Air Filters & MERV Ratings Explained

What it is

MERV (Minimum Efficiency Reporting Value) is a 1-to-16 rating from the test method in ASHRAE Standard 52.2. The test measures how well a filter captures particles in three size ranges: E1 (0.3–1.0 µm), E2 (1.0–3.0 µm) and E3 (3.0–10 µm). A higher MERV means better capture of smaller particles. It usually also means more resistance to airflow, unless the filter is deeper or has more media. Some retailers use their own rating scales (such as MPR or FPR), which are not MERV.

How to identify

• Frame label: shows the nominal size, actual size, MERV and the airflow direction arrow.
• Measure the actual size of the old filter or the filter slot: length × width × depth.
• Filter type: fiberglass panel, pleated, extended-surface (deep media, 4–5 in), bag, rigid box (V-bank) or HEPA. HEPA is rated by a separate test method, not by MERV.

MERV ranges at a glance

Key points

• Nominal vs actual size: a "20 × 25 × 1" filter is a nominal size. The actual frame is usually a little smaller so it fits the slot. The amount varies by manufacturer, so measure.
• Pressure drop matters. A high-MERV 1 in filter can choke a residential blower, causing low airflow, frozen coils, high-limit trips and higher energy use. Check the equipment manufacturer's filter recommendation and the system static pressure.
• Deep (4–5 in) media cabinets allow higher MERV with less pressure drop.
• Design ventilation and filtration levels for buildings come from ASHRAE 62.1/62.2 and local code. The project specification governs on commercial jobs.
• Install the filter with the airflow arrow pointing toward the blower.

Common mistakes

• Treating MPR/FPR as MERV.
• Upgrading to MERV 13 in a 1 in slot without checking static pressure.
• Selling by nominal size only when the slot is a non-standard actual size.
• Assuming a washable filter has a MERV rating. Many are not rated.

Governing standards: ASHRAE 52.2 (test method and MERV); AHRI 680 (residential air filter equipment); ASHRAE 62.1 and 62.2 (ventilation and IAQ); ISO 16890 (international rating system, not directly convertible to MERV).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.