Knudsen Number

When is it useful?

Knudsen number compares molecular mean free path with a device or flow length scale. It helps assess whether a continuum fluid model is a reasonable starting point for rarefied gas, vacuum, or microchannel problems.

Formula and inputs

Enter mean free path λ and characteristic length L in the same length units, here metres. L might be a channel diameter or an object size. Mean free path varies with gas state, including pressure.

Worked example

Interpreting the result

Here the molecular path is much smaller than the chosen device length. That supports considering a continuum approximation, but detailed model choice depends on geometry and accuracy requirements.

Limits and cautions

The calculator’s flow-regime labels are approximate. Published boundaries vary somewhat by application, and this tool does not model wall-slip effects.