Start with a filter that already fits

Turn the system off before opening the user-accessible filter slot. Photograph the edge of the old filter, including its part number, printed size and airflow arrow. If that filter was bent, rattled in its track or left a visible gap, treat its label as a clue. Check the cabinet label or equipment manual for the specified replacement.

Read nominal and actual as two different measurements

The nominal size is the shorthand used to sell a filter. Actual size is the physical length, width and thickness of its frame. Trane gives the example of a nominal 16 × 25 × 1 filter measuring approximately 15.5 × 24.5 × 0.7 inches. That difference is expected for the example. Always use the actual dimensions on the specific product you are considering.

16 × 25 × 1 in. · nominal

The size you might type into a store search.

15.5 × 24.5 × 0.7 in. · actual

Trane’s example of the physical frame measurements.

At checkout

Compare the actual dimensions and cabinet compatibility for the model in your cart.

If the old label is unreadable

Lay the old filter flat and measure its outside frame in three directions, including the thickness at the edge. Write the fractions down before searching. Match the measurements against a manufacturer’s specification and the filter cabinet’s requirements. For a deep media cabinet, use the model number and approved part list, especially when two packages have the same rounded size.

Check the fit before restarting

The correct filter should seat as the equipment instructions describe and allow the access cover to close normally. A filter that needs folding or force warrants a second look at the part number. Keep a photo of the successful replacement with the cabinet label. Next time, you can reorder a known fit without starting the measurement over.

For more detail, see Trane’s furnace filter sizing guide. Related reading: how to orient the filter’s airflow arrow.