Governing Formula
Both spread (pad) footings and strip (continuous) footings reduce to a rectangular prism of concrete. The only difference is engineering intent: a pad spreads a column load, while a strip runs continuously under a wall.
V = L × W × D × n Where:
-
L= Footing plan length (or strip run length) [m] -
W= Footing plan width [m] -
D= Footing depth (thickness) [m] -
n= Number of identical footings [no.]
Derived Equations:
V_each = L × W × D V_total = V_each × n V_ready = V_total × (1 + wastage/100) How the Calculation Works
For a pad footing, L and W are the plan dimensions of the footing pad and D its thickness. For a strip footing, L is the total run length of the foundation wall, W the bearing width, and D the thickness.
Multiplying by the footing count gives the total net volume; a wastage margin is then applied for excavation tolerances, formwork strikes, re-grading, and spillage.
Worked Engineering Example
Design Scenario: Ten Pad Footings
A structural engineer specifies 10 identical pad footings each 1.2 m × 1.2 m × 0.3 m with an 8% wastage allowance.
- Compute per-footing volume:
V_each = 1.2 × 1.2 × 0.3 = 0.432 m³ - Total net volume:
V_total = 0.432 × 10 = 4.32 m³ - Ready-mix order:
V_ready = 4.32 × 1.08 = 4.67 m³
Engineering Notes & Design Benchmarks
| Element | Typical Sizing Rule | Notes |
|---|---|---|
| Pad footing | Plan area sized for allowable bearing pressure | Thickness governs by punching shear (often 0.3 – 0.6 m) |
| Strip footing | Width sized for wall line load / bearing capacity | Typical 0.4 – 0.9 m wide, 0.2 – 0.5 m thick |
| Allowable bearing (safe) | 50 – 300 kPa depending on soil | Verify with geotechnical investigation |
Assumptions & Limitations
- Footings are modeled as uniform rectangular prisms with constant thickness.
- No allowance for stepped/tiered footings, keyways, or dowel provisions.
- The calculator sizes quantities, not the structural dimensions — bearing capacity, settlement, and reinforcement design come from a structural design.
Frequently Asked Questions
What is the difference between pad and strip footings?
A pad (spread) footing is an isolated square or rectangular base that transfers a single column load to the soil. A strip (continuous) footing runs as a long beam beneath a load-bearing wall, distributing a line load over its bearing width.
Why add wastage for footings?
Excavation tolerances, uneven subgrade, formwork deflections, and spillage during pumping all increase the consumed volume above the theoretical prism. Adding 5–10% prevents a short pour, which is difficult to patch in a foundation without cold joints.
Does this replace a geotechnical or structural design?
No. The calculator quantifies concrete for a given geometry. The geometry itself (footing size, depth, steel) must be designed from the verified allowable bearing pressure, settlement limits, and applied loads.
Engineering Disclaimer
Engineering Note: This calculator provides simplified quantity estimates for preliminary planning and educational use. Foundation dimensions and reinforcement must be verified by a qualified structural engineer and may require geotechnical investigation.
Technical References
- ACI 318: Building Code Requirements for Structural Concrete (Chapter 13 – Footings), American Concrete Institute.
- Bowles, J. E., Foundation Analysis and Design, McGraw-Hill.