About this calculator
This calculator takes off a bored (drilled) pile foundation as a whole: the pile layout, the concrete in every pile (with an optional widened bell base), each pile's longitudinal bars and its spiral or stirrup cage, the grade beam that ties the pile heads together — its concrete, longitudinal bars, and stirrups, with the same treatment the strip foundation calculator uses — the formwork for the beam's above-grade portion, and steel mass by element with totals across the whole foundation.
Layout comes from one of two modes. From perimeter places a pile at every
corner of the building's footprint and then along each side at no more than the
entered spacing, deriving the pile count and the beam length from the building's
length and width. Manual count takes the pile count and the total beam length
directly — for a layout with internal beam lines, a non-rectangular footprint, or
piles under isolated columns.
This calculator computes quantities only. It does not design the piles or the beam. Pile diameter, length, bar count and size, tie pitch, cover, the beam section and its reinforcement are all your own inputs — the calculator counts what they add up to, it does not check their adequacy. Bearing capacity, the pile length and diameter actually needed, and every reinforcement detail depend on a soil investigation and must be designed by a qualified geotechnical/structural engineer.
Formula
Layout (perimeter mode). L and W are beam centreline dimensions:
baysAlongLength = ceil(L / spacing); baysAlongWidth = ceil(W / spacing)
pileCount = 2 x (baysAlongLength + baysAlongWidth)
beamLength = 2 x (L + W)
actualSpacing = L / baysAlongLength (and W / baysAlongWidth)
Walking the perimeter, every pile starts exactly one bay, so the bay count is the pile count with each corner counted once. A side that does not divide evenly gets one more bay at a spacing closer than the one entered.
Pile concrete (unchanged from the previous version of this calculator):
shaft = pi x r^2 x (pileLength - bellHeight)
bell (if present) = (pi x bellHeight / 3) x (R^2 + R x r + r^2) (a conical frustum)
volumePerPile = shaft + bell; pilesNet = volumePerPile x pileCount
pilesWaste = pilesNet x waste% / 100; pilesMass = pilesNet x 2500 kg/m3
Per-pile figures are computed unrounded, multiplied by the pile count, and only then rounded once, so rounding error does not accumulate across many piles.
Pile longitudinal bars. Each bar runs from the tip cover to the pile head and
projects BP above it into the beam:
barLength = pileLength - cover + BP
lap = 40 x barDiameter
segments = 1 if barLength <= stockLength
= 1 + ceil((barLength - stockLength) / (stockLength - lap)) otherwise
cutLength = barLength + (segments - 1) x lap
A pile bar is a discrete cut piece, not a continuous run, so the calculator does not buy whole stock bars per segment the way the beam does: every bar's cut length is chunked into stock-length pieces and all the pieces across the whole job are packed into the fewest stock bars by first-fit decreasing bin packing. The remainder pieces of many bars therefore share stock bars — but a piece longer than half a stock bar can never share one with another such piece, which is why the stock-bar count can exceed total length ÷ stock length (see the FAQ). Mass is computed from the cut length actually placed, not from the stock bars bought.
Pile ties. The tie's centreline diameter is the pile diameter less two covers and one tie diameter; the tied length is the bar length inside the pile:
dc = D - 2 x cover - tieDiameter; tiedLength = pileLength - 2 x cover
spiral: length = (tiedLength / pitch) x sqrt((pi x dc)^2 + pitch^2) (one turn per pitch, no extra closing turns)
stirrups: count = floor(tiedLength / pitch) + 1; each = pi x dc + 20 x tieDiameter
Grade beam — identical to the strip foundation calculator, with the pile head assumed flush with the beam underside (no embedment deducted):
beamVolume = width x height x beamLength; beamMass = beamVolume x 2500 kg/m3
bars: segmentsPerRow as above for beamLength; totalBarLength = segmentsPerRow x stockLength x rows
stirrups: length = 2 x ((width - 2C) + (height - 2C)) + 20 x stirrupDiameter
count = floor(beamLength / spacing) + 1
Formwork covers both faces of the beam's above-grade height only (the buried
part is cast against the trench). Boards run horizontally in ceil(HA / boardWidth) courses; every side's run (or the single manual run) on both faces is
chunked into stock-length pieces and packed with first-fit decreasing, plus a
chunked waste allowance.
Steel mass everywhere is length x pi x (d/2)^2 x 7850 kg/m3.
