Short answer. One-fourth of the estate. Article 893 gives the surviving spouse a quarter where the deceased leaves legitimate ascendants but no descendants, and that quarter comes out of the free portion. Your mother's own legitime is a separate half, leaving a quarter freely disposable.
What the law says
If the testator leaves no legitimate descendants, but leaves legitimate ascendants, the surviving spouse shall have a right to one-fourth of the hereditary estate. This fourth shall be taken from the free portion of the estate.
Civil Code, Article 893 — Spouse With Legitimate Ascendants. Read the full provision →
What the law says
The legitime of legitimate parents or ascendants consists of one-half of the hereditary estates of their children and descendants.
Civil Code, Article 889 — Legitime of Legitimate Parents. Read the full provision →
How the estate divides on these facts
Two compulsory heirs share the reserved portions, and the Code gives each its own fraction. Your mother takes half, because the legitime of legitimate parents or ascendants consists of one-half of the hereditary estates of their children and descendants. Your spouse takes a quarter, since if the testator leaves no legitimate descendants, but leaves legitimate ascendants, the surviving spouse shall have a right to one-fourth of the hereditary estate. Three quarters is therefore spoken for, and the last quarter is what a will can give to anyone else.
The quarter comes from the free portion, not from your mother
Article 893 says so in its own second sentence: this fourth shall be taken from the free portion of the estate. The ascendants' half is not reduced to make room for a spouse. What the spouse's share consumes is the testator's liberty, which is why the disposable part here is a quarter rather than a half. The practical consequence is that a childless testator with a living parent and a spouse has far less room to benefit a sibling, a friend or a charity than the bare phrase 'no children' suggests.
Everything turns on there being no descendants
The article applies only where no legitimate descendants survive, and descendants include grandchildren, not just children. A single grandchild through a deceased child moves the estate into an entirely different computation, with the descendants taking the reserved half and your mother taking nothing as of right, since ascendants inherit only in default of descendants. Illegitimate children also change the picture, having legitimes of their own charged to the free portion. Confirm who actually survives before relying on the quarter.
The estate is smaller than you think
Fractions mislead until the base is fixed. If you are married under absolute community or conjugal partnership, that regime is liquidated at death and your spouse's own half of the marital property is not part of your estate at all — the quarter is on top of it. Debts, taxes and settlement expenses are deducted before the shares are computed. If your intention is to provide differently for your spouse or your mother, the room to do it lies in that final quarter, and a will is the only way to direct it.
Related provisions
- Civil Code, Article 893 — Spouse With Legitimate Ascendants
- Civil Code, Article 889 — Legitime of Legitimate Parents
- Civil Code, Article 890 — Legitime Of Legitimate Parents