Short answer. The Civil Code originally fixed this at one-fourth of the estate, but that figure was worked out under a classification of children the Family Code has since replaced. The current rule ties an illegitimate child's legitime to half of what a legitimate child would receive in the same position — not the one-fourth this article states.
What the law says
Illegitimate children who may survive with legitimate parents or ascendants of the deceased shall be entitled to one-fourth of the hereditary estate to be taken from the portion at the free disposal of the testator.
Civil Code, Article 896 — Illegitimate Children With Ascendants. Read the full provision →
What Article 896 originally provided
As written, Article 896 gives an illegitimate child who survives together with the deceased's legitimate parents or ascendants a fixed share: "one-fourth of the hereditary estate to be taken from the portion at the free disposal of the testator." That share came out of the free portion, not out of the parents' own one-half legitime, so on the article's original terms the parents' reserved share was not directly reduced by the illegitimate child's presence.
Why the one-fourth figure can no longer be relied on
This one-fourth share was calculated using the classification of children in force when the Civil Code took effect in 1950, which split illegitimate children into different sub-categories with different shares. The Family Code has since replaced that classification with a single category of illegitimate child, and it fixed a new governing formula: each illegitimate child's legitime is now one-half of the legitime of a legitimate child, with the rest of the Civil Code's succession provisions otherwise remaining in force. Reading Article 896's one-fourth figure as still the law overstates or understates what is actually owed, depending on the facts.
Why this page cannot give you the current fraction
Working out the actual current share requires knowing what a legitimate child would receive as legitime in the same set of circumstances — survived by an illegitimate child and the deceased's parents, but no legitimate children — because the present rule defines the illegitimate child's share as half of that figure. That comparator is governed by other succession provisions, and it is not part of the text this page is built on. Any answer that stated a specific replacement percentage without that figure would be a guess, not the current rule.
What this means for a family working out an estate
Do not divide an estate in this situation using the one-fourth figure Article 896 states — it reflects a classification of children the law no longer uses. The right approach is to work out, under the current Family Code formula, what a legitimate child's legitime would be in the same circumstances, and then take one-half of that for the illegitimate child, rather than defaulting to the number printed in this article.