Worked example
A 10 × 8 m building at 2 m spacing, 600 mm × 12 m piles, 6 ⌀16 bars with a 400 mm projection and a ⌀8 spiral at 200 mm pitch, 50 mm cover, a 600 × 500 mm grade beam 300 mm above grade with 6 rows of ⌀16 and ⌀8 stirrups at 200 mm, 40 mm cover, every other input at its default.
bays = ceil(10/2) + ceil(8/2) = 5 + 4; pileCount = 2 x 9 = 18; beamLength = 36 m
volumePerPile = pi x 0.3^2 x 12 = 3.3929 m3; pilesNet = 61.073 m3; +5% = 64.126 m3
pile bars: barLength = 12,000 - 50 + 400 = 12,350 mm > 11,700 stock
lap = 640; segments = 1 + ceil(650/11,060) = 2; cutLength = 12,990 mm
pieces per bar [11,700, 1,290]; 108 bars -> 108 stock bars for the long pieces,
the 108 short pieces pack 9 to a stock bar -> 12 more: 120 stock bars
total placed = 108 x 12,990 = 1,402.9 m -> 2,214 kg
spiral: dc = 600 - 100 - 8 = 492; tiedLength = 11,900; 59.5 turns x 1,558.5 mm = 92.7 m per pile
beam: 0.6 x 0.5 x 36 = 10.8 m3 (27,000 kg); +5% = 11.34 m3
bars: segments = 1 + ceil((36,000-11,700)/(11,700-640)) = 4; 4 x 11.7 x 6 = 280.8 m -> 443 kg
stirrups: inner 520 x 420 -> 2 x 940 + 160 = 2,040 mm; 181 of them -> 369.2 m -> 146 kg
formwork: 2 courses x both faces of [10, 8, 10, 8] m on 4 m boards + 10% waste -> 40 boards
whole foundation: 61.073 + 10.8 = 71.87 m3 net; 75.47 m3 with waste
For these inputs the calculator returns exactly this: 18 piles, 75.47 m³ of concrete with waste, 120 stock bars for the pile cages, 40 formwork boards, and the steel masses above summed as the total reinforcement mass.
FAQ
Why can the pile bars need more stock bars than total length ÷ stock length? Because a stock bar cannot be cut into two pieces that are each longer than half of it. Four 6.35 m bars (a 6 m pile) need four 11.7 m stock bars — each stock bar yields one 6.35 m piece and a 5.35 m offcut too short for a second — while dividing 25.4 m by 11.7 m says three. The calculator packs the actual pieces.
Is 40 x barDiameter the actual Eurocode 2 lap length?
No — it is a common simplification for good bond conditions. LST EN 1992-1-1 Cl.
8.7 derives lap length from several coefficients (bond conditions, confinement,
the percentage of bars lapped at one section). The calculator uses the simplified
figure because it does not know those; a real design runs the full calculation.
Why is the spiral counted without extra closing turns? Because the number of closing turns is a detailing choice (commonly one to two at each end) that the calculator has no source for. It counts one turn per pitch over the tied length and says so in a note; add your detail's closing turns yourself.
What does the "cage wider than beam" warning mean?
That the pile's bars, at D/2 - cover from the centre, would land outside the
beam's stirrups, at width/2 - cover. The projecting bars would then sit in the
beam's cover or outside the beam entirely. Widen the beam or use a pile cap; the
calculator does not model pile caps.
Why does the 3D view show only one pile? The calculator has no plan positions for piles beyond the rectangular perimeter layout, so one representative pile is modelled, matching the elevation view.
Does the calculator check my piles' bearing capacity? No. It counts concrete and steel for the piles you describe. Capacity, the length and diameter actually needed, and the reinforcement detail all come from a soil investigation and an engineer's design.
Assumptions and limits
Standards cited, and what each actually covers:
- LST EN 206 — the concrete strength-class designations in
concreteClass; nothing else. - LST EN 1992-1-1 (Eurocode 2) — reinforcement lap length in general (Cl.
8.7). The calculator uses the
40 x diametersimplification, not the full calculation. - LST EN 1991-1-1 Annex A (informative) — the 2500 kg/m³ reinforced concrete density used for both piles and beam.
- LST EN 1536 (execution of bored piles) was considered and not cited: it governs how a pile is constructed, and no quantity here is read from it.
Trade practice, not a standard — disclosed here rather than in
meta.standards: the 11.7 m default stock bar length, the 20 x diameter hook
allowance for beam and pile stirrups, the 400 mm default bar projection, the pile
head assumed flush with the beam underside (no embedment deducted), and the
formwork board sizes.
The spiral has no extra closing turns, and ties are counted over the length inside the pile only — the projection into the beam is not tied.
Bar mass is placed steel; stock-bar counts are purchased steel. The two differ by the offcuts the packing could not use.
Perimeter mode models a rectangular perimeter beam only — no internal beam lines. Use manual mode for anything else; it treats the beam as one run for formwork packing and draws no pile positions.
The short-pile warning (under 3 m) states no frost depth of its own — see the frost-depth calculator, which does, with its sources.
This calculator does not design the foundation. Every dimension and reinforcement figure is your input; a qualified engineer must design the real piles and beam.