Soil Bearing Capacity Calculator
Estimate the ultimate and allowable bearing capacity of a shallow foundation from soil cohesion, friction angle and unit weight, using Terzaghi's classical theory — strip, square, circular or rectangular footings, general or local shear failure, SI or US units.
Input
SI metric enters cohesion in kilopascals, unit weight in kN/m³ and dimensions in metres; US customary uses pounds per square foot, pounds per cubic foot and feet.
Terzaghi's shape factors differ by footing geometry. Circular uses the width field as the diameter.
Local shear reduces cohesion and friction to two-thirds of their entered values before computing the bearing factors, per Terzaghi's local-shear model for loose/soft soils.
Soil cohesion (kPa in SI, psf in US customary). Zero for cohesionless (granular) soils.
Soil's angle of internal friction in degrees (0-45°). Zero for purely cohesive (undrained clay) soils.
Bulk unit weight of the soil (kN/m³ in SI, pcf in US customary).
Footing width (metres in SI, feet in US customary). For a circular footing, this is the diameter.
Depth of the footing base below ground surface (metres in SI, feet in US customary).
Safety factor applied to the ultimate bearing capacity — 3 is the conventional value for shallow foundations.
Output
| Metric | Value |
|---|---|
| No data yet | |
One-line overview of the net allowable bearing capacity.
More ways to use this tool
REST API
curl -X POST https://api.iotools.cloud/v1/tool/soil-bearing-capacity-calculator \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"unitSystem": "si",
"footingShape": "strip",
"soilCondition": "general",
"cohesion": "0",
"frictionAngle": "30",
"unitWeight": "18",
"footingWidth": "1.5",
"footingDepth": "1",
"factorOfSafety": "3"
}'Swap in your own key from your account. The tool's fields are the body — no wrapper.
Ask an AI agent
Use the IOTools `soil-bearing-capacity-calculator` tool (Soil Bearing Capacity Calculator) on this input:
YOUR_INPUT_HEREPaste this at any agent connected to the IOTools MCP server, then add your input.
Embed widget
<iframe
src="https://iotools.cloud/embed/soil-bearing-capacity-calculator/"
width="100%" height="520" frameborder="0" scrolling="no" loading="lazy"
title="Soil Bearing Capacity Calculator — iotools.cloud"
sandbox="allow-scripts allow-forms allow-same-origin allow-downloads allow-popups allow-popups-to-escape-sandbox"
allow="clipboard-write"
style="width:100%;border:1px solid #e5e7eb;border-radius:12px;overflow:hidden"></iframe>
<script src="https://iotools.cloud/embed.js" async></script>Drop this into your own page — free, no key required, just a link back.
| Cost per API/MCP call | From 5 credits |
|---|---|
| Need more credits? | View pricing |
Also available with
Guides
The Soil Bearing Capacity Calculator estimates the ultimate and allowable bearing capacity of a shallow foundation from the soil's cohesion, friction angle and unit weight, using Terzaghi's classical bearing capacity theory — the equation geotechnical and structural engineers reach for to size footings before detailed design. It supports strip, square, circular and rectangular footings, general or local shear failure, and SI metric or US customary units.
How to use it
- Pick your unit system — SI metric (kPa, kN/m³, metres) or US customary (psf, pcf, feet).
- Choose the footing shape. Circular uses the width field as the diameter; rectangular reveals a length field.
- Choose the failure mode — general shear for dense sand or stiff clay, local shear for loose sand or soft clay.
- Enter the soil's cohesion and friction angle, its unit weight, the footing width (and length, for rectangular) and depth, and the factor of safety.
Results update instantly: Terzaghi's dimensionless bearing capacity factors (Nc, Nq, Nγ), the effective overburden pressure at the footing base, the ultimate bearing capacity, and both the gross and net allowable bearing capacity. A one-line summary gives the net allowable bearing capacity for quick copying.
What is Terzaghi's bearing capacity equation?
For a strip footing, Terzaghi's equation is:
qu = c·Nc + q·Nq + 0.5·γ·B·Nγwhere c is soil cohesion, q = γ·Df is the effective overburden pressure at the footing base, γ is the soil's unit weight, B is the footing width, and Nc, Nq, Nγ are bearing capacity factors that depend only on the soil's friction angle φ. Square, circular and rectangular footings apply Terzaghi's shape factors to the cohesion and unit-weight terms:
- Square:
qu = 1.3·c·Nc + q·Nq + 0.4·γ·B·Nγ - Circular (B = diameter):
qu = 1.3·c·Nc + q·Nq + 0.3·γ·B·Nγ - Rectangular (B = width, L = length):
qu = c·Nc·(1 + 0.3·B/L) + q·Nq + 0.5·γ·B·Nγ·(1 - 0.2·B/L)
This calculator computes the bearing capacity factors from φ with the widely used closed-form approximations Nq = e^(π·tanφ)·tan²(45° + φ/2), Nc = (Nq - 1)·cotφ, Nγ = 1.5·(Nq - 1)·tanφ (5.7, 1 and 0 respectively at φ = 0), rather than reading them off Terzaghi's original charts.
General shear vs. local shear
Terzaghi distinguished two failure modes. General shear failure applies to dense sand and stiff clay, which mobilize their full strength before failing. Local shear failure applies to loose sand and soft clay, which fail progressively (punching) before reaching peak strength — modeled by reducing the strength parameters before computing the bearing factors: c' = (2/3)·c and tanφ' = (2/3)·tanφ. Local shear always gives a lower, more conservative bearing capacity for the same soil parameters.
Gross vs. net allowable bearing capacity
The gross allowable bearing capacity (qa = qu / FS) is the total pressure the soil can carry, overburden included. The net allowable bearing capacity (qa,net = (qu - q) / FS) is the additional pressure the foundation load can add on top of what the excavation already removed — the value foundation design actually checks the applied load against. A factor of safety of 3 is conventional for shallow foundations.
This calculator doesn't apply a water table correction — it assumes the soil above and below the footing is at the entered unit weight. For a site with a shallow water table, use a reduced (buoyant) unit weight for the affected zone.
Is my data private?
Completely. Every calculation runs entirely in your browser. Nothing you type is uploaded, stored or sent to a server, so you can run as many foundation checks as you like in total privacy.