To measure a fireplace for an insert, you need nine numbers, not one: the front opening width and height, the rear width, the depth at the top and at the bottom, the hearth extension depth, the distance from the opening to the mantel and side trim, the damper opening size and position, and the flue size and total height. Every one comes off the firebox itself, never the decorative surround, written down in inches with the fraction.
The two numbers that kill most insert fits are the rear width and the depth measured at the top of the opening. Masonry fireboxes taper in and up. An opening that reads 36 inches across the front can be 24 inches at the back wall, and a box that is 20 inches deep at the floor may be 14 inches deep under the lintel. Inserts are steel boxes with square corners. They have to fit the smallest part of the firebox, not the widest.
This is the tape measure job. If you are still deciding between a log set, an insert and a built-in, start with our gas logs vs insert vs built-in comparison. If you already know you want doors, our fireplace glass doors guide covers the product. Here we cover how to measure so that whatever you order actually goes in.
The Nine Measurements and Where to Take Each One
Use a steel tape, a flashlight and your phone. Measure twice and round nothing up. Between marks, write the smaller number for anything the product must fit inside and the larger for anything it must cover.
- Front opening width. Inside edge to inside edge of the masonry or metal frame, taken at the top, middle and bottom. Openings are rarely square, so record all three.
- Front opening height. From the firebox floor to the underside of the lintel, the steel angle or masonry spanning the top of the opening, taken at left, center and right. The surround above the lintel does not count.
- Rear width. Across the back wall at mid height. On a tapered masonry box this is the narrowest horizontal dimension, the first number an insert has to beat.
- Depth at the bottom. Front face of the opening straight back to the rear wall along the floor. This is the number people quote, and the less useful of the two.
- Depth at the top. Front face of the lintel straight back to where the sloped rear wall meets the throat. A stiff stick held level helps, because the tape sags. On older boxes this is often several inches less than the floor depth, and that difference decides whether a full height insert slides in.
- Hearth extension depth and width. Face of the opening out to the front edge of the noncombustible hearth, and the hearth width past each side. Carpet or wood where the manual calls for stone is something to know before you order.
- Distance to the mantel and side trim. Top edge of the opening up to the underside of the mantel shelf, and each side edge out to the nearest combustible trim, cabinet or wall. Note how far the mantel projects, since a deeper shelf needs more height.
- Damper opening and position. Open the damper fully and measure the clear opening through the throat, length and width, plus the height from the floor to the frame, and note whether it sits at the front or rear of the throat. A liner has to pass through here.
- Flue size and height. Inside dimensions of the flue tile, from the roof with the cap off or estimated from the throat with a flashlight, and total height from firebox floor to chimney top. Height matters for draft at Denver altitude, and flue size decides which liner fits. Our flue size and appliance match guide explains how the liner and appliance have to agree.
Finish by noting whether the firebox floor sits level with the hearth, above it or below it. A recessed floor changes where the insert face lands, and a raised hearth changes how far a wood insert's blower cavity hangs forward.
Why Masonry Fireboxes Taper and Why the Back Wall Decides the Fit
A masonry firebox is built to burn an open fire well. The side walls angle inward and the back wall leans forward as it rises so heat reflects into the room instead of straight up the chimney. That shape has been the default in Denver brick homes for more than a century.
An insert needs none of that. It is a sealed steel or cast iron box with its own baffles, rectangular in every direction, while the firebox is rectangular only at the face. Picture sliding a shoebox into a wedge. It stops the moment it touches the narrow back or the low top, with the front still sticking out. A wood insert body runs close to full height because it needs room for a load of splits and a baffle above them, so it meets the sloped rear wall first. A gas insert is shallower and shorter, which is one reason it fits many old fireboxes that reject a wood unit of similar output. The wood stove and insert guide for Colorado covers the burn side of that choice.
What Each Product Needs From Your Numbers
Critical means a miss will stop the install.
