Count the remaining intensive degrees of freedom with F = C − P + 2.
| Degrees of freedom F | – |
Use this calculator in a phase-diagram class when you know how many independent components and how many phases sit in equilibrium, and you need the variance F.
The calculation is F = C − P + 2. C is the number of independent components and P is the number of phases. The +2 stands for temperature and pressure. Condensed-only systems are sometimes written with +1 instead; this tool always uses +2.
A negative F means that many phases cannot coexist for that C under the usual two-field constraint; it is not rounded up to zero.
Components C: Number of independent chemical components. Pure water is 1; water–ethanol is 2.
Phases P: Number of phases in equilibrium. Ice + liquid water is 2; ice + liquid + vapour is 3.
Degrees of freedom F: How many intensive variables you may still choose. F = 1 for a one-component two-phase coexistence (pick T or P, not both).
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