Missed Intention to Fast

Q: What do I do if I missed or forgot to make my intention to fast before Fajr?

A: If a person does not form the intention to fast before Fajr, the fast is not valid.

This is because all obligatory fasts require intention from the night, based on the hadith:

“Whoever does not form the intention to fast before dawn, there is no fast for him.”

If someone: forgot to make the intention, was unconscious from sunset until Fajr, or only formed the intention after Fajr, then the fast does not count, even if the person refrains from eating for the rest of the day.

The day would need to be made up.

This applies specifically to obligatory fasts, such as Ramadan, make-ups, vows, and expiations.

Source: Kashshaf al-Qina