What is Footing Size Calculator?
▾
A footing size calculator determines the required dimensions of a concrete foundation element that spreads a structural load over enough soil area to stay within the soil's bearing capacity. Footings are the interface between the structure above and the earth below — if a footing is too small, it sinks (settles) or punches through the soil; too large, it wastes concrete and costs more. The basic bearing pressure equation is: q = P / A, where q is the soil bearing pressure (PSF or ksf), P is the total applied load (lb or kips), and A is the footing area (ft²). The required footing area is: A_req = P / q_allowable. The allowable bearing capacity q_allowable depends on soil type, depth of footing, and foundation width—from 1,000 PSF for soft soil to 4,000+ PSF for dense gravel or shallow bedrock. For a square footing: Side = √(P / q_allow). For a rectangular footing: L × W = P / q_allow with L/W ratio chosen to fit the column or wall spacing. Wall (continuous strip) footings have area expressed per linear foot: w_footing = P_linear / q_allow. Footing depth must be below the frost line—the depth to which soil freezes in winter. Frost heave from freezing water expands soil and can lift or crack a shallow footing. IBC requires footings to extend below the frost depth established by local codes. Frost depths range from 6 inches in southern climates to 60+ inches in northern states and Canada. ACI 318 governs concrete footing design. Footings must be checked for: (1) soil bearing pressure, (2) one-way (beam) shear at d distance from column face, (3) two-way (punching) shear at d/2 from column face, and (4) bending moment at the column face. Minimum footing thickness is 6 inches for unreinforced and 8 inches for reinforced per code. Soil bearing capacity must be confirmed by a geotechnical engineer from borings or load tests on important projects. Presumptive values in IBC Table 1806.2 can be used for light structures in non-critical applications.
DigiCalcs delivers precision-engineered tools for engineers and STEM professionals.
Formula
▾
A_footing = P_total / q_allowable
Side (square) = √(P / q_allow)Variable Legend
▾
| Symbol | Name | Unit | Description |
|---|---|---|---|
| P | — | The power value measured in watts or the applicable unit, representing the rate of energy transfer, consumption, or generation in the system being analyzed |
How to Footing Size Calculator
▾
- 1Gather the required input values: P, q_allow, A, B, L.
- 2Apply the core formula: A_footing = P_total / q_allowable Side (square) = √(P / q_allow).
- 3Compute intermediate values such as Strip footing width if applicable.
- 4Verify that all units are consistent before combining terms.
- 5Calculate the final result and review it for reasonableness.
- 6Check whether any special cases or boundary conditions apply to your inputs.
- 7Interpret the result in context and compare with reference values if available.
Worked Examples
▾
Real-World Applications
▾
Electrical engineers in power distribution companies use Footing Size Calc to size conductors, calculate voltage drop across long cable runs, and verify that circuit breaker ratings provide adequate protection against fault currents in residential, commercial, and industrial installations.
Electronics design engineers apply Footing Size Calc during printed circuit board layout to determine trace widths for required current capacity, calculate impedance matching for high-speed signal traces, and verify thermal dissipation in surface-mount components under worst-case operating conditions.
Maintenance technicians in manufacturing plants use Footing Size Calc to troubleshoot motor control circuits, verify transformer tap settings, and calculate expected current draws when commissioning variable frequency drives and programmable logic controller systems.
Renewable energy system designers rely on Footing Size Calc to size solar panel arrays, calculate battery bank capacity for off-grid installations, and determine inverter ratings that match the expected peak and continuous load demands of residential and commercial photovoltaic systems.
Special Cases
▾
Open circuit or infinite resistance
In practice, this edge case requires careful consideration because standard assumptions may not hold. When encountering this scenario in footing size calculator calculations, practitioners should verify boundary conditions, check for division-by-zero risks, and consider whether the model's assumptions remain valid under these extreme conditions.
Short circuit condition
In practice, this edge case requires careful consideration because standard assumptions may not hold. When encountering this scenario in footing size calculator calculations, practitioners should verify boundary conditions, check for division-by-zero risks, and consider whether the model's assumptions remain valid under these extreme conditions.
Reactive component dominance
In practice, this edge case requires careful consideration because standard assumptions may not hold. When encountering this scenario in footing size calculator calculations, practitioners should verify boundary conditions, check for division-by-zero risks, and consider whether the model's assumptions remain valid under these extreme conditions.
