About this tool Calculate the terminal settling velocity of a spherical particle in a fluid using Stokes' Law. Enter the particle diameter, particle and fluid densities, and dynamic viscosity.
Includes particle size presets from coarse sand to clay, a Reynolds number check to confirm Stokes' Law validity (Re < 1), and the time for a particle to settle through 1 metre of fluid.
Used in hydrometer analysis (BS 1377-2), sedimentation tank design, drilling mud management, and environmental turbidity assessment.
How to use this tool 1. Select a particle size preset or enter a custom diameter — in mm.
2. Check the particle and fluid properties — defaults are quartz in water at 20°C. Adjust for drilling mud, seawater, etc.
3. Read the velocity and Reynolds number — if Re > 1, Stokes' Law is not strictly valid and the result may overestimate velocity.
Technical information v = (d² × (ρs − ρf) × g) / (18 × μ) — Stokes' Law
Re = ρf × v × d / μ — particle Reynolds number
Stokes' Law assumes: spherical particle, laminar flow (Re < 1), infinite fluid extent, no particle interaction. Valid for silt and clay particles in water.
Limitations Stokes' Law is only valid for Re < 1 (laminar flow). For sand-sized particles and larger, the drag coefficient increases and empirical correlations (e.g. Rubey, Dietrich) should be used.
Assumes spherical particles. Real soil particles are angular or platy (especially clays), which increases drag and reduces settling velocity compared to Stokes' prediction.
Does not account for hindered settling in concentrated suspensions, flocculation of clay particles, or convection currents in the settling column.
Revision history 14 July 2026: Initial release
Disclaimer This tool is provided for educational and general information purposes only. It is not a substitute for professional engineering advice, design or verification.
Diggy and its contributors are not licensed engineering consultants and no results generated by this tool should be used directly for construction, design or safety-critical decisions.
All values and outputs are based on published empirical correlations and should be independently checked and confirmed by a qualified geotechnical engineer before use.
By using this tool, you accept full responsibility for how you interpret and apply the information provided.
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