Short answer. Under RA 9255, which amended the Family Code provision on illegitimate children, the legitime of each illegitimate child is one-half of the legitime of a legitimate child. So an illegitimate child's reserved share is half of what a legitimate child of the same parent is guaranteed.
What the law says
The legitime of each illegitimate child shall consist of one-half of the legitime of a legitimate child
RA 9255, Section 1 — Surname Of Illegitimate Children. Read the full provision →
The one-half rule on the legitime
The measure of an illegitimate child's inheritance is stated directly. The law, as amended by RA 9255, provides that The legitime of each illegitimate child shall consist of one-half of the legitime of a legitimate child. The legitime is the portion of a parent's estate reserved by law for compulsory heirs, which the parent cannot freely give away. So the comparison is done at the level of the legitime: whatever a legitimate child is guaranteed as his reserved share, an illegitimate child of the same parent is guaranteed half of that. This is a fixed ratio, applied to each illegitimate child.
What "one-half of the legitime" means in practice
The rule compares reserved shares, not the whole estate. To apply it, you first work out what the legitime of a legitimate child would be in the particular family, then give each illegitimate child half of that figure. Because it operates on the legitime, the result depends on who else are compulsory heirs — a surviving spouse and legitimate children, for instance, all affect how the estate is divided. The one-half proportion is the constant; the actual peso value flows from the size of the estate and the mix of heirs. It is a share protected by law, distinct from any additional amount a parent might freely leave by will.
Recognition of filiation comes first
Before the inheritance ratio matters, the child's status as an illegitimate child of the parent must be established. RA 9255 deals with the surname and recognition of illegitimate children, allowing such a child to use the father's surname where filiation has been expressly recognized — through the record of birth, an admission in a public document, or a private handwritten instrument by the father. The father is given the right to bring an action to prove non-filiation during his lifetime. Recognition or proof of filiation is the gateway: without established filiation to the parent, the succession rights, including the one-half legitime, do not come into play.
Limits and related points
The one-half rule fixes the reserved share, but it is not the whole of inheritance law. It does not address every scenario — such as representation, the interaction with a surviving spouse's share, or gifts a parent makes during life that may be charged against a share. It also concerns the legitime specifically; a parent with disposable free portion may choose to leave more to any child by will, within legal limits. For a family working out actual figures, the ratio here is the starting rule for the illegitimate child's protected minimum, to be combined with the other succession rules that govern the estate as a whole.