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Fireplace/Chimney Chase Framing Clearance Calculator

Calculate the required chase opening dimensions around a chimney or flue, based on the appliance's clearance-to-combustibles requirement.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax
1.97 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Required chase opening area

3.77 ft²

Check your inputs

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
2.3 ft
Chase opening depth
1.64 ft

Running these inputs gives 0.35 m² as the required chase opening area. Expect some drift against the real job; the calculation is solid but conditions on site are not. Currently reading for United States under IRC 2024 — pick a different market above and the figures re-cast accordingly.

Estimated cost — your price

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • 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.

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

Why is the clearance added twice to each dimension?
The required clearance-to-combustibles applies on every side of the chimney or flue, so it is added once on each side of both the width and the depth — doubling the single-side value for each dimension.
Where do I find the required clearance-to-combustibles value?
It is appliance- and chimney-system-specific, set by the manufacturer's listing and the applicable fuel gas/mechanical code (NFPA 211 governs solid-fuel chimneys) — always use the exact figure from your specific unit's installation instructions rather than an assumed value.
Does this account for the chase's exterior finish material thickness?
No — this calculates the required rough framing opening around the chimney/flue based on clearance-to-combustibles only. Add the finish material thickness separately when detailing the overall chase dimensions.