Footing Size Calc reference data
▾
| Soil Type | Presumptive Bearing Capacity (PSF) | Settlement Risk |
|---|---|---|
| Crystalline bedrock | 12,000+ | Negligible |
| Sedimentary bedrock | 4,000–8,000 | Very low |
| Dense gravel/gravel-sand | 3,000–4,000 | Low |
| Loose gravel / dense sand | 2,000–3,000 | Low-moderate |
| Compact fine sand / stiff clay | 1,500–2,000 | Moderate |
| Medium clay | 1,000–1,500 | Moderate-high |
| Soft clay / loose sand | 500–1,000 | High |
| Very soft clay / organic | < 500 | Very high — use piles |
Frequently Asked Questions
▾
How do I calculate the required footing size?
Footing area = Total Load ÷ Soil Bearing Capacity. First, calculate total load: dead load (weight of structure itself — typically 10-15 psf per floor for residential) + live load (occupants, furniture — 40 psf for residential floors, 20 psf for roofs) + snow load (varies by region). For a column supporting a 20×20 ft area over 2 floors: total load ≈ (10+40) × 400 × 2 = 40,000 lbs. With 2,000 psf soil bearing capacity: footing area = 40,000 ÷ 2,000 = 20 sq ft (roughly 4.5 ft × 4.5 ft). Residential strip footings are typically 12-24 inches wide and 6-12 inches deep, depending on load and soil conditions.
How deep should footings be?
Minimum depth: below the frost line to prevent heaving — this ranges from 12 inches in the southern US to 48+ inches in the northern states and Canada. Your local building department specifies the exact depth. General residential guidelines: exterior footings must reach frost depth, interior footings (in heated buildings) can be shallower since the building prevents ground from freezing. Footing thickness: minimum 6 inches for residential, typically the same as the wall thickness above. For standard 8-inch CMU foundation walls on residential loads, a 16-inch wide × 8-inch thick footing is common. Always get a soil test ($300-$500) for new construction — actual soil bearing capacity can vary from 1,000 to 4,000+ psf even on the same property.
What factors affect the bearing capacity of soil?
The bearing capacity of soil is affected by its type, density, and moisture content. For example, clay soils have a lower bearing capacity than sandy soils, with typical values ranging from 1,000 to 4,000 pounds per square foot (psf). A soil's bearing capacity can be estimated using the Terzaghi equation, which takes into account the soil's cohesion, friction angle, and surcharge weight.
How does the type of structure affect footing size?
The type of structure being built determines the load that the footing must support, which in turn affects its required size. For instance, a single-story residential building with a total load of 50 psf may require a footing that is 12 inches wide and 18 inches deep, while a two-story commercial building with a total load of 100 psf may require a footing that is 24 inches wide and 30 inches deep. The load-bearing capacity of the soil must be considered when designing the footing for a specific structure.
Can footing size be adjusted to accommodate different soil conditions?
Yes, footing size can be adjusted to accommodate different soil conditions. For example, if the soil has a low bearing capacity, a wider footing can be used to distribute the load over a larger area, reducing the pressure on the soil. Alternatively, a deeper footing can be used to reach more stable soil or rock, with the depth of the footing typically increasing by 12 to 18 inches for every 1,000 psf of load. This can help to ensure that the footing remains stable and can support the weight of the structure above.
Common Mistakes to Avoid
▾
- !Not adding footing self-weight to the column load before sizing
- !Using net bearing pressure without adding back soil overburden when appropriate
- !Specifying footing depth above the local frost line
- !Sizing footing based on live load only — dead load plus live load (service loads) must be used for bearing checks
- !Skipping punching shear check — column can punch through thin footings even if bearing is OK
Pro Tip
Always specify footings a standard size: width in 6-inch increments and thickness in 2-inch increments (8, 10, 12, 14 in). Non-standard sizes cost more in formwork and rebar detailing.
Did you know?
The Leaning Tower of Pisa began tilting within a decade of the start of construction in 1173 because its foundation rested on soft, unstable soil. Modern geotechnical engineering — which emerged only in the 1920s — would have predicted and prevented this famous structural mistake.
References
Have a question about this calculator? Get a detailed answer.
Get Weekly Math Tips
Join 12,000+ subscribers who get calculator tips every week.