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Underpinning Calculator — Bay Sequence, Volume & Concrete
Calculate underpinning bay count, sequence (1-3-5-2-4-6), pin volume and mass concrete for traditional underpinning.
Depth of current foundations
Required underpinning depth
Thickness of the wall above
Ready-mix £/m³ (materials only)
Extension each side (150mm std)
75mm standard gap
Safety notice
Structural elements must be designed or checked by a structural engineer before construction. Treat these figures as initial guidance for planning and budgeting, not a structural design.
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How We Calculate This
This calculator determines bay count, excavation sequence, concrete volume and dry pack quantities for traditional mass concrete underpinning.
Bay sequence
Bays = ceil(Wall Length / Bay Width)
Sequence: odd bays first (1-3-5), then even bays (2-4-6). Never excavate adjacent bays simultaneously.
Concrete volume
Pin Volume = Bay Width x Pin Width x Pin Depth
Pin width = wall width + 150mm extension each side.
Pin depth = new foundation depth - existing foundation depth. Dry pack at 75mm depth fills the gap under the existing foundation.
Excavation and cost
Excavation (net, in-situ) = bays × bay width × pin width × (pin depth + dry-pack gap)
This is the undisturbed bank volume of the holes — the full pit depth including the 75mm dry-pack zone. Add working space (≈600mm each side is typical up to 3m deep) and spoil bulking on removal (≈15% for gravel up to ≈25–50% for clay) for skip or grab-away quantities.
The cost shown is ready-mix concrete materials only (total concrete × your £/m³). It excludes excavation, labour, spoil disposal, formwork and dry-pack materials. Installed mass-concrete underpinning typically runs £1,000–1,500 per linear metre in the UK (2026).
Frequently Asked Questions
Traditional mass concrete underpinning follows a hit-and-miss sequence: odd-numbered bays first (1, 3, 5...), then even-numbered bays (2, 4, 6...). This ensures you never excavate adjacent bays simultaneously, maintaining wall stability. Each bay is concreted and dry-packed, then left to gain early strength — typically 24-72 hours (commonly 48 hours), not full 28-day cure — before the adjacent bay is opened, per ASUCplus mass-concrete underpinning guidance and BS 8004:2015+A1:2020. The criterion is enough early strength for the dry pack to take load, not full curing.
Standard bay widths are 1.0-1.5m. The maximum bay width should not exceed 1/5 of the wall length or 1.5m, whichever is smaller. Narrower bays (1.0m) are safer for weaker walls. The bay width is measured along the wall face. Each pin should extend 150mm wider than the wall on each side.
Underpinning must reach a depth where the soil has adequate bearing capacity and is unaffected by seasonal moisture changes. In the UK, this is typically 1.0–1.5m below ground level, but may be deeper near trees on clay soil (up to 3m+ in extreme cases). A structural engineer’s assessment and trial holes are essential. See our Foundation Calculator for new foundation design and our Ground Bearing Calculator for soil capacity checks.
Use the grade the structural engineer specifies. Mass concrete underpinning fill is commonly C20/25 (designated GEN 4 under BS 8500-2:2023), with C25/30 (designated concrete RC25/30) used where higher strength is called for, and occasionally C30/37 for heavily loaded or aggressive ground. Note the BS 8500 mapping: GEN 3 is C16/20 and GEN 4 is C20/25 — so C20/25 is not GEN 3. The concrete is poured into the excavated bay and stopped about 75mm short of the underside of the existing foundation; that 75mm gap is later filled with a semi-dry sand:cement dry-pack mortar that is rammed tight to ensure full load contact.
Traditional mass concrete underpinning typically costs £1,000-1,500 per linear metre in the UK (2026 prices). A typical terraced house requiring 6-8m of underpinning costs £6,000-15,000. Costs vary significantly based on depth, access, soil conditions and whether internal floors need to be removed. Mini-piled underpinning is more expensive but faster.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.