Dust Particle Settling Calculator

Stokes' law's velocity term is proportional to diameter squared (Vs = d²(ρp − ρf)g / 18μ), so doubling diameter quadruples the velocity, not just doubles it. This is why a 100 µm dust grain settles vastly faster than a 2.5 µm PM2.5 particle of the same density — the calculator's squared-diameter term dominates the result far more than the linear density term does.

The particle Reynolds number (Re = ρf × Vs × d / μ) tells you whether Stokes' law's laminar-flow assumption still holds — it's only strictly valid for Re < 1. If your particle is large or dense enough to push Re above 1, drag becomes nonlinear and the true settling velocity will actually be slower than what the uncorrected Stokes formula reports, since the calculator doesn't apply a high-Re drag correction.

The Cunningham slip factor corrects for particles small enough to "slip" between air molecules rather than experience continuous drag, and it's built around air's mean free path of 0.066 µm. Below roughly 1 µm diameter the correction becomes significant because the particle's size approaches that of the gaps between molecules; above about 10 µm the correction factor converges toward 1.0 and has negligible effect on the reported velocity.

This calculator models a single particle falling through perfectly still air from your specified room height, but real indoor air has convection currents, HVAC-driven mixing, and general turbulence that keep particles suspended far longer than the still-air math predicts. This gap is largest for fine particles (especially sub-2.5 µm), which settle slowly enough that even mild air movement can keep them aloft indefinitely in practice.