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The two rules measure different things; choose the one that governs where you build.
The IRC sizes the hearth extension in front of a masonry fireplace's opening. Approved Document J sizes the constructional hearth under a solid fuel appliance of up to 50 kW (about 170,000 Btu/h), either in a fireplace recess or standing free. Factory-built fireplaces and listed stoves follow their own listing and installation instructions, which can ask for more or allow less.
Across the front of the firebox opening.
The hearth extension is measured from the edges of the opening itself, not from the outside of the masonry face.
From the hearth to the underside of the lintel.
Width times height is the opening area the code compares with 6 square feet (0.56 m²); at or above it the extension grows from 16 to 20 in (406 to 508 mm) in front and from 8 to 12 in (203 to 305 mm) each side.
Hearth plan area
8.194 sq ft
The opening is 6 sq ft or more, so the extension is the larger one: 20 in in front and 12 in each side.
- Extension width
- 59 in
- Extension in front of the opening
- 20 in
- Extension beyond each side of the opening
- 12 in
- Fireplace opening area
- 7.29 sq ft
- Least hearth extension thickness
- 2 in
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Fireplace Hearth Size Calculator: 8.19 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)
- International Code Council, 2021 International Residential Code, R1001.10 Hearth extension dimensions: at least 16 in (406 mm) in front of and 8 in (203 mm) beyond each side of the fireplace opening; at least 20 in (508 mm) and 12 in (305 mm) where the opening is 6 sq ft (0.6 m²) or larger
- The same code, R1001.9.1 and R1001.9.2: fireplace hearths at least 4 in (102 mm) thick; hearth extensions at least 2 in (51 mm), or 3/8 in (10 mm) where the bottom of the firebox opening is raised at least 8 in (203 mm) above the extension
- HM Government, Approved Document J (2010 edition incorporating 2022 amendments), paragraphs 2.23–2.28 and Diagrams 24–26: a constructional hearth at least 125 mm thick, projecting at least 500 mm from the jamb and 150 mm beyond each side of a fireplace recess, or at least 840 × 840 mm for a free-standing appliance; a hearth surface at least 225 mm (closed appliance) or 300 mm (open appliance) in front and 150 mm at the sides and back
Inputs used
- Fire and rule
- Masonry fireplace — IRC R1001.10 (United States)
- Fireplace opening width
- 35 in
- Fireplace opening height
- 30 in
- Recess opening width
- 35 in
- Recess depth
- 16 in
- Appliance width
- 20 in
- Appliance depth
- 16 in
- Appliance
- Closed appliance, used with its door shut
Intermediate steps
- Extension width
- 59 in
- Extension in front of the opening
- 20 in
- Extension beyond each side of the opening
- 12 in
- Fireplace opening area
- 7.29 sq ft
- Least hearth extension thickness
- 2 in
Confidence note: The opening is 6 sq ft or more, so the extension is the larger one: 20 in in front and 12 in each side.
What this calculation does not cover
- This is the IRC's rule for masonry fireplaces. A factory-built fireplace or a listed stove follows its own listing, which can ask for a larger hearth or a particular insulating value, and a jurisdiction may amend the code it adopts.
- The hearth extension must be at least 2 in (51 mm) thick, or 3/8 in (10 mm) of noncombustible material where the bottom of the firebox opening is raised at least 8 in (203 mm) above it; the hearth inside the firebox must be at least 4 in (102 mm) thick. Combustible forms and framing must not remain against the underside of either after construction.
- Mantels and combustible trim have their own clearances from the opening, and the fireplace's sides and back their own clearances from framing; neither is worked here.
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 2026-09-25 · v1.0.0
Regulatory standards & verification citations3
- International Code Council, 2021 International Residential Code, R1001.10 Hearth extension dimensions: at least 16 in (406 mm) in front of and 8 in (203 mm) beyond each side of the fireplace opening; at least 20 in (508 mm) and 12 in (305 mm) where the opening is 6 sq ft (0.6 m²) or larger
- The same code, R1001.9.1 and R1001.9.2: fireplace hearths at least 4 in (102 mm) thick; hearth extensions at least 2 in (51 mm), or 3/8 in (10 mm) where the bottom of the firebox opening is raised at least 8 in (203 mm) above the extension
- HM Government, Approved Document J (2010 edition incorporating 2022 amendments), paragraphs 2.23–2.28 and Diagrams 24–26: a constructional hearth at least 125 mm thick, projecting at least 500 mm from the jamb and 150 mm beyond each side of a fireplace recess, or at least 840 × 840 mm for a free-standing appliance; a hearth surface at least 225 mm (closed appliance) or 300 mm (open appliance) in front and 150 mm at the sides and back
Which documents these citations point at
- International Residential Code (United States)One- and two-family dwellings and townhouses up to three storeys — structure, fire safety, energy, plumbing, mechanical and electrical in a single document.
- Approved Documents to the Building Regulations (England) — J (United Kingdom)Practical guidance on meeting the Building Regulations, one lettered part per subject — Part A structure, Part B fire, Part K protection from falling, Part L conservation of fuel and power, and the rest.
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.
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