Short answer
In the Hanafi school, once you start a voluntary fast you should complete it, and breaking it without a reason is not allowed in the sounder view. With or without a reason, you make up one day, but there is no kaffarah. The Shafi'i and Hanbali schools say the voluntary faster may break it freely and making it up is only recommended.
Starting a nafl fast makes completing it wajib, so breaking it without an excuse is not permitted in the sounder view, and one day of qada is wajib in every case. Valid excuses include illness, travel and a host or guest who would be hurt if you did not eat, before midday. Kaffarah applies only to breaking a fast of Ramadan itself.
Other madhhabs
- Shafi'i
- Someone keeping a voluntary fast may break it at will, and making it up is recommended, not required.
- Hanbali
- The same as the Shafi'i school.
- Maliki
- Breaking a voluntary fast on purpose without an excuse is forbidden and must be made up; breaking it with an excuse or by forgetfulness needs no make-up.
Explanation
This question has two sides. Breaking a Ramadan fast on purpose is a major sin with qada and, in many cases, kaffarah: see breaking the fast on purpose. Breaking a voluntary fast is lighter, and the schools differ on it.
The Hanafi view. Allah says: do not invalidate your deeds. The Hanafi school holds that a voluntary act of worship, once begun, becomes an obligation to complete, like a voluntary prayer you have started. So breaking a voluntary fast without a reason is not allowed in the sounder view, and you must make up a day for it. If you had a reason, there is no sin, but the make-up day is still owed.
Valid reasons include:
- Illness, or hunger or thirst that harms you.
- Travel.
- Being invited to a meal where the host would be hurt if you did not eat, or being a host whose guest will not eat without you. In the Hanafi texts this excuse applies before midday.
- Parents who are upset by your fasting and ask you to eat.
The other view. Aishah reported that the Prophet ﷺ was once offered some food, said that he had started the day fasting, and then ate. The Shafi'i and Hanbali schools take this as proof that the voluntary faster is free to break the fast. The Hanafi scholars understand that the day was made up, based on other reports, and keep to the general command not to spoil a deed. Great scholars hold both views, so be gentle with those who follow the other.
A make-up day for a voluntary fast can be on any day when fasting is allowed. If you break a Ramadan make-up fast, you simply owe that day again, with no kaffarah.
The evidence
Quran, Surah Muhammad 47:33
۞ يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوٓا۟ أَطِيعُوا۟ ٱللَّهَ وَأَطِيعُوا۟ ٱلرَّسُولَ وَلَا تُبْطِلُوٓا۟ أَعْمَٰلَكُمْ ﴿٣٣﴾
O you who have believed, obey Allah and obey the Messenger and do not invalidate your deeds.
A good deed once begun should be completed, the Hanafi basis for making up a broken voluntary fast.
Sahih Muslim 1154
Sahih... أرينيه فلقد أصبحت صائما فأكل ...
"Show it to me, for I began the day fasting. Then he ate."
The basis of the Shafi'i and Hanbali view that a voluntary fast may be broken.
Where the ruling is recorded
- al-Hidaya, Kitab al-Sawm, chapter on what requires qada and kaffarah. Whoever begins a voluntary fast or prayer and then spoils it must make it up.
- Radd al-Muhtar (Ibn Abidin), Kitab al-Sawm, chapter on what breaks the fast and what does not. Breaking a voluntary fast without an excuse is not permitted in the sound view; hospitality is an excuse.
- al-Mughni (Ibn Qudama), Kitab al-Siyam. The Hanbali view that the voluntary faster may break his fast and making it up is recommended.
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.