Most fireplace conversations in our showroom start with the same question: will it fit? Before we can quote a gas insert or a vented gas log set, we need a handful of numbers from your firebox and a few from the chimney above it. Get them right and the rest of the project is straightforward. Get one wrong (usually the depth or the rear width) and the insert you fell in love with sits half an inch proud of the brick.
This guide walks through every measurement we ask for, shows the real minimum openings published in the installation manuals for the inserts we sell most, explains how log set sizes map to fireplace sizes, and ends with exactly what to send us so we can size it remotely.
The short answer
To size a gas insert or gas log set, measure five things inside the firebox: front width, rear width, depth, front height and rear height, all in inches at the narrowest point. Then measure the hearth, the flat face around the opening, the distance to any mantel, and the chimney flue size and height. Compare those numbers to the minimum fireplace opening in the product's installation manual. For example, a Majestic Ruby 30 insert needs at least 31 5/8 inches across the front, 20 1/8 inches across the back, 21 inches of height and 15 inches of depth.
Key takeaways
- Measure the narrowest point every time. Tapered side walls, sloped back walls and a low lintel are what stop most installs, not the front width.
- Depth decides more inserts than width. Most inserts we sell need roughly 14 to 18 inches; the Kingsman IDV34 needs 17 3/4 inches, while the Ruby and Chaska need 15.
- Gas log sets are sized by burner length. A 24 inch Real Fyre G46 burner needs about 30 inches across the front and 24 across the back.
- The chimney matters as much as the firebox: liner size, flue size and height from hearth to top all appear in the manuals as hard limits.
- Photos of the opening, the face, the damper and the chimney top let us size most homes before anyone visits.
In this guide
- The measurement checklist
- How do I measure the firebox?
- Front opening or back of the firebox?
- Minimum openings for popular gas inserts
- The hearth, the face and the mantel
- The damper, liner and chimney height
- What size gas logs fit my fireplace?
- Can a gas insert go into a prefab fireplace?
- What if my fireplace is too small or too shallow?
- What to send us
- Our recommended models
- Which should you choose?
- Frequently asked questions
The measurement checklist
Print this or keep it open on your phone. A tape measure, a flashlight and five minutes is all it takes. Write every number in inches, rounded down to the nearest eighth.
| Measurement | Where to take it | Why it matters |
|---|---|---|
| Front width | Across the face of the opening, brick to brick, at the narrowest point | Sets the largest insert or log set you can consider |
| Rear width | Across the back wall of the firebox | Tapered fireboxes are where inserts and wide log grates get stuck |
| Depth | From the front edge of the opening to the back wall, at the floor and again at the top | The single most common deal breaker for inserts |
| Front height | Hearth floor to the underside of the lintel (the steel or brick over the opening) | The insert body must slide under the lintel |
| Rear height | Firebox floor to where the back wall starts to slope forward | A sloped back wall can cut usable depth at the top |
| Hearth | How far it projects in front, how high it sits off the floor, what it is made of | Affects surround fit, floor protection and the look |
| Face and mantel | Flat area around the opening; distance from the opening top to the mantel underside | Surrounds must sit flat; mantels have clearance rules |
| Damper and flue | Damper type and throat size; flue tile size; chimney height from hearth to top | Determines liner routing, log set damper clamps and vent limits |
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How do I measure the firebox?
Real Fyre's own guidance for log sets uses the same five numbers we use for inserts: front width, back width, depth, front height and back height. Here is how to take each one without getting fooled by the masonry.
- Front width. Measure across the opening just inside the face brick. If the sides are uneven or the brick is rough, take three readings (bottom, middle, top) and use the smallest.
- Rear width. Measure across the back wall at floor level. Most masonry fireboxes taper, so this number is often 6 to 12 inches smaller than the front.
- Depth. Hook the tape on the front edge of the opening and run it straight back to the rear wall at the floor. Then do it again just under the lintel. If the back wall slopes forward, the top number will be smaller, and that is the one that counts for a box-shaped insert.
- Front height. From the firebox floor (not the room floor) to the lowest point of the lintel. A steel angle iron hanging down an inch steals that inch.
- Rear height. From the floor to the point where the back wall starts leaning toward you.
A quick sketch with the five numbers on it is worth more than any single photo. If the shape is odd, cut a piece of cardboard to the opening, slide it in and trace the profile at the depth an insert would sit.
From our install team
In older Brooklyn and Manhattan brownstones we often find small, shallow fireboxes built for coal, with a back wall that rolls forward a few inches above the floor. The floor depth looks fine, the depth under the lintel does not. Always take both readings before you fall for a specific insert.
Do I measure the front opening or the back of the firebox?