Table 1. Which fireplace measurements are critical for each product type.
| Product | Critical measurements | Usually flexible |
|---|---|---|
| Wood burning insert | Rear width, depth at top, opening height, damper opening, flue size and height, hearth extension | Front width, if the surround covers the gap |
| Gas insert | Rear width, both depths, opening height, damper opening for two liners or a coaxial liner, gas line route | Front width, mantel height within the manual's limits |
| Vented gas log set | Depth at bottom, rear width, opening height, hearth extension, damper that can be held open | Flue diameter, depth at top |
| Glass doors | Front width and height at three points each, squareness, lintel type, hearth level versus firebox floor | All depths, rear width, flue |
| Mesh screen or curtain | Front width and height, lintel type for the rod | Everything else |
Prefab Fireplaces: The Rating Plate Beats the Tape
Many of the fireplaces we work on in newer Aurora, Parker and Highlands Ranch subdivisions are factory built, a metal firebox in a wood framed chase. Measuring one is quick, and the numbers are much less useful than they look, because a prefab fireplace is a listed appliance. Anything you put inside it, insert, log set or doors, has to be the manufacturer's own accessory or a product specifically listed for that model. The clearances, the vent, the refractory panels and the airspace behind the box are all part of the listing. The tape cannot tell you any of that. The model number can.
The plate usually sits low on a side wall of the firebox, on the underside of the top front edge, or inside the lower control compartment on a gas unit. Our prefab fireplace repair guide walks through telling a prefab from masonry and locating that plate, and covers the cracked refractory panels and rusted floors that often surface at this stage and have to be fixed first.
What to write down
Photograph the entire plate, then write out the manufacturer, model number, serial number and any listing agency marks. If the plate is missing, burned off or painted over, tell us that, because a prefab with no readable plate cannot be matched to a listing.
Table 2. How the measuring job differs between masonry and prefab fireplaces.
| Question | Masonry fireplace | Prefab fireplace |
|---|---|---|
| What decides the fit | Your nine measurements plus the flue condition | The model number on the rating plate, then the measurements |
| Can an insert go in | Usually, if the rear width and top depth allow it | Only if the insert is listed for that prefab model, and many are not |
| Vent path | New liner down the existing clay flue | Liner down the existing metal chimney, sized to the listing |
| Common surprise | Flue offset or damaged tiles found on camera | Missing plate, cracked refractory, rusted chase cover |
The Surround and Faceplate Question
An insert is almost always smaller than the opening it goes into. The gap between the steel body and the masonry edge is covered by a surround, also called a faceplate or trim kit, that attaches to the insert face and overlaps the opening.
The surround has to be larger than the opening on all three covered sides so it lands on masonry or metal rather than air. If the opening is 38 inches wide and 30 tall and the standard surround is 40 by 31, you have an inch of overlap per side and on top, which only works if the opening is square. Out of square by an inch, and one corner shows a gap. Size the surround from the largest of your width and height readings, not the average, and check that the corner to corner diagonals match. Then confirm the surround does not reach the mantel or side trim, because it is a heated surface and moves the clearance reference point out to its own edge. Door frames follow a similar overlap or inside fit logic, which the glass doors guide covers in its measuring section.
Clearance to Combustible Trim and Mantels
Every insert, log set and door assembly comes with its own clearances to combustibles in the manual, and those are frequently larger than what the bare fireplace needed, measured from the surround edge rather than the opening. The residential code minimums for the opening, the mantel and the hearth extension are covered in our mantel and hearth clearance guide, which also explains how to check them with a tape. For this job, record the mantel height and projection and the hearth extension dimensions, and let the manual for the specific unit set the real requirement.
The Damper, the Flue and the Chimney Height
Homeowners often forget the damper entirely, and it is a common reason a delivered insert sits in the garage for two weeks. The damper frame is mortared into the throat, and its clear opening is often smaller and further forward than the flue above it. A wood insert liner is typically 6 inches round. A gas insert usually needs two 3 inch flexible liners or one coaxial liner, and both have to fit at once.
Above the damper, the flue tile size decides what liner fits, and the flue is not always straight. Older Denver chimneys frequently jog to clear a room or a roof member, and an offset a flexible liner can negotiate may still pinch it at the bend. That is why we camera the flue, and why a chimney inspection is the first thing on the schedule when an insert is on the table. Height matters twice: a short chimney at altitude pulls weakly and a sealed insert depends on that pull, and the liner is ordered by length.
Photos to Take Before You Shop
Shoot in daylight with the flash on and keep the tape in frame where you can.
- The whole fireplace from across the room, showing surround, mantel, hearth and nearby trim or cabinets.
