Filter sizes are printed on the side of your current filter. If you can't find it, measure the filter slot (Length × Width × Depth).
| Size | Common In | Shop on Amazon |
|---|---|---|
| 16×20×1 | Older furnaces, wall units | Buy → |
| 16×25×1 | Standard furnaces | Buy → |
| 20×20×1 | Square return vents | Buy → |
| 20×25×1 | Most common size | Buy → |
| 20×25×4 | Whole-house media filters | Buy → |
| 24×24×1 | Commercial, some residential | Buy → |
Filter sizes are "nominal" — rounded up. A "20×25×1" filter actually measures about 19.5×24.5×0.75 inches. Always match the nominal size printed on your current filter.
If your system accepts 4-inch filters, use them. They have more surface area, capture more particles, restrict less airflow, and last longer (up to 6 months vs 90 days). They're the upgrade most homeowners should make.