RCC Footing Calculator
Estimate excavation volume, PCC blinding, RCC pour, and steel reinforcement for isolated or raft footings using IS 456 grades and the standard 1.54 dry-volume rule.
How this works
The footing pit is over-excavated by 0.15 m (150 mm) below the founding level for working space; PCC and RCC are sized directly from the plan dimensions:
excavationVol = L × W × (depth + pccThickness + 0.15)
pccVol = L × W × pccThickness
rccVol = L × W × depth
dryVol = rccVol × 1.54
cementBags = ceil(dryVol × c / parts / 0.0347)
steelKg = rccVol × steelKgPerM3Both isolated and raft types use the same math at the planning stage — a raft is effectively one large rectangular footing covering the building footprint.
Worked example
A 1.5 m × 1.5 m × 0.45 m isolated M20 footing with 75 mm PCC and 80 kg/m³ steel:
- Excavation =
1.5 × 1.5 × (0.45 + 0.075 + 0.15) ≈ 1.52 m³ - PCC =
1.5 × 1.5 × 0.075 ≈ 0.169 m³ - RCC =
1.5 × 1.5 × 0.45 = 1.0125 m³ - Cement bags ≈ 9 bags
- Steel ≈ 1.0125 × 80 ≈ 81 kg
Sources
- IS 456:2000 §10.3 (dry-volume bulking); standard PCC blinding thickness of 75–100 mm.
FAQ
Why does the excavation volume add 0.15 m below the founding depth?
Site practice always over-excavates by 100–200 mm to give the formwork carpenter and the steel-fixer working space at the bottom of the pit, and to allow space for any dewatering. The calculator uses 0.15 m (150 mm) — a conservative South Asian field default. If your soil is stable and you're using a small isolated footing, you can subtract this allowance from the excavation figure manually.
What does PCC do in a footing?
Plain Cement Concrete (PCC) is a 75–100 mm blinding layer poured directly on the trimmed soil before the RCC pour. It gives the steel-fixer a clean, level platform to lay reinforcement and prevents soil moisture from drawing cement out of the bottom of the RCC slab. It's structurally non-contributing — the design loads are carried entirely by the RCC above.
How is the steel weight estimated for footings?
We use a steel density of 80 kg per cubic metre of RCC as the default — typical for residential isolated footings with two-way reinforcement at 150 mm centres. Heavier loads or rafts can push to 100 kg/m³; very light pad footings can drop to 60 kg/m³. The exact number comes from the bar bending schedule produced by the structural engineer.
Should I use isolated or raft footing for my house?
Isolated footings (one footing under each column) are the standard choice when the soil bearing capacity is at least 100 kN/m² and the column loads are moderate (up to a 3-storey residence). Raft (mat) footings are required when the soil is poor (under 100 kN/m²), when the building is on filled or recently reclaimed ground, or when adjacent isolated footings would overlap. Always confirm with a soil test report.
Why does this calculator treat raft and isolated footings the same?
At the planning stage the math is identical — a raft is essentially one large rectangular footing for the whole building footprint. The differences (anti-uplift reinforcement, edge thickening, additional cover) live in the structural drawing. The calculator deliberately keeps the formulas simple and the answer conservative; for an authoritative quantity surveyor's BOQ, work from the structural drawings.
What concrete grade should I use for footings?
M20 is the IS 456 minimum for any RCC structural member, including footings. M25 is preferred in seismic zones and for taller buildings. M15 is sometimes used for lightly loaded pad footings supporting boundary walls or single-storey structures, but most engineers avoid it for the main building footings since the marginal cost saving is not worth the lower durability margin.