Particle Settling Velocity Calculator

The Particle Settling Velocity Calculator determines how fast a particle sinks or falls through a fluid — useful in environmental science, filtration design, sediment analysis, and air quality work. Enter your particle diameter, particle density, and select a medium type (air or water) to get the Terminal Settling Velocity in both m/s and mm/s. Secondary outputs include the Reynolds Number and a Stokes Law Validity check to confirm which flow regime applies.

m/s²
m

Diameter of the spherical particle

kg/m³
kg/m³

Density of the fluid medium (air ≈ 1.2 kg/m³)

Pa·s

Dynamic viscosity of the medium (air ≈ 1.849×10⁻⁵ Pa·s)

Results

Terminal Settling Velocity

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Terminal Velocity

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Reynolds Number

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Stokes Law Validity

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Frequently Asked Questions

What is Stokes' law?

Stokes' law describes the drag force acting on spherical particles moving through a viscous fluid at low Reynolds numbers. It's fundamental for calculating terminal settling velocity of particles in air or liquid.

How do I calculate particle settling velocity?

Use the Stokes' law formula: v = gd²(ρp - ρm)/(18μ), where g is gravity, d is particle diameter, ρp is particle density, ρm is medium density, and μ is dynamic viscosity.

What is terminal velocity?

Terminal velocity is the maximum constant speed a particle reaches when falling through a fluid. At this point, the gravitational force equals the drag force, resulting in zero acceleration.

When is Stokes' law valid?

Stokes' law is valid for spherical particles at low Reynolds numbers (Re < 1), in the laminar flow regime. For higher Reynolds numbers, other drag equations should be used.

What factors affect particle settling velocity?

Settling velocity depends on particle size, particle density, fluid density, fluid viscosity, and gravitational acceleration. Larger, denser particles settle faster in less viscous fluids.

How does temperature affect settling velocity?

Temperature affects fluid density and viscosity. Higher temperatures typically decrease viscosity and density, which can increase settling velocity for particles in gases and most liquids.

What are typical values for air properties?

At 25°C, air density is approximately 1.184 kg/m³ and dynamic viscosity is 1.849×10⁻⁵ Pa·s. At standard temperature and pressure (STP), air density is 1.292 kg/m³.