How can calculate the current capacity of 2.5 sq mm wire?

You don't calculate the current-carrying capacity, you find out from the appropriate Tables in your country's wiring regulations. In the UK, this is BS 7671:2008 Requirements for Electrical Installation.

The reason you must do this is that the current-carrying capacity of a cable depends on the conductor type, number of cores, the type of insulation, the method of installation, etc. Allthese factors are taken into account in these Tables.

For a 2.5-mm2 copper cable, the current-carrying capacity can vary from around 22 A to around 36 A, depending on the factors described above.