1. Find your size
Look at the edge of your current filter. The size is printed there as three numbers, such as 20 x 25 x 1 — width x height x depth, in inches.
2. Order by the nominal size
Filters are labeled with a rounded nominal size. The actual size is slightly smaller so the filter slides into its slot. Order the nominal size printed on your old filter.
| Nominal size | Actual size (approx.) |
|---|---|
| 16 x 20 x 1 | 15.5 x 19.5 x 0.75 in |
| 16 x 25 x 1 | 15.5 x 24.5 x 0.75 in |
| 20 x 20 x 1 | 19.5 x 19.5 x 0.75 in |
| 20 x 25 x 1 | 19.5 x 24.5 x 0.75 in |
3. Install it the right way
Find the airflow arrow on the filter frame. Install the filter so the arrow points toward the furnace or air handler — the direction the air is moving.
4. Replace it regularly
Check your filter monthly and replace it about every 90 days. Replace it sooner if you have pets, allergies, recent construction, or heavy system use.
What does MERV 8 mean?
MERV stands for Minimum Efficiency Reporting Value, a filter rating defined by the ASHRAE 52.2 test standard. MERV 8 is a common choice for homes and helps capture everyday particles such as dust, pollen, mold spores, and pet dander. Higher MERV ratings capture finer particles but can restrict airflow in some systems, so check your HVAC manufacturer's recommendation.