People are often surprised that the four Sunni schools of law produce the same inheritance result for the overwhelming majority of families. The fixed shares, blocking, ʿawl and radd are common ground. The schools part company mainly in one scenario: when a paternal grandfather inherits alongside the deceased's brothers and sisters, with no father in between. The classical books call it al-Jadd wa al-Ikhwa.
Two answers
The Ḥanafī school treats the grandfather as standing in for the father. A father excludes the brothers and sisters entirely, so under the Ḥanafī view the grandfather does too — the siblings receive nothing.
The Mālikī, Shāfiʿī and Ḥanbalī schools follow the method attributed to Zayd ibn Thābit: the grandfather shares with the siblings (muqāsama) rather than excluding them. He is given whichever of three outcomes is best for him — a share alongside the siblings as though he were one of them, one third of what remains after the fixed shares, or one sixth of the whole estate.
The most famous case of all
This terrain contains al-Akdariyya — a husband, a mother, a grandfather and a single full sister — long taught as the trickiest single problem in farāʾiḍ. The sister is first given a fixed half, the base is raised by ʿawl, and then the grandfather and sister pool their portions and re-divide them two to one, settling the estate over a base of twenty-seven.
The calculator computes all four schools and tells you, for the family you entered, whether they agree or differ — and if they differ, you can switch schools to see exactly which heir's share changes.