What Is a Footing Calculator?
A footing calculator estimates the bearing area required to transfer a vertical structural load to the supporting soil without exceeding a specified allowable soil bearing pressure. For a concentrically loaded isolated footing, the basic relationship compares service load with available bearing resistance.
This calculator can estimate either a square footing or a rectangular footing. It also uses the selected footing thickness to estimate concrete volume, which can be useful during early quantity planning.
What Does This Calculator Calculate?
- Required footing bearing area
- Square footing side dimension
- Rectangular footing width for a selected length
- Total service load including a user-defined allowance
- Average soil bearing pressure
- Approximate footing concrete volume
- Approximate concrete self-weight
How to Use the Footing Calculator
Enter the Load
Enter the approximate vertical service load transferred from the supported column or structure to the footing.
Enter Soil Pressure
Enter the allowable soil bearing pressure applicable to the proposed foundation location and soil conditions.
Calculate Footing Size
Choose the footing shape and calculate the required area, dimensions, soil pressure, and estimated concrete volume.
How Is Footing Area Calculated?
For a simplified concentrically loaded footing, the minimum required bearing area is obtained by dividing the total service load by the allowable bearing pressure of the supporting soil.
For example, if the total service load is 660 kN and the allowable soil bearing pressure is 150 kPa, the theoretical required footing area is 4.40 m². If a square footing is used, each side would be approximately the square root of 4.40, or about 2.10 m before any practical dimension adjustments or structural design checks.
Square vs. Rectangular Footings
Square Footing
A square isolated footing has equal length and width. Once the required area is known, the theoretical side dimension is calculated as the square root of the required area.
Rectangular Footing
A rectangular footing can be useful when site geometry, column layout, property boundaries, or neighboring foundations limit one direction. In this calculator, you select the footing length and the tool determines the corresponding theoretical width.
What Is Soil Bearing Pressure?
Soil bearing pressure represents the average pressure transmitted from the footing to the supporting ground. In a simplified concentric load calculation, average bearing pressure is total vertical load divided by footing contact area.
The allowable soil pressure used for foundation design depends on site-specific soil conditions and applicable design requirements. A calculated footing area alone does not establish that settlement or other geotechnical performance requirements will be acceptable.
How Is Footing Concrete Volume Calculated?
Once footing length, width, and thickness are known, the approximate concrete volume is calculated by multiplying those three dimensions. The result is shown in cubic meters for metric inputs or cubic feet for imperial inputs.
Actual concrete ordering may need to account for excavation tolerances, construction method, waste, irregular geometry, pedestals, keys, and other project-specific details that are not included in this calculator.
Metric and Imperial Footing Units
| Measurement | Metric Mode | Imperial Mode |
|---|---|---|
| Vertical Load | kN | lb |
| Soil Bearing Pressure | kPa / kN/m² | psf / lb/ft² |
| Dimensions | m | ft |
| Footing Area | m² | ft² |
| Concrete Volume | m³ | ft³ |
Switching the unit selector changes the unit labels and recommended example values. Values already entered should always be checked to ensure they correspond to the selected measurement system.
Footing Sizing Is Not Complete Foundation Design
Required bearing area is only one part of foundation design. Actual footing dimensions and structural reinforcement may depend on column size, load combinations, eccentricity, bending, one-way shear, punching shear, concrete strength, reinforcement properties, cover, settlement, groundwater, soil variability, and other conditions.
This calculator therefore should not be used by itself to determine whether a real foundation is structurally or geotechnically safe. Construction and structural decisions should follow applicable plans, local requirements, site information, and qualified professional review.
Footing Calculator FAQs
It calculates preliminary required footing area, square or rectangular dimensions, average soil pressure, concrete volume, and approximate concrete weight based on the values entered.
For the simplified service-load method used here, divide the total vertical service load by the allowable soil bearing pressure: A = P ÷ q.
Yes. For a square footing, the calculator determines the theoretical side dimension by taking the square root of the required bearing area.
Yes. Choose Rectangular Footing, enter the selected footing length, and the calculator determines the theoretical width needed to provide the required area.
No. Reinforcement design requires structural checks including bending, one-way shear, punching shear, material strengths, load combinations, cover, and applicable design requirements.
The thickness input in this calculator is used for concrete volume estimation. The user-defined foundation allowance can be used to approximately account for additional foundation weight in the preliminary bearing-area calculation.
No. The result is a preliminary mathematical estimate and does not replace structural or geotechnical design, site investigation, approved plans, or applicable building requirements.