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Professional footing calculator for residential and commercial foundations
Calculate concrete volume, excavation depth, and bearing capacity for strip footings, pad footings, and raft foundations. AS 2870 compliant calculations for 2026.
Professional foundation design calculations for all types of shallow footings
Calculate precise concrete volumes, reinforcement requirements, and excavation depths for strip footings, pad footings, and raft foundations. Our calculator uses Australian Standard AS 2870 for residential slab and footing design, ensuring code-compliant results for your construction project.
Determine safe bearing capacity based on soil classification according to Standards Australia guidelines. Calculate footing dimensions required for different soil types, from reactive clay to bedrock, ensuring structural stability for 2026 building requirements.
Get instant cost estimates for concrete, reinforcement steel, excavation, and formwork. Compare different foundation types and understand material requirements with 2026 Australian pricing data for accurate project budgeting and planning.
Select foundation type and enter dimensions below
A shallow foundation calculator is an essential tool for structural engineers, builders, and owner-builders working on residential and commercial construction projects in Australia. Shallow foundations, also known as spread footings, are used when the bearing capacity of surface soils is sufficient to support the structural loads without excessive settlement. The calculator determines the required footing dimensions based on soil bearing capacity, applied loads, and Australian Standards compliance.
According to AS 2870-2011 Residential Slabs and Footings, shallow foundations must be designed to accommodate soil conditions, prevent differential settlement, and provide adequate bearing capacity. For residential construction in 2026, typical shallow foundation depths range from 300mm to 800mm below ground level, depending on soil classification and frost penetration depth.
Typical strip footing showing wall, ground line, and bearing surface
Continuous footings that support load-bearing walls. Common in residential construction with typical widths of 450-600mm and depths of 300-500mm. Strip footings distribute wall loads over a larger soil area, preventing settlement in reactive clay soils classified as Class M, H, or E.
Square or rectangular footings that support individual columns or posts. Used in steel-frame and timber-frame construction, pad footings range from 600mm x 600mm to 1500mm x 1500mm depending on column loads and soil bearing capacity. Critical for bearing pressure calculations.
Large continuous slabs that support the entire building structure. Suitable for poor soil conditions or when individual footings would be uneconomical. Raft foundations typically range from 200-400mm thick with reinforcement mesh and edge beams for additional strength in expansive clay soils.
Soil bearing capacity is the maximum pressure that soil can safely support without excessive settlement or shear failure. Australian Standard AS 2870 classifies residential building sites into five soil categories (A, S, M, H, E) based on reactivity and shrink-swell potential. Understanding your site's soil classification is crucial for proper shallow foundation design.
Professional soil testing is mandatory for all building projects in Australia. A qualified geotechnical engineer must conduct soil classification testing to determine bearing capacity, plasticity index, and reactivity. Using assumed values without proper testing can result in structural failure, non-compliance with building codes, and voided insurance claims.
| Soil Class (AS 2870) | Description | Bearing Capacity | Typical Foundation |
|---|---|---|---|
| Class A | Sand, rock, non-reactive | 400+ kPa | Standard strip footings 300mm deep |
| Class S | Slightly reactive clay | 150-200 kPa | Strip footings 400-450mm deep |
| Class M | Moderately reactive clay | 100-150 kPa | Wider footings 500-600mm deep |
| Class H | Highly reactive clay | 75-100 kPa | Waffle pod slab or stiffened raft |
| Class E | Extremely reactive clay | 50-75 kPa | Engineered pier and beam system |
Calculating shallow foundation requirements involves several key engineering formulas based on structural mechanics and soil mechanics principles. These calculations ensure adequate bearing capacity, prevent settlement, and maintain structural integrity throughout the building's lifespan.
Where:
The calculated bearing pressure must be less than the allowable bearing capacity of the soil, typically with a safety factor of 2.5-3.0 for residential construction.
Where:
Add 5-10% extra for wastage, spillage, and over-excavation. For aggregate quantity calculations, refer to our dedicated calculator.
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Shallow foundation construction requires several key materials including concrete, reinforcement steel, formwork, and excavation services. Understanding material quantities and costs is essential for accurate project budgeting and procurement planning for residential and commercial projects in Australia.
Australian concrete grades for residential foundations follow AS 1379 and AS 3600 standards. N20 concrete (20 MPa compressive strength) is standard for most residential footings, while N25 or N32 may be required for heavier loads or adverse soil conditions. Concrete should have maximum aggregate size of 20mm and slump of 80-120mm for easy placement in footing trenches.
Steel reinforcement in shallow foundations prevents cracking from shrinkage, temperature changes, and differential settlement. Typical residential strip footings use N12 or N16 deformed bars at 450-600mm spacing in both longitudinal and transverse directions. Pad footings require reinforcement mesh or bar grid in both directions, with minimum cover of 50-75mm to prevent corrosion.
Proper excavation and site preparation are critical for successful shallow foundation construction. Excavation depth must extend below the depth of seasonal moisture variation, typically 300-800mm depending on soil classification and climate zone. In reactive clay soils (Class H and E), deeper excavation or alternative foundation systems may be required to reach stable bearing strata.
Site clearing should remove all vegetation, roots, and organic topsoil to prevent future settlement. The bearing surface must be level, compact, and free of loose material. For sites with fill or disturbed soil, compaction testing to 95% Standard Maximum Dry Density (SMDD) as per AS 3798 may be required before footing construction.
