Landslide Risk Calculator
Slope stability factor from soil, angle, and water table.
About this calculator
This calculator applies the infinite slope model, the standard engineering-geology tool for shallow, planar landslides where the soil layer is thin relative to the slope's length, so edge effects can be ignored — the same slope-stability formulation the U.S. Geological Survey uses in its TRIGRS rainfall-triggered-landslide model (Baum, Savage, and Godt, 2008). The Factor of Safety is FOS = (c′ + (γz·cos²α − u)·tanφ′) / (γz·sinα·cosα), comparing the shear strength resisting failure (cohesion plus friction reduced by pore pressure) against the gravitational shear stress driving it. Pore water pressure u is calculated from how far up the soil column the water table sits (your saturation ratio input, from 0=dry to 1=fully saturated), and it matters enormously: rising water table reduces the effective normal stress holding grains together, which is exactly why most landslides happen during or right after heavy rain, not during dry spells.
The model uses a fixed typical soil unit weight of 18 kN/m³ rather than a user input, so results are only as good as that assumption fits your actual soil. A Factor of Safety above 1.5 indicates a stable slope, 1.25–1.5 is marginally stable, 1.0–1.25 is conditionally unstable, and below 1.0 means the driving forces already exceed the resisting forces — failure is expected, not just possible. The calculator also numerically searches for the critical slope angle (the steepest angle that still holds at FOS=1.0 given your soil and water conditions) and a critical soil depth, both useful for excavation or cut-slope planning. Because this is a simplified 2D model, it doesn't account for vegetation root reinforcement, layered stratigraphy, or three-dimensional slope geometry, all of which can meaningfully shift real-world stability in either direction.
Inputs
Results
Factor of Safety
0.88
How to Use This Calculator
- Measure or estimate the Slope angle (°) from topographic data or a clinometer.
- Enter Soil thickness (m) above the potential slip surface — from field investigation or auger logs.
- Set the Friction angle (°) and Cohesion (kPa) from laboratory shear-strength tests or published soil tables.
- Adjust the Saturation ratio (0–1) to represent dry (0), partially saturated (0.5), or fully saturated (1) conditions after heavy rainfall.
- Read the Factor of Safety: values above 1.5 indicate stability, 1.0–1.25 are conditionally unstable, and below 1.0 means failure is expected.
How the result changes with Slope angle (°)
| Slope angle (°) | Factor of Safety |
|---|---|
| 15 | 1.81 |
| 23 | 1.17 |
| 45 | 0.57 |
| 75 | 0.47 |
What each input means
- Slope angle (°)
- Angle of the hillslope from horizontal in degrees.
- Soil thickness (m)
- Depth of soil above the bedrock or potential slip surface.
- Friction angle (°)
- Internal friction angle of the soil. Clay ~15-25°, sand ~30-40°, gravel ~35-45°.
- Cohesion (kPa)
- Effective soil cohesion. Sand/gravel ~0, silty clay ~5-20, stiff clay ~25-50.
- Saturation ratio (0-1)
- Water table height as fraction of soil thickness. 0=dry, 0.5=half saturated, 1=fully saturated.
What each result means
- Factor of Safety
- Ratio of resisting to driving forces. Below 1.0 = failure expected, above 1.5 = stable.
- Stability (0-3)
- 0=Unstable (<1.0), 1=Conditionally unstable (1.0-1.25), 2=Marginal (1.25-1.5), 3=Stable (>1.5).
- Shear strength (kPa)
- Total resisting shear strength along the slip surface.
- Shear stress (kPa)
- Gravitational driving shear stress along the slip surface.
- Critical slope angle (°)
- Maximum slope angle before failure occurs with current soil and water conditions.
- Critical soil depth (m)
- Maximum soil thickness before failure at the current slope angle.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersSlope angle (°) = 30, Soil thickness (m) = 3, Friction angle (°) = 28, Cohesion (kPa) = 5 = 5 input(s) provided
- Calculate Factor of Safety0.884 = 0.884
- Calculate StabilityStability0 = 0
- Calculate Shear strength20.67 = 20.67
Figures and sources
- USGS infinite-slope stability model — Factor of Safety formula for shallow, rainfall-induced landslides (2008) — Baum, R.L., Savage, W.Z., and Godt, J.W., 2008, TRIGRS — A Fortran Program for Transient Rainfall Infiltration and Grid-Based Regional Slope-Stability Analysis, Version 2.0: U.S. Geological Survey Open-File Report 2008-1159, which computes factor of safety cell-by-cell using an infinite-slope analysis following Iverson (2000) and Taylor (1948).
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the Saturation ratio input have such a large effect on the Factor of Safety?
Raising the water table increases pore water pressure u at the slip surface, and that pressure is subtracted directly from the normal stress before the friction term is applied — so a wetter slope loses effective friction even though nothing about the soil's cohesion or friction angle changed. This is the same mechanism the USGS's TRIGRS model (Baum, Savage, and Godt, 2008, Open-File Report 2008-1159) uses to compute how transient rainfall infiltration lowers the infinite-slope factor of safety, and it's the calculator's built-in explanation for why so many real landslides are triggered by heavy rainfall rather than occurring during dry conditions.
What is the Critical slope angle output, and how is it different from my entered Slope angle?
The calculator numerically searches, in half-degree steps, for the steepest angle at which the resisting shear strength still equals or exceeds the driving shear stress under your current soil thickness, cohesion, friction angle, and water saturation. It answers 'how much steeper could this slope get before failing,' which is useful for grading or cut-slope planning, separate from evaluating the angle you actually entered.
Why is the soil unit weight not one of the inputs I can adjust?
The calculator uses a fixed typical value of 18 kN/m³ for soil unit weight rather than exposing it as an input, so the Factor of Safety and related outputs are only as accurate as that assumption is for your actual soil. A much denser or lighter soil than 18 kN/m³ would shift both the driving and resisting stresses and could change the result meaningfully.
What does a Factor of Safety exactly at 1.0 mean in practice?
It means the calculated resisting shear strength exactly equals the driving shear stress along the slip surface — the theoretical threshold between stability and failure in this model. The calculator's four-tier classification treats anything below 1.0 as unstable (failure expected), so a result hovering near 1.0 should be treated as a warning sign given the model's simplifications around soil layering and vegetation effects.
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