In air at sea level a molecule travels only about 68 nm between collisions – but in a vacuum chamber it can be metres. The mean free path decides whether gases behave as continuous fluids.
How to use the Mean Free Path Calculator
- Gas
- Kinetic diameter
- Pressure
- Temperature
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose the gas (or enter its kinetic diameter), the pressure and temperature.
Tips
- Use absolute temperature (K) in gas laws and efficiency formulas.
- Temperature differences are the same in K and °C.
- Heat = mass × specific heat × temperature change when no phase change occurs.
- Phase changes need latent heat at a constant temperature.
Common mistakes to avoid
- Using °C in pV = nRT.
- Forgetting latent heat during melting or boiling.
- Confusing heat capacity (J/K) with specific heat (J/(kg·K)).
Inputs explained
| Field | What to enter |
|---|---|
| Gas | Choose one: Air, Nitrogen, Oxygen, Argon … |
| Kinetic diameter | Units: nm, Å, pm |
| Pressure | Units: kPa, Pa, mbar, Torr, atm |
| Temperature | Units: K, °C, °F |
Example values
How the main result changes when Kinetic diameter changes and everything else stays at the example values above:
| Kinetic diameter (nm) | Mean free path λ (nm) |
|---|---|
| 0.182 | 67.8561 |
| 0.273 | 67.8561 |
| 0.364 | 67.8561 |
| 0.455 | 67.8561 |
| 0.546 | 67.8561 |
| 0.728 | 67.8561 |
Formula
λ = k_B·T / (√2·π·d²·p)Worked example
Inputs — Gas: Air; Pressure: 101.325 kPa; Temperature: 293.15 K.
Results — Mean free path λ = 67.85611 nm; Number density = 2.5035e25 m⁻³.
Key terms
- Mean free path
- Average distance between collisions.
Frequently asked questions
What is the mean free path in air?
About 68 nm at sea level and 20 °C.
How does it depend on pressure?
Inversely – halving the pressure doubles it.
What is the kinetic diameter?
An effective size of a molecule for collisions.
Why does it matter for vacuum systems?
Above about 1 cm, gas behaves as free molecules.
What formula does the Mean Free Path Calculator use?
λ = k_B·T / (√2·π·d²·p).