Find your size (it's printed on the frame)
Filters list nominal size on the cardboard edge — like 16x25x1. Buy that exact size; gaps around an undersized filter let dust bypass straight into the coil. No filter installed at all? That's how coils end up needing professional cleaning.
MERV: higher is not automatically better
MERV rates how fine a particle the filter catches. MERV 8–11 suits most homes: real filtration without strangling airflow. MERV 13+ catches more but many residential blowers can't push air through it — recreating the clogged-filter problem with a clean filter. If allergies push you higher, ask whether your system's blower is rated for it.
How often, really
- 1-inch filters: every 1–3 months — monthly in heavy cooling season, with pets, or during pollen waves.
- 4–5 inch media filters: every 6–12 months.
- Rule of thumb: check monthly; if light won't pass through, it's done.
Set a recurring phone reminder for the 1st of each summer month — the habit outperforms every gadget.
Sources
Get a plan for your exact situation
The free FixAC First-Aid wizard turns your symptom, system age, and ZIP into safe step-by-step checks — plus a briefing on what a fair repair should cost.