A footing needs enough area to transfer the building load to the soil without exceeding the allowable soil bearing pressure. For a simple preliminary calculation, divide the supported load by the allowable soil bearing pressure. A square footing then uses the square root of the required area to find its side length.
The basic relationship is:
For example, a 600 kN supported load on soil with an allowable bearing pressure of 150 kN/m² gives a preliminary required area of 4.0 m². A square footing with this plan area has sides of 2.0 m.
Table of Contents
- What Foundation Area Means
- Foundation Area vs Footing Size
- Basic Footing Area Formula
- How Soil Bearing Pressure Affects Footing Area
- How to Calculate Footing Area From Load
- How to Calculate Square Footing Dimensions
- How to Calculate Rectangular Footing Dimensions
- Worked Footing Size Example
- Multiple Columns and Footings
- Strip Foundation Area and Width
- Converting Foundation Area to Dimensions
- Common Footing Calculation Mistakes
- Foundation Area Calculation Method
- Frequently Asked Questions
What Foundation Area Means
Foundation area is the portion of the foundation that transfers structural load into the supporting soil. For an isolated pad footing, the plan area is usually expressed as length × width.
The required area depends heavily on the load reaching the footing and the allowable soil bearing pressure. A heavier load generally requires more bearing area. Soil with a lower allowable bearing pressure also requires more area for the same load.
For a rectangular footing:
For a square footing:
Foundation Area vs Footing Size
Foundation area and footing size describe related but different measurements.
| Term | Meaning |
|---|---|
| Foundation area | The horizontal plan area transferring load to the soil. |
| Footing length | One horizontal dimension of a rectangular footing. |
| Footing width | The other horizontal dimension of a rectangular footing. |
| Footing thickness | The vertical depth of the concrete footing. |
| Footing volume | Plan area multiplied by footing thickness, subject to the actual footing geometry. |
A footing with an area of 4 m² might measure 2 m × 2 m, 1.6 m × 2.5 m, or another combination producing the same area. The final dimensions depend on structural layout, column position, loads, soil conditions and design requirements.
Basic Footing Area Formula
The simplified preliminary formula uses supported load and allowable soil bearing pressure:
A = required footing area
P = supported load
qa = allowable soil bearing pressure
Use matching units. For example, when the load is in kN and the allowable soil pressure is in kN/m², the resulting footing area is in m².
Example
Suppose:
- Supported load = 600 kN
- Allowable soil bearing pressure = 150 kN/m²
Apply the formula:
A = 4.0 m²
The preliminary required bearing area is 4.0 m².
How Soil Bearing Pressure Affects Footing Area
Allowable soil bearing pressure represents the pressure used for foundation design without exceeding the applicable soil performance limits. The value comes from geotechnical information, site investigation, engineering assessment or applicable project requirements.
The relationship is direct:
Lower allowable bearing pressure → larger required area for the same load
| Supported Load | Allowable Bearing Pressure | Calculated Area |
|---|---|---|
| 300 kN | 100 kN/m² | 3.00 m² |
| 300 kN | 150 kN/m² | 2.00 m² |
| 600 kN | 100 kN/m² | 6.00 m² |
| 600 kN | 150 kN/m² | 4.00 m² |
| 900 kN | 200 kN/m² | 4.50 m² |
The table shows the mathematical relationship only. The listed areas are not recommendations for actual buildings.
How to Calculate Footing Area From Load
Start by identifying the load supported by the footing and the allowable soil bearing pressure used for the project.
Step 1: Determine the supported load
Identify the load transferred to the foundation support. Depending on the design, this includes relevant permanent loads, imposed loads and other applicable actions.
Step 2: Establish the allowable soil bearing pressure
Use the value provided by the appropriate geotechnical or structural design process. Do not assume a soil pressure from appearance, soil color or a generic online table.
Step 3: Divide load by soil pressure
Step 4: Select a practical footing shape
Convert the required area into square or rectangular dimensions while considering the column location, property boundaries, adjacent foundations and structural layout.
Step 5: Complete structural checks
The preliminary bearing-area calculation is only one part of footing design. Structural checks determine the required thickness, reinforcement, shear resistance, bending resistance and other details.
How to Calculate Square Footing Dimensions
When the footing is square, both sides have the same length.
Example: Square Footing
Required area = 4.0 m²
Side = 2.0 m
The corresponding square plan is 2.0 m × 2.0 m.
If the calculated area is 6.25 m²:
The square plan is therefore 2.5 m × 2.5 m for the mathematical example.
How to Calculate Rectangular Footing Dimensions
A rectangular footing has two different plan dimensions. The product of the length and width must equal the required area.
If one dimension is selected first, calculate the other dimension by dividing the area by the known dimension.
Example: Rectangular Footing
Suppose the required bearing area is 6 m² and the selected footing length is 3 m.
Width = 2 m
The resulting rectangular plan is 3 m × 2 m, giving an area of 6 m².
