How to Measure Your Furnace Filter Size
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Why Filter Size Matters
Installing the wrong size filter is one of the most common HVAC mistakes homeowners make. A filter that's too small leaves gaps for unfiltered air to pass through. One that's too large simply won't fit. Getting the right size is the first step to cleaner air and a longer-lasting HVAC system.
Step 1: Check Your Existing Filter First
The easiest way to find your filter size is to look at the filter currently in your system. Most filters have their dimensions printed on the cardboard frame, listed as Length × Width × Depth (in inches). For example: 20 × 25 × 1.
If the label is worn or missing, move on to Step 2.
Step 2: Measure the Filter Slot
Use a tape measure to measure the inside of your filter slot or return air vent — not the filter itself. Measure:
- Length (the vertical dimension)
- Width (the horizontal dimension)
- Depth (how deep the slot is — typically 1")
Subtract 1/4" from the Length and Width of the filter slot or housing size. This will allow the filter to be removed easily.
Step 3: Understand Nominal vs. Actual Size
Most filters are sold by their nominal size, which is slightly larger than the actual physical size. For example, a filter labeled 20 × 25 × 1 typically measures about 19.5 × 24.5 × 0.875 inches. This is normal — don't be alarmed if your tape measure shows a slightly smaller number than what's printed on the box.
We also specialize in custom sizes. These are ordered based on the exact or actual size of the filter. Sometimes this number is printed on the filter, but it is always best to physically measure your filter to find the exact custom size.
Choosing the Right Filter Once You Have Your Size
Once you know your size, the next decision is which filter type is right for your home. NeverBuyAnotherFilter.com offers three tiers of washable electrostatic filters:
- Silver Filter — Great everyday performance for standard homes
- Gold Filter — Enhanced filtration for homes with pets or mild allergies
- Platinum Filter — Maximum filtration for allergy sufferers and sensitive households
Unlike disposable filters, our washable filters are designed to last a lifetime — so you only need to measure once and buy once.
Still not sure? Contact us and we'll help you find the perfect fit.
