According to the Quran, the share of one boy is equal to that of two girls (4:11).
Allah enjoins you concerning your children: The male shall have the equal of the portion of two females; then if they are more than two females, they shall have two-thirds of what the deceased has left, and if there is one, she shall have the half; and as for his parents, each of them shall have the sixth of what he has left if he has a child, but if he has no child and (only) his two parents inherit him, then his mother shall have the third; but if he has brothers, then his mother shall have the sixth after (the payment of) a bequest he may have bequeathed or a debt; your parents and your children, you know not which of them is the nearer to you in usefulness; this is an ordinance from Allah: Surely Allah is Knowing, Wise.[4:11]
The Quran establishes a society in which men are the main earners and are responsible for providing the means of subsistence (4:34), because the duties and responsibilities assigned to the women do not leave them with enough time to earn their own living. In such a society in which the man is responsible for earning livelihood and providing the means of maintenance, he should get a larger portion when making an economic division. That is why, in inheritance, a boy’s share has been kept double that of a girl.
So if on one hand a man gets double than a woman from the inheritance, on the other hand, he has the responsibility for providing the means of maintenance to the family. Similarly, if on one hand a woman gets half than a man from the inheritance, she is exempt from any responsibility of earning and providing the means of support to the family.
According to the Quran, girls are not responsible for providing the means of maintenance of their own, or of their family. On the contrary, a boy has to bear his own expenses in addition to those expenses of his wife and children. As such, he should be getting the greater share.
On the other hand, in a situation in which men are neglecting their duties and it is feared that the girls would be rendered destitute, then the Quran gives authority that, in a will, a person can divide his property as he pleases and according to the circumstances. Writing a will is mandatory according to the Quran.
The division of inheritance as given in the Quran (4:11) is only applicable if a person dies without leaving a will, or if his will does not include all his property. The division as given in 4:11 is only from that property that the deceased did not include in his will, (i.e. the property that is left after the payment of legacies) and/or what is left after the payment of debt, if any. In his will the person could give more share to the girl than the boy if he deemed appropriate according to the circumstances.
The division given by the Quran in 4:11 is in view of another principle given by the Quran that men are responsible for bearing the expenses for maintaining the family.
Another guidelines while dividing the inheritance is that there is no loss to anyone (ghayra mudarrin) (4:12).
Another point that should be mentioned here is that although the main responsibility of providing the means of maintenance is on the male but if female wishes or there is need or appropriateness, female can also earn and own. (4:32)