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Pile Group Efficiency Calculator (Converse-Labarre)

Estimate the reduction in per-pile capacity when piles are installed close together in a group, using the Converse-Labarre formula.

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
1.31 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Group efficiency factor

0.73 (Eg)

Check your inputs

The Converse-Labarre formula is a widely-used but empirical approximation — some geotechnical engineers consider group efficiency for friction piles in clay more reliably assessed with a block failure check instead, especially for closely-spaced groups.

θ = arctan(d/s)
18.43 °

At the values currently entered, the group efficiency factor works out to 0.73 (Eg). Confidence is moderate: the method is sound, but real materials and site conditions vary. Figures are shown for United States, where IRC 2024 is the governing residential reference; switch the market above if you are building elsewhere.

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

  • Converse-Labarre pile group efficiency formula: Eg = 1 − θ x [(n−1)m + (m−1)n] / (90mn), where θ = arctan(d/s) in degrees, d is pile diameter, s is center-to-center spacing, m is the number of rows, and n is the number of columns

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Frequently asked questions

Why do piles in a group have less capacity per pile than an isolated pile?
Each pile's zone of soil stress overlaps with its neighbors' zones — closely-spaced piles effectively compete for the same soil resistance, reducing the group's total capacity below the simple sum of each pile's individual capacity.
How is group capacity actually used?
Multiply the group efficiency factor by (number of piles x individual pile capacity) to get the group's usable capacity — or compare that against the equivalent "block" capacity of the whole group treated as one large pier, and use whichever governs (is lower).
Does wider spacing always mean better efficiency?
Yes — efficiency approaches 100% as spacing increases relative to pile diameter, since the piles' stress zones stop overlapping — but wider spacing also means a bigger, more expensive pile cap and foundation footprint.