Calculate excavation volume by multiplying footing dimensions by a factor of 1.2-1.5 to account for working space, battered sides, and over-dig. For strip footings, excavate trenches 150-200mm wider than the footing width to allow for formwork installation and inspection. Small excavators (1.7-3.5 tonne) are typical for residential foundation excavation, costing $80-$150 per hour including operator.
| Foundation Type | Typical Dimensions | Concrete Volume | Est. Cost (2026) |
|---|---|---|---|
| Strip footing (10m length) | 450mm W × 400mm D | 1.8 m³ | $500-$750 |
| Pad footing (single post) | 800mm × 800mm × 400mm | 0.26 m³ | $150-$250 |
| Raft slab (10m × 8m) | 200mm thick + edge beams | 18-22 m³ | $6,500-$9,000 |
| House perimeter (150m²) | 500mm W × 450mm D | 12-15 m³ | $4,500-$6,500 |
Understanding potential foundation issues helps prevent costly repairs and structural damage. Shallow foundations in reactive clay soils are particularly susceptible to differential settlement, heave, and cracking due to seasonal moisture variations. Proper design, construction, and site drainage are essential for long-term foundation performance.
Uneven settlement occurs when different parts of the foundation settle at different rates, causing structural cracking and distortion. Common causes include variable soil conditions, inadequate bearing capacity, poor compaction, tree root activity, and water infiltration. Maximum allowable differential settlement for residential structures is typically 20-40mm over 10 metres.
Expansive clay soils (Classes M, H, E) undergo significant volume changes with moisture variation, causing foundation heave in wet periods and subsidence during drought. Proper footing depth below the zone of seasonal moisture change is critical. For basement construction, additional waterproofing and drainage measures are essential.
If you observe these signs, consult a structural engineer immediately. Early intervention prevents progressive damage and reduces repair costs.
A shallow foundation calculator is a digital tool that computes concrete volume, bearing pressure, excavation depth, reinforcement requirements, and cost estimates for strip footings, pad footings, and raft foundations. It uses Australian Standard AS 2870 guidelines to ensure code-compliant designs based on soil classification, structural loads, and site-specific conditions.
Shallow foundation depth in Australia ranges from 300mm to 800mm below ground level, depending on soil classification. Class A (sand/rock) sites typically require 300-400mm depth. Class S and M (reactive clays) need 400-600mm. Class H and E (highly reactive) soils may require depths of 600-800mm or alternative foundation systems like waffle pod slabs or pier and beam.
Minimum strip footing width in Australia is typically 450mm for single-storey residential construction on Class A or S soils. For Class M soils, 500-600mm width is standard. Class H and E soils often require 600-800mm or wider footings. Width increases with building height, wall load, and decreasing soil bearing capacity. Always follow engineer specifications based on soil test results.
Concrete volume = Length × Width × Depth. For example, a 10-metre strip footing that's 500mm wide and 450mm deep requires: 10m × 0.5m × 0.45m = 2.25 m³ of concrete. Add 10% for wastage, bringing the order to 2.5 m³. For a typical 150m² house with perimeter footings, expect 12-18 m³ of N20 concrete costing $3,500-$5,500 delivered in 2026.
Strip footings are continuous linear footings that support load-bearing walls, distributing weight along their length. They're common in brick and masonry construction. Pad footings are isolated square or rectangular footings that support individual columns or posts, used in steel-frame and timber-frame buildings. Strip footings are more economical for continuous wall loads; pad footings are better for point loads.
Yes, reinforcement is required in most shallow foundations in Australia. AS 2870 mandates minimum reinforcement to control cracking from shrinkage and thermal movement. Typical strip footings use N12 or N16 bars at 600mm centres longitudinally, with N12 cross bars at 1200mm centres. Pad footings need reinforcement mesh or bar grid in both directions. Proper reinforcement with adequate cover (50-75mm) prevents corrosion and ensures structural integrity.
Bearing pressure = Total load (kN) ÷ Contact area (m²). For example, a column load of 180 kN on an 800mm × 800mm pad footing: Bearing pressure = 180 kN ÷ 0.64 m² = 281 kPa. This must be less than the soil's allowable bearing capacity with appropriate safety factor (typically 2.5-3.0). If calculated pressure exceeds allowable capacity, increase footing dimensions or improve soil conditions.
N20 concrete (20 MPa compressive strength) is standard for most residential footings in Australia. N25 may be specified for heavier loads, adverse soil conditions, or engineering requirements. N32 or N40 is used for commercial applications or high-rise buildings. All footing concrete must meet AS 1379 standards with maximum aggregate size of 20mm and appropriate workability (80-120mm slump) for placement in trenches.
Raft foundations are preferable when: 1) Soil bearing capacity is poor or variable (Class H/E soils), 2) Individual footings would be uneconomical or too close together, 3) Differential settlement risk is high, 4) The building is located on fill or soft clay, 5) Water table is high. Raft slabs distribute loads over a larger area, reducing bearing pressure and minimizing differential settlement in challenging soil conditions.
2026 shallow foundation costs range from $150-$250 per lineal metre for standard strip footings, including excavation, concrete, reinforcement, and labour. A typical 150m² single-storey house requires $5,000-$8,500 for perimeter strip footings. Pad footings cost $200-$400 each. Raft slabs range from $80-$150 per m² depending on thickness and complexity. Add $800-$2,500 for engineering certification and inspection fees.
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