People usually arrive at this question having been told the first half of the answer and not the second, and it lands badly. The fixed shares do not reach an adopted child. That is true, and stopping there makes the scheme look as though it has nothing to say to a family that has raised a child for twenty years.
Why the shares run on lineage
The relevant passage is 33:4-5, which addresses the practice directly and instructs that such children be called by the names of their own fathers. What is being ruled out is not the raising of a child who needs a home — that is encouraged in the strongest terms — but the legal fiction that severs the child from their origins and substitutes a new parentage.
The reason is that lineage does structural work throughout the law. It determines who may marry whom. It determines who is mahram, and therefore what the household looks like as the child grows up. It determines who owes maintenance to whom. And it determines inheritance. A fiction that rewrites lineage does not rewrite one thing; it silently rewrites all four.
What Islamic law offers in its place is kafala — guardianship, provision, care, a home, everything a child needs, without the substitution of parentage. The reward described for it is considerable. What it does not carry is an automatic share of the estate.
Which is exactly what the third is for
The bequest may be made to anyone who is not already an heir, up to one third of the net estate. The restriction that catches most people — no bequest to an heir without the other heirs' consent — does not apply here, because an adopted or foster child is not among the heirs.
So the child you raised can be left up to a third of everything you own, freely, by a document you write yourself. That is not a workaround. It is the mechanism working as intended: the fixed shares handle the relations the law recognises automatically, and the third handles the ones it does not.
The same is true of a step-child, who does not inherit from a step-parent, and of a foster child, and of anyone else whose claim on you is real and whom the scheme does not reach. Note the one thing people forget: a step-child still inherits from their own biological parents in the ordinary way, so the question is only about the step-parent's estate.
The provisions outside the estate
Several routes do not touch the estate at all, and they are worth knowing because the third is a ceiling on the will, not a ceiling on what you can provide.
- A lifetime gift — property transferred while you are alive is not part of the estate at death, and there is no third to work within.
- A takaful policy or a pension with a nominated beneficiary, which in many jurisdictions passes outside the estate entirely. Whether that is treated as estate property is a genuine question scholars differ on, so it is worth asking about rather than assuming.
- A trust, where the local law offers one, holding property for the child directly.
The trap worth naming
In Britain, the United States, Canada and Australia, a legally adopted child is your child for every purpose the courts recognise — including inheritance, and including a claim against your estate if a will excludes them. An Islamic will drafted without regard to that can be challenged, varied, or set aside, and the family is left with litigation instead of a distribution.
The reverse trap is just as common: a will that satisfies the local court but distributes on lines the fixed shares do not recognise. Both failures come from treating this as a document you can copy from a website.
Work out the shares first so you know what the scheme gives, decide what the third should do, and then have the document drafted by someone who practises succession law where you actually live. The calculators here will do the first two. The third part is not a calculation.
This is not a fatwa. It sets out the positions scholars hold and where they part, so that you know what you are asking about. Your own case turns on facts a page cannot see — put it to someone qualified.