How the shares are worked out
Islamic inheritance runs in a fixed order, and the order is the whole of it. What the deceased owned is gathered, the funeral is paid for, the debts are settled in full, and any bequest is honoured up to a third. Only then is there an estate to divide.
What remains goes first to the heirs who hold a fixed share — the ashab al-furud. A husband takes a half, or a quarter if his wife left children. A wife takes a quarter, or an eighth if there are children, and co-wives divide that one portion between them. A mother takes a third, falling to a sixth if her child left children of their own or if two or more siblings survive. These fractions are named in the Qur’an and are not open to negotiation.
Whatever the fixed shares leave passes to the residuary heirs, the asaba — the sons first, then the son’s sons, then the father, then the brothers. It is here that the two-to-one ratio between a son and a daughter applies, and it is here that most estates are actually settled, because the fixed shares rarely consume everything.
Blocking: why a named relative sometimes receives nothing
The hardest part of faraid for most people is hajb, the rule that a nearer heir shuts out a more distant one. A surviving son excludes the deceased’s brothers and sisters completely, however close they were. A mother excludes both grandmothers. A father excludes the grandfather, and also excludes his own mother. Two daughters take the entire two thirds set aside for daughters, which leaves a son’s daughters with nothing unless a son’s son survives to bring them back in beside him.
The calculator lists everyone it has excluded and names the relative who excluded them, because an unexplained zero in a family settlement causes more harm than the arithmetic ever does.
When the fractions do not add up
Fixed shares are not guaranteed to total one. A husband, two daughters, a father and a mother between them claim a quarter, two thirds and two sixths — five quarters of an estate that only has four. The classical answer is awl: raise the denominator so every share shrinks by the same proportion. Twelve becomes fifteen, and each heir receives a little less than the fraction named, with the ratios between them untouched.
The opposite case is radd. A single daughter takes a half; if nobody else survives, the other half has nowhere to go, since there is no residuary heir waiting. The surplus returns to the sharers in proportion, so she takes the whole estate. A surviving spouse is excluded from that return on the majority view, which is why a wife with one daughter keeps exactly her eighth while the daughter takes the remaining seven eighths.
A calculator, not a ruling
This page applies the majority Sunni position and is checked against the worked examples in the standard texts, including the classical awl cases of twelve to fifteen, six to eight and twenty-four to twenty-seven, and the two Umariyyatan rulings on a spouse with both parents. That makes it a reliable way to learn the system and to sense-check what you have been told.
It does not make it a ruling. Estates carry facts no form can hold — a missing heir, an unborn child, a contested debt, property held in common, relatives who died in the same accident, or a question on which the schools of law genuinely differ. Take the result to a scholar. The arithmetic is the easy part; the facts are not.