Short answer
A sin in Ramadan is recorded as one sin, not multiplied in number, but it is graver in weight because it violates a blessed time and the fast itself. The Prophet ﷺ warned that Allah has no need of the fast of someone who does not leave false speech and acting on it. Good deeds, on the other hand, are multiplied.
Sins remain sins in every month; in Ramadan they are more serious because of the sanctity of the time, and they reduce or ruin the reward of the fast without breaking it in the fiqh sense, except for acts that break the fast itself.
The four schools agree on this.
Explanation
Two principles answer this. First, Allah says that whoever comes with a good deed will have ten times its like, and whoever comes with an evil deed will be recompensed only with its like. So a sin is not counted as many sins because it was done in Ramadan.
Second, the weight of a sin grows with the sanctity of the time and place in which it is done. Allah told us not to wrong ourselves in the sacred months, and the scholars applied the same reasoning to Ramadan, the month in which the Quran was revealed. A sin committed while fasting is also an insult to the worship you are in. So it is not more in number, but it is heavier.
What it does to the fast. Lying, backbiting, swearing and looking at what is haram do not make you eat or drink, so your fast is valid and you do not repeat it. But the Prophet ﷺ said Allah has no need for a person to leave food and drink if he does not leave false speech and acting on it. You may be left with nothing but hunger and thirst. See swearing and lying.
- Some sins also break the fast, such as intercourse or zina. See zina and the fast.
- If you slip, repent at once. Ramadan is the month of forgiveness.
- Guard the tongue, eyes and phone as carefully as the stomach.
See also why people still sin in Ramadan and deeds that wipe away sins.
The evidence
Quran, Surah Al-An'aam 6:160
مَن جَآءَ بِٱلْحَسَنَةِ فَلَهُۥ عَشْرُ أَمْثَالِهَا ۖ وَمَن جَآءَ بِٱلسَّيِّئَةِ فَلَا يُجْزَىٰٓ إِلَّا مِثْلَهَا وَهُمْ لَا يُظْلَمُونَ ﴿١٦٠﴾
Whoever comes [on the Day of Judgement] with a good deed will have ten times the like thereof [to his credit], and whoever comes with an evil deed will not be recompensed except the like thereof; and they will not be wronged.
A sin is recorded as one, not multiplied.
Quran, Surah At-Tawba 9:36
إِنَّ عِدَّةَ ٱلشُّهُورِ عِندَ ٱللَّهِ ٱثْنَا عَشَرَ شَهْرًۭا فِى كِتَٰبِ ٱللَّهِ يَوْمَ خَلَقَ ٱلسَّمَٰوَٰتِ وَٱلْأَرْضَ مِنْهَآ أَرْبَعَةٌ حُرُمٌۭ ۚ ذَٰلِكَ ٱلدِّينُ ٱلْقَيِّمُ ۚ فَلَا تَظْلِمُوا۟ فِيهِنَّ أَنفُسَكُمْ ۚ وَقَٰتِلُوا۟ ٱلْمُشْرِكِينَ كَآفَّةًۭ كَمَا يُقَٰتِلُونَكُمْ كَآفَّةًۭ ۚ وَٱعْلَمُوٓا۟ أَنَّ ٱللَّهَ مَعَ ٱلْمُتَّقِينَ ﴿٣٦﴾
Indeed, the number of months with Allah is twelve [lunar] months in the register of Allah [from] the day He created the heavens and the earth; of these, four are sacred. That is the correct religion, so do not wrong yourselves during them. And fight against the disbelievers collectively as they fight against you collectively. And know that Allah is with the righteous [who fear Him].
Sins weigh more in sacred times.
Sahih al-Bukhari 1903
Sahih... من لم يدع قول الزور والعمل به فليس لله حاجة في أن يدع طعامه وشرابه ...
"Whoever does not leave false speech and acting on it, Allah has no need of his leaving his food and drink."
Sins while fasting empty the fast of its reward.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Sawm, chapter on what is disliked for the fasting person. Sins such as backbiting do not break the fast but are graver while fasting.
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.