SettingsSettings for this calculationUS
The width of the chimney or flue assembly itself.
Use the overall width of the chimney pipe, factory-built chimney assembly, or masonry flue — not the chase opening.
The depth of the chimney or flue assembly itself.
Use the overall depth of the chimney pipe, factory-built chimney assembly, or masonry flue — not the chase opening.
The minimum clearance from the chimney/flue to any combustible framing, per the manufacturer's listing.
Clearance-to-combustibles is appliance- and chimney-system-specific per the manufacturer's listing and the applicable fuel gas/mechanical code (NFPA 211 governs solid-fuel chimneys) — always use the exact clearance from your specific unit's installation instructions, never an assumed value.
Required chase opening area
3.89 sq ft
Clearance-to-combustibles is appliance- and chimney-system-specific per the manufacturer's listing and the applicable fuel gas/mechanical code (NFPA 211 governs solid-fuel chimneys) — always use the exact clearance from your specific unit's installation instructions, never an assumed value.
- Chase opening width
- 2.33 ft
- Chase opening depth
- 1.67 ft
They open the calculator with your figures already in it
Fireplace/Chimney Chase Framing Clearance Calculator: 3.89 sq ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Chase opening width/depth = Chimney/Flue Width or Depth + 2 × Required Clearance-to-Combustibles — clearance-to-combustibles is always specified by the specific appliance or chimney system manufacturer's listing (per NFPA 211 and the applicable mechanical/fuel gas code), and varies significantly by appliance type and chimney system; this calculator applies the clearance value you supply.
Inputs used
- Chimney/Flue Width
- 2 ft
- Chimney/Flue Depth
- 16 in
- Required Clearance-to-Combustibles (per Manufacturer's Listing)
- 2 in
Intermediate steps
- Chase opening width
- 2.33 ft
- Chase opening depth
- 1.67 ft
Confidence note: Clearance-to-combustibles is appliance- and chimney-system-specific per the manufacturer's listing and the applicable fuel gas/mechanical code (NFPA 211 governs solid-fuel chimneys) — always use the exact clearance from your specific unit's installation instructions, never an assumed value.
What this calculation does not cover
- The airspace this opening creates has to stay empty for the life of the chase. It is the ventilated gap that keeps framing below its ignition temperature, so blowing attic insulation across it, or packing mineral wool into it because the chase feels cold, voids the very listing the clearance figure came from — the opening measures right and the framing still chars.
- Nothing here firestops the chase. Where it passes a floor or ceiling the gap around the chimney has to be closed at each level with the material the listing names, usually sheet metal or a listed firestop spacer, and a chase left open top to bottom is a vertical shaft that takes a basement fire into the attic in minutes.
- Flue termination height is a separate rule this page does not reach: the chimney has to finish at least 3 ft (0.91 m) above the roof where it passes through and 2 ft (0.61 m) above anything within 10 ft (3 m) of it. A chase framed to exactly the right clearance around a flue that stops short still pushes smoke back into the room every time the wind swings.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Chase opening width/depth = Chimney/Flue Width or Depth + 2 × Required Clearance-to-Combustibles — clearance-to-combustibles is always specified by the specific appliance or chimney system manufacturer's listing (per NFPA 211 and the applicable mechanical/fuel gas code), and varies significantly by appliance type and chimney system; this calculator applies the clearance value you supply.
Which documents these citations point at
- Standard for Chimneys, Fireplaces, Vents and Solid Fuel-Burning Appliances (NFPA 211) (United States)Chimney and vent sizing, clearances and the installation of solid-fuel appliances.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
Cite this page
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