), if your system can accommodate it |
EPA notes that higher-MERV filters tend to restrict airflow, and recommends using the highest rating your system fan and filter slot can accommodate. Most systems handle MERV 13 if the filter is replaced frequently. Thin 1-inch slots are the most restrictive; deeper 4-inch media filters generally create less static pressure. Weak airflow from vents, a system that runs longer, or a filter that bows are signs to step down a grade or ask an HVAC technician.
How often to replace
Check monthly; replace when dirty and at least as often as the filter and equipment maker specify. ENERGY STAR's general rule is at least every three months; Filterbuy rates its 4-inch filters for 6 to 12 months.
Equivalent & nearby sizes
There is no exact substitute for a different size: length, width and depth must all match. These nearby sizes are commonly confused with 20x20x4:
FAQs
What is the actual size of a 20x20x4 air filter?
20x20x4 is the nominal (ordering) size. Filterbuy's 20x20x4 measures an actual 19.5 x 19.5 x 3.63 inches. Filterbuy says the half-inch clearance on the face is deliberate, so the filter slides into slots that are slightly out of square without leaving room for air to slip around it.
Does a 20x20x4 fit a Honeywell or Lennox media cabinet?
Often not. Filterbuy says Honeywell F100, F150 and F200 media cabinets and Lennox X0585, X8305 and X8308 cabinets take a filter about 19.63 x 19.88 x 4.38 inches, sold as a 20x20x5 replacement. Filterbuy lists White Rodgers ACM1600M and FR1600 series cabinets as taking the 19.5 x 19.5 x 3.63 size. Check the model on your cabinet and measure the depth before you order.
Can I use a 20x20x1 instead of a 20x20x4?
No. A 4-inch filter goes in a media cabinet, and a 1-inch filter in that cabinet leaves room for air to bypass it. Filterbuy also notes a 4-inch filter won't fit a 1-inch slot, and EPA says a filter must fit snugly so air doesn't leak around it.
How often should I replace a 20x20x4 filter?
ENERGY STAR says to check your filter monthly and change it when dirty, at least every three months. Filterbuy rates its 20x20x4 at 6 to 12 months, against 1 to 3 months for a 1-inch filter, because the deeper frame holds roughly four times the media. Follow your filter maker's rating and replace it sooner if it looks dirty, especially with pets, smoke or remodeling.
Is MERV 13 safe for my system?
EPA says most furnaces and HVAC systems can accommodate a MERV 13 filter without equipment problems if it is replaced frequently, but higher-MERV filters do tend to restrict airflow. If you notice weak airflow or are unsure, ask an HVAC technician.
My filter says MPR or FPR, not MERV. What do I buy?
MPR (3M/Filtrete) and FPR (Home Depot) are proprietary scales, and EPA notes MERV is the national consensus standard, so conversions are approximate. Filterbuy's guide maps roughly MPR 600 or FPR 5 to MERV 8, MPR 1000–1200 or FPR 7 to MERV 11, and MPR 1500–1900 or FPR 10 to MERV 13.
Which way does the arrow go?
Per ENERGY STAR, the airflow arrow on the filter frame should point toward the furnace or blower unit.
Sources
- Filterbuy — 20x20x4 air filters (actual size, compatible cabinets, service life)
- Filterbuy — Air filter sizes chart (nominal vs actual)
- Aerostar — Air filter sizes: nominal vs actual
- US EPA — What is a MERV rating?
- US EPA — Guide to Air Cleaners in the Home
- US EPA Indoor airPLUS — Filtration technical bulletin
- ENERGY STAR — How to keep your HVAC system working efficiently
- ENERGY STAR — Certified Homes maintenance guide