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Slope Stability Factor Calculator

Work out slope stability factor instantly with clear inputs, formula shown and shareable results.

About Slope Stability Factor Calculator

For a long slope with a shallow planar slip surface, the factor of safety is the available shear strength divided by the driving shear stress: (c + gamma z cos²beta tan phi) / (gamma z cos beta sin beta). Cohesion helps most at shallow depth, which is why a cohesive slope can stand steeply until it is undercut.

Formula

Infinite slope stability

FoS = (c + gamma z cos²beta tan phi) / (gamma z cos beta sin beta)

beta
Slope angle
z
Depth to slip plane
phi
Friction angle

Worked example

With the default values below, Slope Stability Factor Calculator returns Factor of safety of 1.624. Change any input above and every figure here is recalculated in your browser.

Inputs used

Slope angleaccepts 1–60 °
25 °
Soil cohesionminimum 0 kPa
10 kPa
Angle of internal frictionaccepts 1–45 °
28 °
Depth to slip surfaceminimum 0.1 m
3 m

Results

Factor of safety
1.624
Driving shear stress
20.68 kPa
Available shear strength
33.58 kPa
Assessment
Adequate for permanent works

Step by step

  1. 1Values used
  2. 2Infinite slope stability
  3. 3Factor of safety= 1.624
  4. 4Driving shear stress= 20.68 kPa
  5. 5Available shear strength= 33.58 kPa
  6. 6Assessment= Adequate for permanent works

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Slope stability assessment must be carried out by a qualified geotechnical engineer with groundwater data and appropriate analysis methods.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Slope stability assessment must be carried out by a qualified geotechnical engineer with groundwater data and appropriate analysis methods.

Frequently Asked Questions

How is the Slope Stability Factor figure worked out?

FoS = (c + gamma z cos²beta tan phi) / (gamma z cos beta sin beta). For a long slope with a shallow planar slip surface, the factor of safety is the available shear strength divided by the driving shear stress: (c + gamma z cos²beta tan phi) / (gamma z cos beta sin beta).

What does water do to this?

Positive pore pressure reduces effective stress and therefore the frictional term. A slope stable when dry can fail after rainfall with no change in geometry at all.

What values do I need to enter?

This calculator takes 4 inputs: Slope angle, Soil cohesion, Angle of internal friction, Depth to slip surface. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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