Practical footing dimensions also depend on the position of the supported column and the direction of the applied loads. A mathematical rectangle is not automatically a suitable structural arrangement.
Worked Footing Size Example
Consider a simplified isolated column footing example:
- Supported load = 600 kN
- Allowable soil bearing pressure = 150 kN/m²
- Footing shape = square
1. Calculate the required area
A = 600 ÷ 150
A = 4.0 m²
2. Calculate the square side
3. Check the plan area
The mathematical result is a 2.0 m × 2.0 m square plan.
Multiple Columns and Footings
A building with several columns often uses several isolated footings. Each footing needs to be considered according to the load reaching its support.
Do not automatically divide the total building load by the number of columns. Loads are often distributed unevenly because of different column positions, spans, floor areas, wall loads and structural framing.
For example, an interior column might carry load from a larger tributary area than an edge column. A corner column often carries a different load again.
For each footing, the design process should establish the relevant support load before calculating the preliminary bearing area.
Strip Foundation Area and Width
Strip footings support continuous walls or lines of columns. Their calculation differs from an isolated square footing because the load is commonly expressed per unit length.
For a simplified preliminary calculation:
If the wall load is expressed in kN/m and soil pressure in kN/m², the result is in metres.
Example: Strip Footing Width
Suppose:
- Wall load = 180 kN/m
- Allowable soil bearing pressure = 120 kN/m²
Width = 1.5 m
The simplified calculation gives a preliminary bearing width of 1.5 m.
Actual strip footing design also considers wall geometry, eccentricity, footing weight, soil settlement, shear, bending, reinforcement and applicable design rules.
Converting Foundation Area to Dimensions
Once the required area is known, several dimension combinations are possible.
| Required Area | Possible Length | Corresponding Width |
|---|---|---|
| 4 m² | 2 m | 2 m |
| 4 m² | 2.5 m | 1.6 m |
| 6 m² | 3 m | 2 m |
| 8 m² | 4 m | 2 m |
| 10 m² | 5 m | 2 m |
Use:
The chosen dimensions need to fit the structural layout and site conditions. Property lines, neighboring footings, column position and construction access often affect the final arrangement.
Common Footing Calculation Mistakes
Using the wrong soil pressure
A footing calculation is only as reliable as the soil information used. A generic soil value should not replace project-specific geotechnical information.
Mixing units
Keep load and pressure units compatible. A load in kN combined with soil pressure in kN/m² produces area in m².
Confusing area with volume
Foundation bearing area describes the horizontal contact area. Concrete volume also depends on footing thickness and shape.
Dividing total building load equally
Individual supports often carry different loads. Each footing needs the appropriate support load for its location.
Ignoring eccentric loading
A column positioned away from the footing center produces an uneven pressure pattern. A simple load-area calculation does not capture every eccentricity effect.
Treating preliminary sizing as final design
Bearing pressure is only one part of foundation design. Structural resistance, settlement and construction requirements also affect the final footing.
Foundation Area Calculation Method
For a quick preliminary calculation, use these steps:
- Enter the supported load.
- Enter the allowable soil bearing pressure.
- Divide the load by the soil pressure.
- Record the required bearing area.
- For a square footing, calculate the square root of the area.
- For a rectangular footing, divide the area by the selected length.
- Review the result against the structural and geotechnical design requirements.
Required area = Load ÷ Allowable soil bearing pressure
Square footing:
Side = √Required area
Rectangular footing:
Width = Required area ÷ Length
Strip footing:
Width = Load per unit length ÷ Allowable soil bearing pressure
For other construction calculations, see the BuildersMeasures construction calculators.
Frequently Asked Questions
How do you calculate foundation area?
For a simplified preliminary calculation, divide the supported load by the allowable soil bearing pressure. The result gives the required bearing area when compatible units are used.
What is the formula for footing size?
The basic bearing-area formula is footing area = supported load ÷ allowable soil bearing pressure. A square footing uses the square root of the area to find its side length.
How do you calculate square footing size?
First calculate the required area. Then calculate the square root of the area. For example, 4 m² gives a square side of 2 m.
How does soil bearing pressure affect footing area?
A lower allowable soil bearing pressure requires a larger bearing area for the same supported load. A higher allowable pressure requires less area for the same simplified calculation.
How do you calculate strip footing width?
For a simplified preliminary calculation, divide the wall or support load per unit length by the allowable soil bearing pressure. When the load is in kN/m and pressure is in kN/m², the result is in metres.
Does footing area equal footing size?
Footing area describes the horizontal plan area. Footing size often refers to the length and width dimensions. A 2 m × 2 m footing has a plan area of 4 m².
Does footing thickness affect footing area?
Footing thickness is a separate vertical dimension. Thickness affects concrete volume and structural capacity, while the plan area describes the soil contact area.
Is a calculated footing size ready for construction?
No. A simple load-area calculation provides preliminary bearing-area information. Final footing design requires appropriate structural and geotechnical checks, project loads, soil conditions and applicable building requirements.
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