Assalaamu ‘Alaykum,
How will good and bad deeds be weighed on the Day of Resurrection? If one committed grave major sins without being penalized with the prescribed corporal punishment and then repented, will these sins be weighed with his bad deeds? Will Allaah The Almighty forgive him by virtue of his repentance? How will the good and bad deeds of a person who died but did not repent be weighed on the Day of Resurrection? If such a person has a lot of good deeds, will he be saved from Hell?
Thank you, and may Allaah reward you.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.

Belief in the Last Day entails belief in the Scales and believing that they are real scales that weigh deeds. However, the scholars have provided three different views regarding what will be weighed:

First: One’s deeds will be the weighed object. They will be materialized on the Day of Resurrection and will be put in the Scale. This view is supported by many authentic Hadeeths. In a Hadeeth, the Prophet, sallallaahu ‘alayhi wa sallam, said: “There are two words which are beloved to the Most Merciful, light on the tongue, and heavy in the Scale: Subhaan Allaah wa bihamdihi, (and) Subhaan Allaah Al-‘Atheem (Glory and praise be to Allaah, glory be to Allaah, the Great).”  [Al-Bukhari and other narrators]

Second: The person himself will be the weighed object. Everyone will be put in the scale and his faith and deeds, not his body, will determine his weight. There is evidence behind this view, for Abu Hurayrah narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: “On the Day of Resurrection, a huge fat man will come who will not weigh as much as the wing of a mosquito.” He added: “Read, if you want, the verse in which Allaah The Almighty Says (what means): {And We will not assign to them on the Day of Resurrection any weight.} [Quran 18:105]” [Al-Bukhari]

Third: The records of deeds will be the weighed object. This is supported by a Hadeeth in which it is mentioned that on the Day of Resurrection there will come a person with records of his deeds which will be weighed against a card in which the testimony of faith will be written. The Prophet, sallallaahu ‘alayhi wa sallam, said: “The records will be put in one hand of the Scale and the card will be put on the other hand.” [At-Tirmithi and other narrators]

Each of these three views is adopted by a group of scholars. The three views are all accepted since all of them are based on evidence from the authentic Sunnah. So, it is possible that the three objects will be weighed. In his famous book Ma‘aarij Al-Qabool, Shaykh Haafith Al-Hakami stated that the preponderant opinion is that all three are possible.

Allaah The Almighty will expiate the sin of a person who sincerely repented from it and fulfilled all the conditions of repentance. In a Hadeeth, the Prophet, sallallaahu ‘alayhi wa sallam, said: “A person who repents from a sin is as if he did not commit it at all.” [Ibn Maajah]

Regardless of how grave the sin is, when a person sincerely repents, Allaah forgives his sin, even if it is Shirk (polytheism). Allaah The Almighty Says (what means): {Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allaah. Indeed, Allaah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful."} [Quran 39:53]

This verse applies to sins from which the doer repented. The general indication of the verse includes Shirk.

Regarding a person who committed grave major sins other than Shirk and died without repentance, his position exclusively is determined according to the will of Allaah. If Allaah wills he would punish him; and if He wills, He would forgive him. Allaah The Almighty Says (what means): {Indeed, Allaah does not forgive association with Him, but He forgives what is less than that for whom He wills.} [Quran 4:48]

A person whose good deeds are more than his bad deeds will be saved from Hell by the permission of Allaah The Almighty and His bounty.

Allaah Knows best.