How to Measure a Fireplace for a Replacement Inset Gas Fire

Accurate fireplace measurements are the starting point for choosing a replacement inset gas fire. A burner that is too large will not fit; one that leaves a large unexplained gap may need engineer review.

Enter every measurement in millimetres (mm), not centimetres.

Why clearance matters

Our burner finder looks for a burner that is smaller than the opening. The usual standard-fit range is 1–30mm smaller on each required dimension. A burner must not be larger than the opening.

If any required dimension is an exact match (0mm clearance), the finder flags that for checking rather than treating it as a confirmed standard fit.

Opening shapes

Rectangular or square

A — front / width
B — depth

Tapered

A — front width
B — depth
C — rear width

Curved front

Measurements A, B, C and D as shown on the finder diagrams

Six sided

A (front) plus B, C, D and E as shown on the finder

Available burner height

Measure from the floor or base of the fireplace to the top of the existing grate or burner. Choose 107mm, 142mm, 179mm or 218mm — the finder uses the largest of those heights that does not exceed what you enter.

Use the Inset Gas Fires burner finder after measuring.

Gas appliances must be installed and commissioned by a suitably qualified Gas Safe registered engineer. This guide does not cover gas connections, conversion or combustion adjustment.

FAQs

Do I measure in centimetres or millimetres?

Millimetres only. The finder and our range are specified in mm.

How much clearance does an inset gas fire need?

A typical standard fit is 1–30mm smaller than the opening on each required dimension. The burner must not be larger than the opening.

Can the burner be exactly the same size as my opening?

An exact size (0mm clearance) is not treated as a confirmed standard fit. It needs checking.

What measurements do I need for a tapered fireplace?

Front width (A), depth (B) and rear width (C), all in millimetres.

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