How this calculator works
In a dilute gas of identical hard spheres, a collision sweeps out a cross-sectional area pi d². Accounting for the motion of other molecules introduces the square-root-of-two factor. Combining this collision geometry with number density p/(kBT) gives the mean free path. Kinetic-diameter presets are tabulated in the cited US DOE storage report. They are approximate effective molecular sizes, not temperature-specific transport fits. The custom default remains illustrative; supply a suitable collision diameter for precision transport calculations.
Formula
lambda = 1/(sqrt(2) pi d² N/V) = kBT/(sqrt(2) pi d² p).
Worked example
With d = 1e-10 m, T = 300 K and p = 100000 Pa, the model gives 9.322656e-7 m. At twice the pressure, the path is halved to 4.661328e-7 m.
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Frequently asked questions
Is a kinetic diameter an atomic radius?
No. It is an effective collision diameter for a transport model.
What happens in a vacuum?
The mean free path grows as pressure falls. At zero pressure there is no finite collision spacing in this model.
Can I use it for liquids?
No. Dense fluids have correlated interactions that invalidate the dilute hard-sphere derivation.