Question

what is the dimensional formula for charge how is it find?

Answer 1

q = ne
e = coulomb 
I × t = q
[A] × [T] = q
Therefore, the dimensional formula of charge is AT.

Answer 2

The dimensional formula for charge is [AT].

Every physical quantity has a unit and the unit has the Dimensional formula which is derived from the dimensions known for the fundamental quantities which are defined and not derived.

As we know that charge is the total number of electron present. And the formula for charge is Q = ne, where 'e' has the unit of Coulomb.  

Also, $q=I \times t$; that is rate of current flowing is also charge.

$\begin{aligned} Q &=I \times t \\ \mathrm{Q} &=[A] \times[T] \\ Q &=[A T] \end{aligned}$.