Short answer
No, biting your nails does not break the fast as long as you spit the pieces out. If you deliberately swallow a piece of nail, the Hanafi school counts the fast as broken and you make up one day. It is better to drop the habit while fasting.
Biting the nails does not break the fast. Swallowing a piece of nail on purpose breaks it, like swallowing a pebble, and one day of qada is due with no kaffarah, since a nail is not food. Putting things in the mouth without need is makruh for the fasting person.
The four schools agree on this.
Explanation
Biting your nails puts your fingers and a little nail in your mouth, but nothing goes down to the stomach, so the fast is intact. The question is what happens to the bits.
Swallowing a piece. The fast is broken when something from outside the mouth is swallowed on purpose, even if it is not food, such as a pebble, a bead or a scrap of paper. A piece of fingernail is the same: once it is off the finger and in your mouth, swallowing it breaks the fast. In the Hanafi school you make up one day. There is no kaffarah, because a nail is not something people eat.
What if it slipped down? A tiny fleck that goes down with your saliva before you notice it is hard to avoid, like dust, and does not need to worry you. A piece you could feel and chose not to spit out is a different matter. Spit the bits out straight away.
A good time to stop. The Hanafi scholars dislike chewing or putting things in the mouth without need while fasting, because it risks swallowing something. Many people find Ramadan the easiest time to break the habit: keep your nails short, keep your hands busy with dhikr beads, and notice when stress or boredom sets you off.
The four schools agree that deliberately swallowing something that is not food breaks the fast. Cutting your nails is a different matter and does not affect the fast at all: see cutting hair and nails while fasting.
The evidence
Quran, Surah Al-Baqara 2:187
أُحِلَّ لَكُمْ لَيْلَةَ ٱلصِّيَامِ ٱلرَّفَثُ إِلَىٰ نِسَآئِكُمْ ۚ هُنَّ لِبَاسٌۭ لَّكُمْ وَأَنتُمْ لِبَاسٌۭ لَّهُنَّ ۗ عَلِمَ ٱللَّهُ أَنَّكُمْ كُنتُمْ تَخْتَانُونَ أَنفُسَكُمْ فَتَابَ عَلَيْكُمْ وَعَفَا عَنكُمْ ۖ فَٱلْـَٰٔنَ بَٰشِرُوهُنَّ وَٱبْتَغُوا۟ مَا كَتَبَ ٱللَّهُ لَكُمْ ۚ وَكُلُوا۟ وَٱشْرَبُوا۟ حَتَّىٰ يَتَبَيَّنَ لَكُمُ ٱلْخَيْطُ ٱلْأَبْيَضُ مِنَ ٱلْخَيْطِ ٱلْأَسْوَدِ مِنَ ٱلْفَجْرِ ۖ ثُمَّ أَتِمُّوا۟ ٱلصِّيَامَ إِلَى ٱلَّيْلِ ۚ وَلَا تُبَٰشِرُوهُنَّ وَأَنتُمْ عَٰكِفُونَ فِى ٱلْمَسَٰجِدِ ۗ تِلْكَ حُدُودُ ٱللَّهِ فَلَا تَقْرَبُوهَا ۗ كَذَٰلِكَ يُبَيِّنُ ٱللَّهُ ءَايَٰتِهِۦ لِلنَّاسِ لَعَلَّهُمْ يَتَّقُونَ ﴿١٨٧﴾
It has been made permissible for you the night preceding fasting to go to your wives [for sexual relations]. They are clothing for you and you are clothing for them. Allah knows that you used to deceive yourselves, so He accepted your repentance and forgave you. So now, have relations with them and seek that which Allah has decreed for you. And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]. Then complete the fast until the sunset. And do not have relations with them as long as you are staying for worship in the mosques. These are the limits [set by] Allah, so do not approach them. Thus does Allah make clear His ordinances to the people that they may become righteous.
The fast means nothing is taken in through the mouth from dawn to sunset.
Where the ruling is recorded
- al-Hidaya, Kitab al-Sawm, chapter on what requires qada and kaffarah. Whoever swallows a pebble or a piece of iron makes up the day without kaffarah.
- Radd al-Muhtar (Ibn Abidin), Kitab al-Sawm, chapter on what breaks the fast and what does not. What cannot be avoided, such as dust reaching the throat, does not break the fast.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.