How to Measure for an 18 Inch Electric Fireplace Insert? | Get the Fit Right

Measure the firebox opening itself—not the trim—in width, height, and depth, then compare to the insert’s required specs.

The label “18 inch” rarely means the opening is exactly 18 inches wide. That number usually refers to a product class or series, so sizing an electric fireplace insert comes down to one thing: the real dimensions of your existing firebox. Grab a tape measure and follow this sequence to avoid a costly return.

What Does “18 Inch” Actually Refer To?

Manufacturers group electric inserts into nominal size classes, and the actual unit dimensions differ from the label. One 18-inch model, for instance, lists item dimensions of 18.70 x 17.64 x 5.20 inches, with a wooden frame mounting size of 17.99 x 17.32 inches.

That means an insert sold as “18 inch” can be wider or narrower than the label suggests. The only reliable way to know if a unit fits is to measure your opening and compare those numbers against the specific product’s specs, not the series name.

How to Measure the Firebox Opening Correctly

Fireplace openings frequently taper—they are wider at the front than at the back, or taller at one side than the other. A single measurement point can miss that variation and cause a fit failure. Measure in three directions and use the smallest numbers for your fit check.

• Width: Measure across the opening at the front edge and again at the back. Record both.
• Height: Measure from the hearth floor to the top of the opening at the front and at the back. Record both.
• Depth: Measure from the front lip straight back to the rear wall of the firebox. This is the measurement people skip most often.

Use the smaller width and the smaller height when comparing against the insert’s minimum opening requirements. If the back of the opening is narrower than the insert’s width, the unit will not slide in regardless of what the front measurement says.

Clearances, Power, and Other Fit Factors

Electric inserts generally need less clearance than wood or gas units—some require little or none—but the exact spacing depends on the model’s installation instructions. Check the manual for required gaps around heat-producing components before you commit to a fit.

Power access matters just as much as physical dimensions. The insert needs to reach an outlet, and the opening must allow the power cord to route out without pinching. Confirm outlet location and wiring access before purchase so you are not left with a unit that fits but cannot be plugged in.

Manufacturer resources consistently stress using the product-specific manual for exact dimensions and installation steps rather than generic fit rules. Both Touchstone’s installation manual library and FireplaceX’s owner resources pages carry the per-model documentation you need. Dimplex’s Nova line, which includes plug-in inserts in 23″, 26″, 28″, and 33″ sizes, similarly points buyers to model-specific specs for supplemental warmth coverage and installation requirements.

Installation Requirements After the Fit Check

Once the measurements confirm the unit will fit, installation follows a standard sequence: slide the firebox into the opening, center it, level it, then attach the trim or frame pieces to cover any gaps around the edges.

Before sliding the unit in, most installation guides recommend closing and sealing the damper, and in some cases adding a chimney cover plate. These steps prevent drafts and debris from affecting the insert’s performance. Skip them and you may end up with cold air spilling down the chimney around the unit.

If your opening is irregular, tapered, or obstructed in any way, have a professional installer verify the fit before you buy. That is the safest path when the firebox does not match a clean rectangular shape.

Ready to compare models that will fit your space? Our tested roundup of the best 18 inch electric fireplace inserts breaks down the real specs side by side.