Enter the Stokes number, air viscosity, particle density, and flow velocity into the calculator to determine the aerodynamic diameter of a particle.

Aerodynamic Diameter Formula

The following formula is used to calculate the aerodynamic diameter of a particle:

d_a = √((18 * St * μ) / (ρ_p * v))


  • d_a is the aerodynamic diameter (μm)
  • St is the Stokes number (dimensionless)
  • μ is the air viscosity (μPa·s)
  • ρ_p is the particle density (kg/m³)
  • v is the flow velocity (m/s)

To calculate the aerodynamic diameter, take the square root of the product of 18 times the Stokes number times the air viscosity, divided by the product of the particle density and the flow velocity. The result is then converted to micrometers (μm).

What is Aerodynamic Diameter?

Aerodynamic diameter is a measure used to describe the size of aerosol particles based on their behavior in the air. It is defined as the diameter of a spherical particle with a density of 1 g/cm³ that has the same settling velocity as the particle in question. This measure is important in various fields such as environmental health, aerosol science, and air quality monitoring, as it affects how particles behave in the atmosphere, including their transport, deposition, and respiratory deposition in human lungs.

How to Calculate Aerodynamic Diameter?

The following steps outline how to calculate the Aerodynamic Diameter:

  1. First, determine the Stokes number (St).
  2. Next, determine the air viscosity (μ) in micro-pascal seconds (μPa·s).
  3. Next, determine the particle density (ρ_p) in kilograms per cubic meter (kg/m³).
  4. Next, determine the flow velocity (v) in meters per second (m/s).
  5. Use the formula to calculate the aerodynamic diameter (d_a) in micrometers (μm).
  6. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Stokes number (St) = 0.2

Air viscosity (μ) = 18.5 μPa·s

Particle density (ρ_p) = 1200 kg/m³

Flow velocity (v) = 1.5 m/s