20x25x4 Air Filter: Actual Size, MERV Guide & Replacement
Nominal vs actual size
| Nominal (order this) | 20 x 25 x 4 in |
|---|---|
| Actual | 19.5 x 24.5 x 3.63 in (Filterbuy spec) |
A 4-inch media-cabinet size used on whole-home furnace, heat pump and central AC systems. Depth varies more between brands than on 1-inch filters. 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 20x25x4:
FAQs
What is the actual size of a 20x25x4 air filter?
20x25x4 is the nominal (ordering) size. Filterbuy cuts its 20x25x4 to an actual 19.5 x 24.5 x 3.63 inches. Other brands are usually within about 1/4 to 1/2 inch of the nominal length and width, so check the small print on your old frame.
Are all 20x25x4 filters the same depth?
No. Filterbuy cuts its 20x25x4 to 3.625 inches deep and notes that some brands print 20x25x4 on filters that are about 4.38 inches deep. Measure your old filter's depth and your cabinet before ordering.
Can I use a 20x25x1 instead of a 20x25x4?
No. Depth must match the slot or cabinet. A 1-inch slot cannot take a 4-inch filter, and a 1-inch filter in a 4-inch cabinet leaves gaps for air to bypass the filter.
How often should I replace a 20x25x4 filter?
ENERGY STAR says to check your filter monthly and change it when dirty, at least every three months. Filterbuy rates its 20x25x4 for 6 to 12 months; follow your filter and equipment maker's guidance and check it monthly.
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.
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 — 20x25x4 air filters (actual size)
- 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