Measure length (l) & width (w).
If both sides are not equal, use the smaller measurement as your length.
Measure the depth (D) by measuring the thickness of your filter.
Put measurements together - l x w x d
Actual Size vs. Nominal Size
Actual size is the exact dimensions of your filter, often shown in precise fractions. Nominal size is the actual size rounded up to the nearest whole inch and is typically printed in a larger print above the actual size. The nominal size is what you should typically use when shopping for your replacement filter.
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