Size Bearing Area Before You Estimate Concrete
Estimate preliminary footing dimensions from service load and allowable soil-bearing pressure. Check rectangular footing pressure, size continuous strip footings, estimate round pier pads, and calculate concrete volume, order allowance, bags and optional ready-mix cost.
Choose a footing type, enter the project load and soil value, then calculate preliminary bearing geometry and concrete quantity. Results stay closed until Calculate is pressed.
Estimate the plan area and rounded-up square side needed to keep average service bearing pressure at or below the soil value you enter.
1 kip = 1,000 lb
For quantity only; not structural design.
Enter an existing or proposed footing length and width to compare average soil pressure with the allowable value you provide.
Estimate preliminary footing width from a service line load expressed in pounds per linear foot and an allowable soil-bearing value.
Estimate a preliminary circular footing-pad diameter for a centered vertical service load using the soil-bearing value entered.
A footing spreads concentrated or line loads over enough soil area that the average contact pressure is compatible with the allowable soil-bearing value used for the project. The simplest preliminary relationship is required bearing area = service load ÷ allowable soil-bearing pressure. For a square footing, the square root of that required area gives the theoretical side dimension. For a circular footing, the equivalent diameter comes from the area of a circle. For a continuous wall footing, line load divided by allowable soil pressure gives a theoretical footing width.
This Concrete Footing Size Calculator automates that arithmetic and then rounds the theoretical dimension upward to a practical increment selected by the user. It also uses an entered footing thickness to estimate concrete volume. The thickness input is a quantity input only—it is not calculated as an adequate structural depth.
For a continuous footing: required width (ft) = line load (lb/ft) ÷ allowable soil pressure (lb/ft²).
Convert a centered concentrated service load into preliminary square plan dimensions.
Estimate continuous footing width from a wall or other service line load.
Estimate equivalent circular bearing area and diameter for a centered pier load.
Convert entered geometry and thickness into cubic feet, cubic yards and bag estimates.
Allowable soil-bearing pressure is a project-specific value used to limit contact pressure between a footing and supporting soil. It should not be guessed from appearance alone. Soil type, density, moisture, fill history, groundwater, nearby slopes, settlement sensitivity and local code provisions can all affect the value appropriate for a site.
The 2024 International Residential Code states that footings must transmit loads to the soil within limitations determined from the character of the soil, and that footings should bear on undisturbed natural soils or engineered fill. Its residential footing provisions also tie footing width to soil load-bearing value. Those prescriptive provisions apply only where the code and project conditions permit them; they are not a universal replacement for geotechnical or structural design.
When reliable geotechnical information is available, enter the allowable bearing value specified for the relevant footing condition. If the project uses a prescriptive code table, use the value and minimum dimensions required by that adopted code. Never increase the soil value merely to make a footing smaller.
This calculator labels the load input as service load because allowable soil-bearing values are commonly paired with service-level loading in preliminary bearing checks. Do not mix a factored structural load with an allowable soil pressure unless the governing design method specifically requires that combination. Mixing incompatible load and resistance formats can produce misleading footing dimensions.
A column service load may include dead load, live load, snow load and other applicable effects transferred to the foundation. A strip footing line load may represent the service reaction from a wall and supported floors or roof. The correct load path should come from structural analysis or approved construction documents, not from a generic online estimator.
Suppose a centered column service load is 18 kips, or 18,000 lb, and the allowable soil-bearing value entered is 2,000 psf. The theoretical area is 18,000 ÷ 2,000 = 9 ft². A square with 9 ft² of area is 3 ft by 3 ft. If the theoretical side were 3.08 ft, a practical layout would normally round upward rather than downward, while also checking all required code and structural minimums.
The calculator then uses the entered thickness to estimate concrete. For example, a 3 ft × 3 ft footing entered as 12 inches thick contains 9 ft³, or about 0.33 yd³, before any ordering allowance. That volume calculation says nothing about whether 12 inches is structurally adequate.
A continuous footing distributes a wall or line load along its length. If a wall delivers 2,800 lb per linear foot and the allowable soil pressure is 2,000 psf, the theoretical width is 2,800 ÷ 2,000 = 1.4 ft, or 16.8 inches. The practical footing must then satisfy the applicable code, projection, construction and structural requirements.
For quantity planning, the calculator multiplies the rounded footing width by total footing length and the thickness you enter. Strip footings can consume substantial concrete on long wall runs, so accurate total length measurements, corners, steps and changes in width should be accounted for separately when they occur.
Round concrete pads are common below some piers, posts and columns. The required bearing area is calculated the same way as for a square pad, but the area is converted into a circular diameter. If required area is A, the theoretical diameter is √(4A/π). The calculator rounds that diameter upward to the selected increment and then reports the average soil pressure under the rounded footprint.
A round bearing pad should not be confused with the diameter of a pier shaft or sonotube. A narrow shaft may transfer load into a wider base, bell, pad or engineered footing. The final geometry depends on the structural system, frost requirements and soil conditions.
The footing spreads a structural reaction over a larger soil area. Plan dimensions control average contact pressure; footing depth and reinforcement require separate structural checks.
