The reducing effect of the nuclear charge by the inner electrons for an outer electron is termed as shielding (or screening). As a result of shielding, the outer electrons in an atom always experience less nuclear charge than the actual nuclear charge Z. The effective nuclear charge (Z*) as experienced by an electron is then obtained by subtracting the total shielding contributions from all the other electron (i.e., except the one under consideration) from the actual nuclear charge.
 $Z*=Z-s$
Where s = sum of the shielding contributions. The rules for estimating contributions to s are as follows (Slater’s rule)
Contribution to shielding by each electron is:

Electron group	All higher group	Same group	Group $n-1$	Group  $\le n-2$
1s	0	0.30	-	-
(ns, np)	0	0.35	0.85	1.00
(nd) or (nf)	0	0.35	1.00	1.00