Two rules do almost all the work
A Muslim’s will is a narrower instrument than the one most legal systems imagine. It cannot decide who gets what across the whole estate, because most of that is already decided. What it can do is direct a bounded portion — and the boundary is set by two rules that between them settle nearly every question people arrive with.
The first is the third. Sa’d ibn Abi Waqqas, gravely ill, asked the Prophet whether he could give away two thirds of his wealth. No. A half? No. A third — “and a third is a lot.” The reason given was that leaving your heirs provided for is better than leaving them dependent on other people. That is the ceiling, measured on what survives your funeral costs and your debts, and it is a ceiling rather than a target.
The second is no bequest to an heir. Allah has already given each entitled person their due, so a bequest cannot be used to top up a son, a wife or a parent. A bequest of that kind is not ignored outright: it takes effect if the other heirs agree to it after death. Without their agreement it fails, however small it is.
Debts are not bequests
The order is fixed: burial, then debts in full, then the bequest up to a third, then the heirs. What people most often miss is how wide the second step is. It covers ordinary debts, but also unpaid zakat from earlier years, an unpaid mahr, and expiation owed for obligations missed. These are debts owed by the estate, not gifts from it, and they are not limited to a third.
An estate whose debts exceed its value therefore has nothing to will and nothing to inherit. The calculator says so plainly rather than producing a third of a negative number.
What the third is actually for
Because it cannot go to an heir, the third exists for everyone faraid leaves out. A mosque or a school. A charity. An adopted child, who does not inherit as a natural child does. A non-Muslim parent or spouse, who would otherwise take nothing. A relative in need who is too distant to be an heir. A debt of gratitude with no legal name.
It is also where a continuing charity — a sadaqah jariyah — is usually placed, since it is the one part of the estate a person still chooses.
A plan is not a will
This page works out the shape. It does not produce a document a court will act on, and the gap between the two is where most Muslim estates in the West go wrong. In England and Wales, in most US states and across most of Europe, a will that satisfies the shariah but not the local formalities of signing and witnessing is simply disregarded, and the estate is distributed under default statutory rules that bear no relation to faraid at all.
The usual answer is one properly executed will, drawn by a solicitor or estate attorney, that expresses Islamic distribution in a form the local court will enforce. Plan the shape here; have it drafted there; have it checked by someone qualified in both.