Soil Bearing Capacity Load Calculator

Calculate allowable foundation load, required footing size, and safety margin based on soil type and structural load.

Soil Type

Enter a known bearing capacity below, or select a soil type to auto-fill.

Ultimate Bearing Capacity (ksf or kPa)

Please enter a valid bearing capacity > 0.

Units

Factor of Safety

3.0

Safety factor must be ≥ 1.0.

Total Structural Load (kips)

Please enter a valid load > 0.

Footing Shape

Footing Width (B) (ft)

Must be > 0.

Footing Length (L) (ft)

Must be > 0.

Embedment Depth (D) (ft)

Soil Unit Weight (pcf)

How to Use This Soil Bearing Capacity Calculator

Select your soil type (or enter a known ultimate bearing capacity), choose your unit system, set the factor of safety, and enter your total structural load and footing dimensions. Click Calculate to instantly see the allowable bearing capacity, actual soil pressure, required footing area, and a pass/fail status for your design.

You can also enter known footing dimensions to check whether an existing design is adequate, or leave dimensions blank to find the minimum required footing size automatically.

Why This Matters

Soil bearing capacity is the most fundamental parameter in foundation design. Get it wrong, and you risk differential settlement, structural cracking, or catastrophic foundation failure — even if the structure above is perfectly designed.

Consider a two-story house with a 120-kip column load on medium dense sand (ultimate capacity ~4.5 ksf). With a 3.0 factor of safety, the allowable capacity is only 1.5 ksf. A 6×6 ft footing (36 sq ft) produces just 3.3 ksf of pressure — far exceeding what's safe. You'd need at least a 9×9 ft footing. This kind of error is shockingly common on residential projects where soil testing is skipped.

Engineers use this calculation on every foundation design. Contractors reference it when verifying shop drawings. Home inspectors flag it when existing footings are undersized. Getting the numbers right before you pour is always cheaper than underpinning later.

How It's Calculated

The core formula chain is:

Allowable Bearing Capacity: q_allow = q_ultimate ÷ FS
Footing Area Required: A_required = P ÷ q_allow
Actual Soil Pressure: q_actual = P ÷ A_provided
Overburden Pressure: q_overburden = γ × D (soil unit weight × depth)
Net Allowable: q_net = q_allow − q_overburden

Where P = total structural load, FS = factor of safety (typically 2.5–3.0 for most soils), γ = soil unit weight, and D = embedment depth. The net allowable capacity accounts for the weight of soil already present at the foundation depth.

Typical ultimate bearing capacities range from ~0.5 ksf for soft clay to over 100 ksf for hard rock. Always verify with a geotechnical report for critical structures.

Tips & Common Mistakes

Always get a soil test for commercial projects. Published soil type values are rough estimates. Cone penetration tests (CPT) or Standard Penetration Tests (SPT) provide site-specific data.
Don't confuse ultimate and allowable capacity. If your geotech report says "allowable bearing = 2 ksf," that's already factored — don't divide by safety factor again.
Include the footing self-weight. A 6×6×2 ft concrete footing weighs about 10.8 kips. Add this to your column load for accurate pressure calculations.
Depth matters. Deeper footings benefit from overburden confining pressure, especially in sand. Going from 2 ft to 4 ft depth can meaningfully increase allowable capacity.
Eccentric loads reduce effective area. If your load isn't centered on the footing, use the effective footing dimensions (B' = B − 2e) instead of the full dimensions.

Frequently Asked Questions

What safety factor should I use for soil bearing capacity?

For most residential and commercial buildings, a factor of safety of 2.5 to 3.0 is standard. Use 3.0 or higher when soil data is uncertain, loads are variable, or the structure is critical (hospitals, schools). For temporary structures with well-defined loads, 2.0 may be acceptable.

What's the difference between gross and net bearing capacity?

Gross bearing capacity is the total pressure the soil can support, including the weight of the overburden soil above the footing. Net bearing capacity subtracts that overburden, representing only the additional load the footing can safely apply. For shallow footings, the difference is small; for deep footings, it becomes significant.

Can I use this calculator for retaining wall footings?

Yes, with care. For retaining walls, you also need to check sliding and overturning stability, not just bearing capacity. Enter the total vertical reaction (dead load + wall weight + soil weight on heel) as your structural load, and use a strip footing shape matching your wall length.

What if my calculated required area is larger than practical?

If the required footing is too large, consider: (1) going deeper to access better soil, (2) a mat/raft foundation that spreads load over the entire building footprint, (3) pile foundations to transfer load to deeper competent strata, or (4) ground improvement techniques like compaction grouting.