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Brick Arch Rise-to-Span Ratio Calculator

Classify a masonry arch's shape (flat, segmental, or semicircular) from its rise-to-span ratio.

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
3.94 ft0.49 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Rise-to-span ratio

0.13 (rise/span)

High confidence

Classification: Flat / jack arch.

With the figures above, the rise-to-span ratio comes to 0.13 (rise/span). Note that arch rise sits at the low end of the range this calculator was checked against, so treat the output as indicative rather than settled. This is calculated for United States and cites IRC 2024. Building in another market? Change the selector above so the units and code reference follow.

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

  • Arch classification by rise-to-span ratio: flat/jack arch ≈ 1/8 or less, segmental arch ≈ 1/8 to 1/2, semicircular arch = 1/2

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Frequently asked questions

Why does arch shape matter structurally?
Flatter arches (low rise-to-span ratio) generate much higher horizontal thrust at their supports for the same vertical load than a semicircular arch — flat and jack arches need substantially stronger abutments or a supporting steel angle to resist that thrust.
What's a 'jack arch'?
A nearly flat arch (very low rise) commonly used over door and window openings where a rounded arch shape isn't desired — jack arches rely heavily on their abutments to resist the high horizontal thrust they generate.
Does a semicircular arch need less structural support?
It has more favorable (mostly vertical, lower horizontal thrust) load paths than a flat arch of the same span, but still needs adequate abutment resistance — no masonry arch is entirely self-supporting without adequate end restraint.