Short answer
All of it. Riba is any increase a lender requires on a loan, whether the rate is 0.5% or 50%. The Quran's words "doubled and multiplied" describe the worst practice of the time, not a level below which interest is fine. The four schools and the modern fiqh academies agree.
Any stipulated increase on a loan is riba and haram, whatever the rate and whoever the lender: a bank, a government or a friend. There is no permitted "reasonable" rate, and linking repayment to inflation is also not allowed.
The four schools agree on this.
Explanation
In older English, usury meant any interest on a loan. Today it usually means excessive interest. That shift leads some people to argue that the Quran forbids only loan-shark rates, so a normal mortgage or savings rate is fine. The texts do not support this.
The Quran draws the line at the principal. Allah tells the believers to "give up what remains due to you of interest" (2:278) and, if they repent, "you may have your principal" (2:279). The lender keeps exactly what he lent. Any amount above it, small or large, is what must be given up.
"Doubled and multiplied" (3:130) describes how pre-Islamic lenders doubled a debt each time it was rolled over. The commentators explain that it names the ugliest form, not a condition; the later verses of Surah al-Baqarah state the rule for all riba.
The Prophet ﷺ cancelled all of it. At the Farewell Pilgrimage he declared that the riba of the Jahiliyyah was abolished, starting with the riba owed to his own uncle al-Abbas, without asking whether any of it was at a modest rate.
A small number of modern writers have argued that bank interest is a fee for investment or a fair return. The great majority of scholars in all four schools, and the international fiqh academies, have rejected this and ruled that bank interest is riba. Inflation does not change the rule either: the borrower repays the same amount he borrowed. For why riba is forbidden, see Why is interest haram?
The evidence
Quran, Surah Al-Baqara 2:279
فَإِن لَّمْ تَفْعَلُوا۟ فَأْذَنُوا۟ بِحَرْبٍۢ مِّنَ ٱللَّهِ وَرَسُولِهِۦ ۖ وَإِن تُبْتُمْ فَلَكُمْ رُءُوسُ أَمْوَٰلِكُمْ لَا تَظْلِمُونَ وَلَا تُظْلَمُونَ ﴿٢٧٩﴾
And if you do not, then be informed of a war [against you] from Allah and His Messenger. But if you repent, you may have your principal - [thus] you do no wrong, nor are you wronged.
The lender is entitled to the principal only, so any increase is riba.
Quran, Surah Al-Baqara 2:278
يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ ٱتَّقُوا۟ ٱللَّهَ وَذَرُوا۟ مَا بَقِىَ مِنَ ٱلرِّبَوٰٓا۟ إِن كُنتُم مُّؤْمِنِينَ ﴿٢٧٨﴾
O you who have believed, fear Allah and give up what remains [due to you] of interest, if you should be believers.
All remaining interest must be dropped, with no exception for low rates.
Quran, Surah Aal-i-Imraan 3:130
يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ لَا تَأْكُلُوا۟ ٱلرِّبَوٰٓا۟ أَضْعَٰفًۭا مُّضَٰعَفَةًۭ ۖ وَٱتَّقُوا۟ ٱللَّهَ لَعَلَّكُمْ تُفْلِحُونَ ﴿١٣٠﴾
O you who have believed, do not consume usury, doubled and multiplied, but fear Allah that you may be successful.
Describes the worst form of riba practised at the time.
Sahih Muslim 1218
Sahih... وربا الجاهلية موضوع وأول ربا أضع ربانا ...
"The riba of the Jahiliyyah is abolished, and the first riba I abolish is our own."
All outstanding interest was cancelled, whatever its rate.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Buyu', section on loans (fasl fi al-qard). Every loan that brings the lender a stipulated benefit is riba.
- The Historic Judgment on Interest (Mufti Taqi Usmani), Sections on the meaning of riba and the claim that only usury is forbidden. Answers the arguments that modern bank interest is not riba.
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.