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The diameter of each pile in the group.
The shaft diameter, used here only as the yardstick for spacing — group efficiency is governed by the spacing-to-diameter ratio, so this figure and the spacing have to be in the same terms. Groups are commonly spaced at three diameters; below about 2.5 the interaction is severe enough that the Converse-Labarre arrangement is being asked a question outside where it is usually applied.
The center-to-center spacing between adjacent piles.
Closer spacing (smaller ratio to diameter) reduces group efficiency, since neighboring piles' stress zones overlap more.
The number of pile rows in the group.
Rows in one direction of the grid, with columns counted in the other. It does not matter which direction you call rows as long as the two numbers describe the grid — three by four and four by three give the same efficiency. What does matter is that this describes a RECTANGULAR grid: a group with a pile missing, or one arranged in a circle or a triangle, is not what this formula counts.
The number of pile columns in the group.
The other dimension of the grid. A single row of piles — any number by one — has less interaction than a square group of the same count, because each pile has fewer neighbours: nine piles in a line are more efficient than nine in a three-by-three. If the count is not a rectangle, round to the grid the piles are actually set out on rather than forcing the total into two factors.
Group efficiency factor
0.727 (Eg)
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 °
They open the calculator with your figures already in it
Pile Group Efficiency Calculator (Converse-Labarre): 0.7269 (Eg) — 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)
- 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
Inputs used
- Pile Diameter (d)
- 16 in
- Center-to-Center Spacing (s)
- 4 ft
- Number of pile rows m
- 3
- Number of Columns (n)
- 3
Intermediate steps
- θ = arctan(d/s)
- 18.43 °
Confidence note: 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.
What this calculation does not cover
- The result is the Converse-Labarre per-pile efficiency and nothing else. It does not check block failure, where the piles and the soil between them fail as one mass — for friction piles in clay at close spacing that check often governs and gives the lower capacity, so it has to be run separately and the lower of the two used.
- The formula reads only diameter, spacing and grid size. It knows nothing about soil type, pile length or installation method, so it returns the same reduction for a driven pile in dense sand as for a bored pile in soft clay. Driven piles in loose to medium sand can reach an efficiency of 1.0 or higher because driving densifies the ground between them, and piles end-bearing on rock see essentially no group reduction — this calculator never returns either.
- This is a capacity reduction factor, not a foundation design. It applies no factor of safety or resistance factor, and it covers no settlement check — a pile group settles more than a single pile carrying the same load per pile, and settlement rather than capacity often governs the design.
- It assumes one rectangular grid of identical vertical piles at a single uniform spacing in both directions. Staggered or triangular layouts, raked piles, mixed pile diameters within the group, irregular group outlines and different row and column spacings are all outside the model.
- It applies no minimum spacing check. Buildable centre-to-centre spacing is set by the governing code and by piling rig tolerance, and this calculator will return a number for spacings closer than those permit — including spacings below one pile diameter, which is not a pile group at all.
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 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- 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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