The Four Measurements
Front width, measured across the opening at the front. Front height, floor of the firebox to the top of the opening. Back width, across the rear wall of the firebox. Depth, front edge to back wall.
Write all four down. The back width is the one people skip and the one that rules units out.
Why the Taper Matters
Masonry fireboxes were built to angle inward, which reflects heat into the room. It also means a unit that fits the front may not clear the back.
This single measurement is the most common reason a customer's first choice turns out not to fit.
While You're in There
Note whether there is a gas line already in the firebox, and whether the damper is intact. Photograph the opening straight on and from a low angle.
If the chimney has a visible liner, mention it—it changes the venting conversation.
When to Stop and Call
If the firebox is cracked, if there's water staining, or if you can see daylight, stop measuring and have someone look at it. Those are chimney questions before they're fireplace questions, and every shop in this network can tell you which you have.