Stokes’ Law Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

Share Print Save

Fluid calculations are especially sensitive to density, viscosity, geometry, pressure conventions, and unit systems. Stokes’ Law Calculator narrows that problem to the relationship used on this page, making the displayed terminal velocity easier to audit against the inputs and the governing equation.

What this calculator does

The Stokes’ Law Calculator uses Acceleration of gravity (g), Medium viscosity (μ), Medium density (ρm), Particle density (ρp), Particle diameter (d) to estimate the page’s Terminal velocity from the fluid-mechanics relationship below. It is meant for a defined geometry and property set, so the useful part is not just the headline number but also whether your density, viscosity, dimensions, pressure reference, and flow convention match the model.

How to use it

Start with the fields that actually drive this result: Acceleration of gravity (g), Medium viscosity (μ), Medium density (ρm), Particle density (ρp), Particle diameter (d). Keep units consistent with the menus beside the fields and avoid mixing values measured under different conditions. After calculating, change one input at a time if you are comparing scenarios; that makes cause-and-effect much easier to see.

How the calculation works

For a small sphere in creeping flow, terminal speed is v = g d²(ρp−ρf)/(18μ). The relation assumes low Reynolds number and a Newtonian fluid around an approximately spherical particle.

Example

Using the page’s default example (Acceleration of gravity (g) = 9.80665 m/s²; Medium viscosity (μ) = 0.001 Pa·s; Medium density (ρm) = 1000 kg/m³; Particle density (ρp) = 2500 kg/m³), the calculator reports Terminal velocity of 0.817221 m/s. Change one driving input at a time and confirm the result moves in the direction predicted by the equation; that is a quick way to catch a unit or mode mistake.

How to interpret the result

Interpret the Terminal velocity within the fluid, geometry, pressure reference, and property values you entered. A numerically plausible answer can still be physically wrong if gauge/absolute pressure, diameter/radius, viscosity type, or unit convention is mismatched, so compare the result with the assumptions as well as the formula.

Limitations and notes

Stokes terminal velocity requires creeping flow around a small sphere. If the resulting Reynolds number is not small, drag is no longer proportional to velocity and this expression can be inaccurate.