A footing can have enough plan area for average soil pressure and still be structurally inadequate. Concrete thickness may be controlled by one-way shear, punching shear around a column, bending, reinforcement development, minimum cover, column or wall geometry, construction tolerances and local code requirements. This is why the calculator asks for thickness only to estimate concrete volume.
For residential work within its scope, ACI 332 addresses concrete footings and foundation construction. ACI guidance also emphasizes that project-specific codes and construction documents take precedence over generic illustrations. For engineered buildings, the governing structural concrete code, geotechnical report and construction documents should be followed.
Prescriptive residential codes may impose minimum footing dimensions independent of the mathematical bearing area. For example, the 2024 IRC includes minimum footing widths and thicknesses in Section R403 for qualifying residential construction and states that footing sizes for piers and columns are based on tributary load and allowable soil pressure. Local amendments, frost rules, seismic provisions, expansive soils and other site conditions can alter what is required.
Therefore, if the calculator returns a dimension smaller than a code minimum, the code minimum does not disappear. Likewise, a larger bearing area does not automatically satisfy structural thickness, reinforcement or embedment requirements. Treat the result as a preliminary planning number that must be reconciled with the adopted code and approved design.
In freezing climates, foundation elevation and frost protection can be as important as footing width. Footings may need to extend below local frost depth or use an approved frost-protected shallow foundation system. In warm climates, other embedment or expansive-soil rules may control. The calculator intentionally does not select footing depth below grade because these requirements are location- and project-specific.
The bottom of the excavation should provide competent support consistent with the project requirements. Loose material, uncontrolled fill, standing water, organic soil and disturbed bearing surfaces can invalidate assumptions used in a simple pressure calculation. Where engineered fill is specified, placement and compaction should follow the project geotechnical requirements.
Footing work frequently involves excavations, and excavation hazards can be severe. OSHA excavation rules address issues such as adjacent-structure stability and circumstances where excavation below an existing foundation could endanger workers or the structure. Where excavation could affect an adjacent footing, wall or building, appropriate support, engineering review or other protective measures may be required.
This calculator does not evaluate trench stability, protective systems, utilities, access/egress, spoil placement, groundwater or nearby structures. Those must be managed as separate site-safety and construction-planning tasks.
Once final footing dimensions are known, concrete quantity is straightforward geometry. A rectangular footing volume is length × width × thickness. A round footing uses π × radius² × thickness. Convert inches to feet before multiplying, then divide cubic feet by 27 to obtain cubic yards.
The calculator offers an optional ordering allowance. Extra concrete can help cover minor excavation irregularities, dimensional variation and unavoidable handling losses, but the appropriate allowance is project-specific. Very small pours may be more practical with bagged concrete, while larger footings often use ready-mix. Always verify bag yield and supplier minimum-order or short-load policies.
| Footing Type | Primary Inputs | Preliminary Output | Still Requires Verification |
|---|---|---|---|
| Square isolated | Service load + soil bearing | Required area and square side | Thickness, shear, bending, reinforcement |
| Rectangular check | Load + actual length/width | Average bearing pressure | Eccentricity, settlement, structural capacity |
| Strip footing | Line load + soil bearing | Required theoretical width | Code minimums, projections, reinforcement |
| Round footing | Service load + soil bearing | Required area and diameter | Pier connection, frost, thickness, reinforcement |
Use service reactions or line loads from approved design information.
Use the allowable bearing pressure applicable to the actual site.
Calculate preliminary square, strip or round dimensions.
Check code minimums, thickness, reinforcement, frost and settlement.
Use final approved dimensions to estimate volume and delivery quantity.
The calculated footing area is directly proportional to the soil value. An unrealistically high input can make the footing look dangerously small. Use geotechnical or code-permitted values appropriate to the project.
The service reaction used in design may need to include multiple load components. Use the load combination and foundation reaction required by the project design method.
Plan area is not a substitute for checking concrete shear, bending, reinforcement, column transfer, settlement or eccentricity.
If a theoretical side is 35.2 inches, using 35 inches reduces area. Preliminary construction sizing should normally round up, subject to project requirements.
Quantity should reflect the actual concrete geometry. If concrete is cast against earth, field excavation irregularities may increase volume; if formed, use the formed dimensions.
Estimate general slab, wall, pier and footing concrete quantity.
Open calculator →Plan pier geometry and concrete volume for column-style foundations.
Open calculator →Estimate reinforcement quantities after the structural layout is known.
Open calculator →Convert specified reinforcement spacing into bar counts and lengths.
Open calculator →Convert final footing geometry into cubic yards and cubic meters.
Open calculator →Plan placement duration once the final concrete quantity is known.
Open calculator →For final requirements, consult the adopted building code and project documents. Useful authoritative resources include the 2024 International Residential Code foundation provisions, ACI 332 Code Requirements for Residential Concrete, and OSHA excavation requirements. These resources do not replace project-specific engineering, geotechnical recommendations or local amendments.
Common questions about footing area, soil bearing, dimensions and concrete quantity.