how is first ionisation potential  calculated?

first ionisation potential means, energy reqd. to exite an  electron from ground state to leave that atom.

jest put n1= 1 and n2 = infinity. in the eqn En=R(1/n1² -1/n2²)Z²

wher Z is atomic no. of considered atom 

You can find it using

E_n = (-2*pi*K² *Z² *m * e⁴)/n²h²...(this formula is derived using coulombic force = centripetal force on electron)

Just put n as 1..

E = (-2*pi*K² *Z² *m * e⁴)/h²...

Where

K is Couloumbs law constant=9* 10⁹Nm²/C²

e = magnitude of charge of proton

Substituting values for all constants, you get

E= [-2.18 * 10^{-18 *} Z² / n²  ] J/atom..