Electric Fireplace Insert Size Guide

Matching an insert to your fireplace opening requires measuring its width, height, and depth, then comparing those numbers to the unit’s required dimensions.

Choosing the right electric fireplace insert size keeps you from wrestling a unit that doesn’t fit or leaving an ugly gap your surround can’t cover. The process is straightforward: measure the opening at several points, compare those numbers to a manufacturer’s minimum specifications, and confirm clearance and power requirements before you commit. This guide walks through the exact measurements and official numbers from real product documentation.

Electric Fireplace Insert Sizes: What The Numbers Mean

The insert’s name usually references the front width. SimpliFire’s SF-INS series is a good example, with three common sizes: the 25-inch, 30-inch, and 35-inch. The table below shows the minimum fireplace opening each requires, along with the physical size of the insert itself.

A larger opening doesn’t automatically mean you need the largest insert. The front width must slip through the opening, but the back width and depth dictate whether it fits the firebox shape. A fireplace that narrows toward the back, common in older homes, can reject an insert that looks right from the front.

How To Measure A Fireplace Opening For An Insert

Measure at multiple points because fireplace openings are rarely perfectly square. Grab a steel tape measure and a notepad, then follow this sequence:

1. Width: Measure the opening width at the front, middle, and back. Record the narrowest value for each position.
2. Height: Measure from the hearth floor to the top of the opening, both at the front edge and back. The lower number is your usable height.
3. Depth: Measure from the front face to the back wall at the left, center, and right. Use the shallowest measurement as your minimum depth.
4. Surroundings: Note the hearth depth, mantel height above the opening, and any obstructions like a gas line stub or protruding bricks.

Compare your smallest measurements to the manufacturer’s minimum opening dimensions. If your opening is narrower, shorter, or shallower in any spot, that insert size won’t fit without modification.

Clearance, Ventilation, And Power: The Requirements People Skip

Fit isn’t just about opening dimensions—the manuals list clearance and power requirements that matter just as much. The product also needs ventilation between the inside and outside of the fireplace, and the manual warns against letting the bottom of the insert touch the mantel during movement.

Power is the other common miss. A long extension cord is a fire hazard, so confirm an outlet is reachable before you pick a spot. If installing into an existing fireplace, also confirm the opening has no debris or obstructions that would block airflow around the insert.

Once you’ve confirmed fit and power requirements, the last decision is the surround. The surround fills the gap between the insert and the opening, and manufacturers like SimpliFire offer 3-sided and 4-sided options in small and large sizes to match the 25″, 30″, and 35″ inserts. If you’re working with a non-standard opening, or ready to look at bigger 40-inch options for a large wall, our tested roundup of the top 40-inch electric fireplace inserts covers the models worth buying. For most standard fireplaces, though, one of the three sizes above will be the right fit, and the final steps are just confirming clearances and outlet access before you order.

FAQs

Can I install an electric insert into any existing fireplace?

In most cases, yes, as long as the opening meets the insert’s minimum width, height, and depth at the narrowest point, and you have access to a standard 120-volt outlet. The firebox must also be clean and free of obstructions that would block the ventilation the insert requires.

What happens if my fireplace opening is too deep?

A deeper opening is generally fine because the insert sits forward and the surround covers the gap. The minimum depth spec is the critical number; as long as the opening is at least that deep, the unit will fit. The required depth is typically around 11 inches for most inserts.

How much space should there be between the insert and the fireplace?

Manufacturers specify the clearance, and it’s usually small. These gaps allow for airflow and prevent heat buildup, so follow the exact numbers in your insert’s manual.