Both. Manufacturers publish minimums for the front and the back separately because an insert is a box that is narrower at the back than at the face. The Majestic Ruby 30, for example, needs 31 5/8 inches across the front but only 20 1/8 inches across the back, so it will drop into a tapered firebox that would never accept a straight-sided unit of the same front width.
Log sets work the same way. The grate and burner sit toward the back, so a narrow rear wall can limit the set size even when the front looks generous. When the front and rear numbers point to different sizes, the smaller one wins.
Minimum fireplace openings for popular gas inserts
These are the minimum fireplace openings published in each manufacturer's manual or specification sheet. Your firebox has to meet or beat every number in the row. All of these are direct vent inserts that vent through liners in your chimney, so they are legal in New York City, where vent-free gas appliances are not permitted.
| Insert | Front width | Rear width | Height | Depth | Max input (NG) |
|---|---|---|---|---|---|
| Majestic Ruby 25 | 26 5/8 in | 15 1/8 in | 17 5/16 in | 15 in | 27,000 BTU |
| Majestic Ruby 30 | 31 5/8 in | 20 1/8 in | 21 in | 15 in | 32,700 BTU |
| Majestic Ruby 35 | 36 5/8 in | 25 1/8 in | 24 1/4 in | 15 in | 35,000 BTU |
| Majestic Trilliant 30 | 31 5/8 in | 20 1/8 in | 21 in | 15 in | 32,700 BTU |
| Majestic Trilliant 35 | 36 5/8 in | 25 1/8 in | 24 1/4 in | 15 in | 35,000 BTU |
| Napoleon Oakville X3 | 29 in | Not listed | 21 1/2 in | 14 3/8 in (more with blower) | 30,000 BTU |
| Kozy Heat Chaska 29 | 29 1/2 in | 18 7/8 in | 19 1/4 in | 15 in | About 30,000 BTU |
| Kingsman IDV34 | 28 5/8 in | 21 1/4 in | 21 1/2 in | 17 3/4 in | 31,000 BTU |
| Kingsman IDV44 | 32 7/8 in | 25 3/4 in | 25 3/4 in | 18 in | 34,250 BTU |
Two patterns jump out. First, the Majestic Ruby and Trilliant families share the same opening sizes, so the choice between them is about the look (the Trilliant is the more traditional face), not the fit. Second, depth varies more than width. The Kingsman units are wonderful heaters but want a deeper box than the Ruby, Oakville or Chaska.
The Napoleon Rosedale 3 body measures about 29 inches wide, 21 inches high and 15 inches deep; Napoleon's installation manual gives its minimum opening in a figure, so we confirm that one against your numbers before quoting.
Notice the Majestic manual's footnote too: those openings assume about a quarter inch of clearance on every side and the standard surround. Choose the deeper 2 inch surround and the required depth drops to 14 inches, which can rescue a shallow firebox.
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How do the hearth, the face and the mantel affect fit?
The firebox numbers decide whether the insert fits. The face numbers decide whether it looks right. An insert covers the gap between its body and the brick with a surround (a flat metal frame), and that surround has to sit flat against the face. Majestic sells Ruby surrounds in sizes like 40 by 27 inches and 42 by 30 inches; Kingsman's Clean View fronts for the IDV34 cover roughly 35 by 24 inches. Measure the flat area around the opening and note anything that sticks out: raised brick, a marble slip, tile edges, a mantel leg.
Mantels have their own rules. Napoleon's Oakville manual, for example, wants at least 12 inches from the top of the insert to the underside of a combustible mantel that projects up to 3 inches, with more clearance for deeper mantels. Majestic publishes a similar chart for the Ruby. Measure from the top of the opening to the underside of the mantel shelf, and how far the shelf sticks out from the wall.
For the hearth, note how far it projects into the room, how high it sits above the finished floor, and what it is made of. A raised hearth changes where the surround lands. If you are going with an open log set, the hearth also serves as floor protection: the model residential code calls for a hearth extension at least 16 inches in front and 8 inches past each side of an opening under 6 square feet, and 20 and 12 inches for larger openings. Older homes often fall short.
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What about the damper, chimney liner and chimney height?
Look up through the damper with a flashlight and note three things: the damper type (a cast iron plate with a handle, or a cap damper at the top of the chimney), the size of the throat, and the size of the flue tile above it. A photo looking straight up is perfect.
For a gas insert, the damper plate is usually removed or locked fully open so two flexible liners can pass through. The Ruby, Oakville and Rosedale manuals all call for two 3 inch liners, one for exhaust and one for combustion air, and the Chaska uses the same 3 inch and 3 inch pairing. Manufacturers also set a minimum flue: Napoleon lists a 6 inch round minimum for the Oakville, and Kozy Heat asks for at least 6 by 8 inches in masonry or a 7 inch factory-built chimney for the Chaska 29.
