Short answer
Because Islam places the financial duties of the family on men. A son must pay mahr and support his wife, children and, when needed, his parents and sisters, while a daughter's share is hers alone with no duty to spend it on anyone. The rule applies to sons and daughters inheriting together, not to every man and woman.
When sons and daughters inherit together, they take the residue with each son receiving twice a daughter's share, by the clear text of the Quran (4:11). This is fard to follow; changing it by will or pressure is not allowed. A daughter's share is her full property.
The four schools agree on this.
Explanation
This is one of the most asked questions about Islamic inheritance, by Muslims and by people curious about Islam. The rule is in the Quran itself: "for the male, what is equal to the share of two females." The four schools agree on it. Understanding the reason helps.
The system is about duties as well as shares.
- A man must pay mahr when he marries. A woman receives it.
- A man must provide housing, food and clothing for his wife and children, even if his wife is rich. A woman's wealth is hers; she never has to spend it on the family.
- A son must support his parents when they are in need, and a brother may have to support an unmarried sister.
So the son's larger share comes with obligations that eat into it, while the daughter's share grows untouched. The Quran links men's role in the family to "what they spend from their wealth."
It is not a rule that women always get half.
- A mother and father each get one sixth when the deceased had children: equal shares.
- Brothers and sisters from the mother's side share equally.
- In some families, a daughter or sister receives more than a male relative, or inherits where he does not.
What Islam does not allow is cutting women out, as many cultures did before the Quran and some still do; see denying sisters their inheritance. Parents may give gifts in life, and the Hanafi fatwa is to give sons and daughters equally when doing so, but no one may undo the shares at death; see dividing wealth before death. To see the shares in your family, use the inheritance calculator.
Ask a mufti about your own case. This answer explains the general ruling. When the details of your situation matter, as they do here, ask a qualified mufti and give them the full facts. Where to ask a scholar.
The evidence
Quran, Surah An-Nisaa 4:11
يُوصِيكُمُ ٱللَّهُ فِىٓ أَوْلَٰدِكُمْ ۖ لِلذَّكَرِ مِثْلُ حَظِّ ٱلْأُنثَيَيْنِ ۚ فَإِن كُنَّ نِسَآءًۭ فَوْقَ ٱثْنَتَيْنِ فَلَهُنَّ ثُلُثَا مَا تَرَكَ ۖ وَإِن كَانَتْ وَٰحِدَةًۭ فَلَهَا ٱلنِّصْفُ ۚ وَلِأَبَوَيْهِ لِكُلِّ وَٰحِدٍۢ مِّنْهُمَا ٱلسُّدُسُ مِمَّا تَرَكَ إِن كَانَ لَهُۥ وَلَدٌۭ ۚ فَإِن لَّمْ يَكُن لَّهُۥ وَلَدٌۭ وَوَرِثَهُۥٓ أَبَوَاهُ فَلِأُمِّهِ ٱلثُّلُثُ ۚ فَإِن كَانَ لَهُۥٓ إِخْوَةٌۭ فَلِأُمِّهِ ٱلسُّدُسُ ۚ مِنۢ بَعْدِ وَصِيَّةٍۢ يُوصِى بِهَآ أَوْ دَيْنٍ ۗ ءَابَآؤُكُمْ وَأَبْنَآؤُكُمْ لَا تَدْرُونَ أَيُّهُمْ أَقْرَبُ لَكُمْ نَفْعًۭا ۚ فَرِيضَةًۭ مِّنَ ٱللَّهِ ۗ إِنَّ ٱللَّهَ كَانَ عَلِيمًا حَكِيمًۭا ﴿١١﴾
Allah instructs you concerning your children: for the male, what is equal to the share of two females. But if there are [only] daughters, two or more, for them is two thirds of one's estate. And if there is only one, for her is half. And for one's parents, to each one of them is a sixth of his estate if he left children. But if he had no children and the parents [alone] inherit from him, then for his mother is one third. And if he had brothers [or sisters], for his mother is a sixth, after any bequest he [may have] made or debt. Your parents or your children - you know not which of them are nearest to you in benefit. [These shares are] an obligation [imposed] by Allah. Indeed, Allah is ever Knowing and Wise.
The rule for sons and daughters inheriting together.
Quran, Surah An-Nisaa 4:34
ٱلرِّجَالُ قَوَّٰمُونَ عَلَى ٱلنِّسَآءِ بِمَا فَضَّلَ ٱللَّهُ بَعْضَهُمْ عَلَىٰ بَعْضٍۢ وَبِمَآ أَنفَقُوا۟ مِنْ أَمْوَٰلِهِمْ ۚ فَٱلصَّٰلِحَٰتُ قَٰنِتَٰتٌ حَٰفِظَٰتٌۭ لِّلْغَيْبِ بِمَا حَفِظَ ٱللَّهُ ۚ وَٱلَّٰتِى تَخَافُونَ نُشُوزَهُنَّ فَعِظُوهُنَّ وَٱهْجُرُوهُنَّ فِى ٱلْمَضَاجِعِ وَٱضْرِبُوهُنَّ ۖ فَإِنْ أَطَعْنَكُمْ فَلَا تَبْغُوا۟ عَلَيْهِنَّ سَبِيلًا ۗ إِنَّ ٱللَّهَ كَانَ عَلِيًّۭا كَبِيرًۭا ﴿٣٤﴾
Men are in charge of women by [right of] what Allah has given one over the other and what they spend [for maintenance] from their wealth. So righteous women are devoutly obedient, guarding in [the husband's] absence what Allah would have them guard. But those [wives] from whom you fear arrogance - [first] advise them; [then if they persist], forsake them in bed; and [finally], strike them. But if they obey you [once more], seek no means against them. Indeed, Allah is ever Exalted and Grand.
Men carry the financial duty of the family.
Sahih al-Bukhari 6732
Sahih... ألحقوا الفرائض بأهلها فما بقي فهو لأولى رجل ذكر ...
"Give the fixed shares to those entitled to them, and what remains goes to the nearest male relative."
Fixed shares come first, then the residue goes to the residuary heirs.
Where the ruling is recorded
- al-Hidaya, Kitab al-Fara'id, section on the residuaries. Sons make daughters residuaries with them, the male taking two shares.
- al-Ikhtiyar, Kitab al-Fara'id. Daughters with sons, and the maintenance duties of men.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.