- The opening straight on, with the tape held across the width at the top, then again at the bottom.
- The opening from the side, showing hearth depth and how far the mantel projects.
- The back wall, then each side wall, so the taper and any cracks or missing mortar are visible.
- The damper closed, then fully open, shot from below with a flashlight up the throat.
- The rating plate on a prefab, close enough to read every character, plus a wider shot of where it sits.
- The chimney from outside, showing the top, the cap and how far it rises above the roof.
- Any existing gas line, valve or outlet near the fireplace, since inserts need gas, power or both.
Save the photos and the numbers in one note on your phone. That note answers the first fifteen questions from any showroom or installer.
The Measuring Mistakes We See Most
- Measuring the surround instead of the opening. The tile or marble face is usually larger than the firebox opening behind it, and an insert sized to it will not go in.
- Forgetting the damper frame. The unit fits the firebox on paper and stops at the damper on delivery day.
- Assuming the flue is straight. Offsets, ledges and broken tiles reduce the usable diameter. Only a camera tells the truth.
- Quoting one depth. Floor depth is the generous number. Top depth is the one the insert hits first.
- Rounding up. A fraction of an inch is the difference between a slide in fit and a return shipment.
- Skipping the rating plate. On a prefab, without the model number nobody can confirm what is allowed inside.
Firebox condition belongs here too. Cracked firebrick, a sagging lintel or a separating back wall has to be dealt with before an insert covers it. Our fireplace repair and installation service handles that work so the insert goes into a sound box.
Why a Site Visit Still Happens Before Any Order
Good homeowner measurements save everyone time, and we ask for them. They do not replace the visit, which adds what the tape cannot.
- A camera run of the flue to find offsets, gaps between tiles and damage that decide whether the liner goes in or a relining comes first.
- A check of the damper frame against the actual liner or liners for the unit you want, including whether it needs cutting.
- Our own measurements, so that if the order is wrong the return is on us, not on you.
The measuring visit is a standard part of how we quote a gas insert installation. What we charge for the visit, the inspection and the install lives on our chimney and fireplace pricing page, where the numbers stay current.
Frequently Asked Questions
Do I measure the fireplace opening or the surround?
Measure the opening in the firebox itself, from masonry edge to masonry edge, or from the metal frame edge on a prefab. The marble, tile or brick surround sits outside that opening and is usually larger than it. A product sized from the surround will not fit.
How much smaller than the opening should an insert be?
The insert body has to be smaller than the smallest part of the firebox with some working room on every side, and it has to clear the damper frame on the way in. The exact allowance is in the manufacturer's installation manual for that model, which is why we size from the manual and your numbers together.
Can an insert go into a prefab fireplace?
Sometimes, but only when the insert manufacturer has tested and listed the insert for use in prefab fireplaces, and only in a prefab that is intact and correctly installed. The model number on your rating plate decides it.
What if my firebox is too shallow at the top for a wood insert?
You have three realistic paths. Choose a shallower insert model, choose a hearth stove that sits in front of the fireplace and vents up the chimney, or have a mason modify the firebox. A gas insert is often shallower than a wood insert of similar output, so it is worth pricing both.
Do I need to measure the flue for a gas log set?
You need to confirm the flue exists, is clear, and that the damper can be held open or fitted with a clamp for vented logs. Flue diameter matters far less for logs than it does for an insert, where a new liner has to fit inside the old flue.
Why does a professional still need to visit if I have all the measurements?
Because the tape cannot see everything. We check the flue for offsets and damage with a camera, confirm the damper frame will pass the liner, verify the hearth extension and mantel clearances against the specific unit, and take our own measurements so the order is on us if it is wrong.
What tools do I need to measure a fireplace?
A steel tape, a flashlight, a phone camera, and a short level to check the firebox floor against the hearth. A straight stick helps you find the depth at the top of the opening, where the tape wants to sag.
Ready to Measure? Or Let Us Do It
Grab a tape, a flashlight and your phone, work down the nine measurements, take the eight photos, and read the rating plate if you have a prefab. Put it all in one note, then call the shop at (720) 207-9232 and read it to us. We will tell you whether the product you have in mind is a likely fit, an unlikely one, or a definite no, and schedule the site visit that turns a likely fit into an order you can trust.
