20x20x1 air filter: actual size

Short answer: a filter sold as 20x20x1 is a little smaller than its label. Published actual sizes run from about 19.5 × 19.5 to 19.75 × 19.75 inches on the face and about 0.75 to 0.78 inch thick, depending on the brand.^{[1 2nd-hand 2 · Oct 28, 2026]}

Published actual sizes for 20x20x1

Brand / product family	Published actual size (in)	Source
Honeywell 1-inch (High and Ultra Efficiency, MERV 11 and 13)	19.75 × 19.75 × 0.75	maker store specs, compiled^{[1 2nd-hand · Oct 28, 2026]}
Amazon Basics 1-inch MERV 11	19.75 × 19.75 × 0.75	product listing, compiled^{[1 2nd-hand · Oct 28, 2026]}
Filtrete MERV 13 / MPR 1900	19.62 × 19.62 × 0.78 in the title; 19.69 × 19.69 × 0.78 in the dimensions field	product listing, compiled^{[1 2nd-hand · Oct 28, 2026]}
Nordic Pure pleated MERV 13	about 19.5 × 19.5 × 0.75	retailer listing, compiled^{[1 2nd-hand · Oct 28, 2026]}
Filter King pleated (MERV 8, 11, 13)	19.5 × 19.5 × 0.75	product page^{[2 · Oct 28, 2026]}

Will a different brand fit my slot?

Usually yes. The brands above differ by about ¼ inch per side. Going from a 19.5-inch cut to a 19.75-inch cut means a snugger slide-in; measure the old filter first if your slot is tight.^{[1 2nd-hand · Oct 28, 2026]}
One listing, two sizes. The Filtrete listing above shows two slightly different figures, so measure if the fit matters.^{[1 2nd-hand · Oct 28, 2026]}
Measured about 19½ × 19½ × ¾? That is a 20x20x1. Round the face numbers up to the nearest whole inch to get the size printed on the box.^{[2 · Oct 28, 2026]} Full method: nominal vs. actual filter size.
A 20x20x2 will not fit a 1-inch slot. It is too thick; match the third number to your slot.^{[2 · Oct 28, 2026]}
Arrow toward the fan. The airflow arrow on the frame points toward the blower; in a wall or ceiling return grille it points inward.^{[2 · Oct 28, 2026]}

Picking a MERV rating

20x20x1 filters are widely sold in MERV 8, 11 and 13.^{[1 2nd-hand 2 · Oct 28, 2026]} MERV comes from an ASHRAE test; a higher number captures more particles in the 0.3 to 10 micron test range but adds airflow resistance. The EPA suggests using the highest rating your system's fan and filter slot can accommodate, and asking an HVAC technician if unsure.^{[3 · Oct 28, 2026]} A 1-inch MERV 13 is not a HEPA filter.^{[1 2nd-hand · Oct 28, 2026]} Also see 20x25x1 actual size and 16x25x1 actual size.

Sources: 1. FilterBySize, 20x20x1 actual size by brand (compiles makers' and sellers' published actual sizes; second-hand). 2. Filter King, 20x20x1 air filters. 3. US EPA, What is a MERV rating? Checked Oct 28, 2026.