Chimney height is the number homeowners forget. Majestic lists a vertical run of 10 to 50 feet for the Ruby, Napoleon gives 10 to 45 feet of vent length for the Oakville and Rosedale, and Kozy Heat lists 10 to 50 feet for the Chaska. Measure from the firebox floor to the top of the chimney crown (a rough count of stories works: about 9 to 10 feet each, plus the part above the roof).
For a vented log set, the damper stays and must be permanently clamped open. Real Fyre's G46 manual requires a damper clamp, a minimum chimney height of 15 feet from hearth to top, and a minimum permanent free opening that grows with burner size and shrinks as the chimney gets taller. A 24 inch burner in a 15 foot masonry chimney needs 35 square inches of permanent opening, for example.
From our install team
The damper is where we find the most surprises. Dampers rusted shut, throats too narrow for two liners to pass side by side, and cap dampers someone added years ago that need to come off. A flashlight photo straight up the flue answers most of it before we ever roll a truck.
Not sure what fits your space? Send us your five firebox numbers and a photo up the flue, and we will tell you which inserts or log sets fit without a site visit in most cases. Book a free design consultation and we will help you narrow it down.
What size gas logs fit my fireplace?
Gas log sets are named by burner length (18, 24, 30 inches and so on), not by the fireplace they fit. The firebox needs to be noticeably larger than the burner so the flame does not lick the side walls and so the valve has room. Real Fyre's G46 manual publishes these minimums:
| Real Fyre G46 burner | Min front width | Min rear width | Min depth | Min height | Input |
|---|---|---|---|---|---|
| 18/20 inch | 26 in | 20 in | 15 in | 17 in | 40,000 BTU |
| 24 inch | 30 in | 24 in | 15 in | 17 in | 55,000 BTU |
| 30 inch | 36 in | 29 in | 15 in | 17 in | 65,000 BTU |
Real Fyre adds 6 inches to the front width and 4 inches to the rear width for its "02" valve models, because that control sits beside the burner. A quick rule: plan for at least 6 inches more front width than the log set length, then check the rear width.
So what fits a 36 inch fireplace? If the rear wall is 29 inches or wider and the depth is at least 15 inches, a 30 inch set like the Real Fyre Charred American Oak with G46 burner fits. If the back tapers to 24 or 26 inches, step down to the 24 inch set. Napoleon's Fiberglow vented log set comes in 18 and 24 inch sizes, and Hargrove's 18 to 21 inch Fresh Cut set suits the small fireboxes we see in city apartments and older row houses. Every log set we link here is vented. Vent-free logs would fit more fireplaces but cannot be installed in New York City.
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Can a gas insert go into a prefab fireplace?
Often, yes, but it depends on the paperwork as much as the tape measure. A prefab (factory-built or zero clearance) wood fireplace has a metal firebox and a listed metal chimney. Napoleon's manuals say the Oakville and Rosedale must be recessed into a vented, non-combustible wood-burning appliance, prefabricated or masonry, which allows a prefab box. Majestic points installers to its manual for zero clearance guidelines. Kozy Heat's Chaska 29 lists a 7 inch minimum for a factory-built chimney.
For a prefab, add three things to your list: the brand and model from the rating plate (usually behind the screen or on the side wall), the inside diameter of the round metal chimney, and a photo of the top of the chimney. Some prefab manufacturers restrict what can go inside their boxes, and we check both listings before we quote.
What if my fireplace is too small or too shallow?
You still have options. In rough order of what we try first:
- A smaller insert. The Ruby 25 needs only 26 5/8 inches across the front and 17 5/16 inches of height, and the Kingsman IDV26 is built for compact openings.
- A deeper surround option. On the Ruby, the 2 inch surround reduces the depth requirement from 15 to 14 inches.
- A vented log set. Log sets need about 15 inches of depth and are far more forgiving of tapered walls, as long as the chimney drafts and meets the height and damper rules.
- An electric firebox insert. No chimney requirements at all, just a nearby outlet. Browse electric firebox inserts when the masonry or chimney rules out gas.
From our install team
When a firebox is a quarter inch short, the answer is almost never grinding brick. It is usually the next size down, a different surround, or a different brand whose box is shaped differently. That is exactly why we compare three or four manuals against your numbers instead of one.
What to send us: photos and numbers
Email or text these and we can usually narrow it to two or three options before anyone visits:
- The five firebox numbers: front width, rear width, depth at floor and under the lintel, front height, rear height.
- A straight-on photo of the whole fireplace wall, including the mantel and hearth, with a tape measure visible across the opening.
- A photo inside the firebox from each front corner, showing the side and back walls.
- A flashlight photo looking straight up through the damper.
- A photo of the chimney top from outside, plus how many stories the house is.
- Gas: is there a gas line nearby, natural gas or propane, and where the meter is.
