Most furnace and HVAC return filters are sized by the nominal dimensions printed on the frame (for example 16×25×1). Those numbers are usually a little larger than the exact cardboard. Order by the printed size on your old filter — not by a tape measure alone.
What you need
- The filter currently in the slot (best), or a tape measure if the slot is empty
- A phone to photograph the printed size
- About two minutes
Step 1 — Find the filter slot
Look for a thin rectangular grille or a slot near the furnace / air handler, or a wall/ceiling return. Slide or unclip the cover. Do not force a sealed panel; if you are not sure which grille holds the filter, stop and check the furnace manual or a tech.
Step 2 — Pull the old filter and read the frame
On one edge of the cardboard frame you will usually see a printed size like 16x25x1 or 20×25×1. That printed size is what you order. Photograph it.
Also note the arrow on the frame. The arrow points with the airflow (toward the furnace / blower). The new pack goes in the same direction.
Step 3 — If there is no printed size
- Measure width × height × thickness of the filter (or the empty slot) in inches.
- Round to the nearest common nominal size we stock (for example a filter that measures about 15.5 × 24.5 × 0.75 is usually ordered as 16×25×1).
- When in doubt, use the closest printed size from a hardware photo of the same house, or email us a photo of the frame before you buy.
Step 4 — Pick MERV from the pack and the furnace manual
Caliza HVAC packs are offered in MERV 11 and MERV 13 where we stock that size. Use the MERV printed on the pack and what your furnace / air-handler manual allows. Denser is not always better for every blower. If the manual is silent, start with the same MERV you already run.
Step 5 — Order a 6-pack and write the size on the frame
Write the size on the new frame with a marker after install. Swap on about a 90-day cadence for many homes (sooner if the pack looks gray or airflow feels restricted). Wrong size is not a shipping defect if we shipped the size you ordered — measure twice.
Common sizes we stock (1-inch packs)
Examples include 16×25×1, 20×25×1, 16×20×1, 20×20×1, 14×20×1, 14×25×1, 18×24×1, 12×24×1, and many other 1-inch nominal sizes in MERV 11 and MERV 13 6-packs. Browse the HVAC collection or use the size finder on the homepage.
What this guide is not
This is hardware sizing help for a household furnace / return filter. It is not medical advice, not an allergy or asthma guide, and not a claim about purifying whole-home air. We sell replacement filters sized to the slot.