An analytical expression is derived for the net amount of charges penetrating into a membrane, which is immersed in an electrolyte solution. The membrane carries fixed charges due to, for example, the dissociation of the functional groups it carries. Planar, cylindrical, and spherical membranes and a general electrolyte solution are considered. The estimation of the amount of penetration charges is essential to the description of membrane properties. We show that the amount of fixed charges and the potential gradient at the membrane-liquid interface are two key factors to determine the net amount of charges penetrating into a membrane. The deviation of the present approximate result from the corresponding exact value, which needs to be evaluated numerically, is on the order of 5%.