- Building type: house, co-op or condo. In NYC, co-op and condo boards often need to approve chimney work, and gas piping must be done by a DOB-licensed master plumber.
Send it all with a consultation request, or bring the photos to our Maspeth showroom and we will pull the manuals with you at the counter.
Our recommended models
Chosen by how many real openings they fit, what ships in the box and how they heat. Inserts first, then log sets for owners who want an open flame.
Which should you choose?
Choose a gas insert if:
- Your firebox meets the manual's depth and rear width, usually 14 to 18 inches deep.
- You want real heat, a sealed glass front and a remote.
- The chimney can take two 3 inch liners and is within the vent length limits.
Choose a vented log set if:
- Your firebox is too shallow or tapered for an insert but at least 15 inches deep.
- You want an open flame and mostly use it for ambiance.
- The chimney is at least 15 feet tall and drafts well, with the damper clamped open.
Choose an electric insert if:
- The chimney is damaged, blocked or not worth relining.
- Your co-op or condo board will not approve chimney work.
- You want flame on demand with no gas line and no venting.
For most masonry fireplaces in houses across Queens, Long Island and Westchester, a gas insert is the right answer and the Majestic Ruby 30 is where we start, because its opening fits so many fireboxes. If the depth is not there, we look at the Ruby 25 or a Real Fyre log set before anything else. Measure first, then shop. It is the cheapest step in the whole project.
Frequently asked questions
How do I measure my fireplace for a gas insert?
Measure the front width, rear width, depth (at the floor and under the lintel), front height and rear height, all at the narrowest point. Then measure the flat face around the opening, the distance to the mantel, and the chimney height and flue size. Compare every number to the minimum opening in the insert's installation manual.
What measurements do I need for a gas log set?
Front width, rear width, depth and height of the firebox, plus chimney height and the damper type. Real Fyre's G46 manual, for example, asks for at least 30 inches of front width, 24 of rear width, 15 of depth and 17 of height for a 24 inch set, and a chimney at least 15 feet tall.
What size gas logs fit a 36 inch fireplace?
Usually a 30 inch set if the back wall is at least 29 inches wide, and a 24 inch set if the firebox tapers more than that. Plan on about 6 inches more front width than the log set length, and more if the valve sits beside the burner.
Will a gas insert fit a shallow fireplace?
Often. Many inserts need about 15 inches of depth, and the Majestic Ruby needs only 14 inches with its 2 inch surround. The Kingsman IDV34 needs 17 3/4 inches. If your firebox is shallower than about 14 inches, a vented log set or an electric insert is usually the better fit.
Do I measure the front opening or the back of the firebox?
Both. Manufacturers list separate minimums for front width and rear width because fireboxes taper. Measure the back wall at the floor, and measure depth at both the floor and the top, since a sloped back wall can reduce usable depth.
How much clearance does a gas insert need around it?
The insert needs the manual's minimum opening, which already includes a small gap (Majestic assumes about a quarter inch on each side). Combustible mantels need more: Napoleon's Oakville manual asks for at least 12 inches from the insert top to a mantel projecting up to 3 inches, with more for deeper mantels.
Can a gas insert go into a prefab fireplace?
Many can. Napoleon's Oakville and Rosedale are approved for vented, non-combustible wood-burning fireplaces, prefabricated or masonry. You will need the prefab's brand, model and chimney diameter so we can confirm both listings allow it.
What if my fireplace is too small for an insert?
Try a smaller insert such as the Majestic Ruby 25 or Kingsman IDV26, a different surround, or a vented gas log set. If the chimney is unusable, an electric firebox insert needs no venting at all. In New York City, vent-free gas logs are not an option because they are not permitted.
Talk to a specialist
Send us your measurements and we will match the insert
We have been designing, selling, installing and servicing fire features and outdoor kitchens for over 30 years across New York City, Long Island, Westchester, New Jersey and South Florida. Text or email your five firebox numbers and a few photos, and we will check them against the manuals for the inserts and log sets that actually fit your fireplace.
Book a free consultationCall 718-326-4328Showroom: 58-30 Maspeth Ave, Maspeth, Queens.
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Written by the NYC Fireplaces & Outdoor Kitchens team. Last reviewed October 2026. We sell, install and service the products discussed here.
Sources:Majestic Ruby 25/30/35 installation manual, Majestic metal inserts brochure (Ruby and Trilliant), Napoleon Oakville X3, Napoleon Oakville X3 specification sheet, Napoleon Rosedale installation manual, Kozy Heat Chaska 29 manual, Kingsman IDV34/IDV44 manual, Real Fyre: Choosing the right log, Real Fyre G46 burner installation manual, ICC International Residential Code, Chapter 10. Specifications change; always confirm with the manufacturer's current installation manual.