According to the Qur'an, every action, whether good or bad, has some weight. God says: So whoever does an atom’s weight of good will see it, and whoever does an atom’s weight of evil will see it (99:7-8).
Because the actions will have some weight, they will be measured or weighed as mentioned in the following verses: The weight [of deeds] on that day is the truth. As for those whose scales are heavy, it is they who are the felicitous. As for those whose scales are light—it is they who have ruined their souls, because they used to wrong Our signs (7:7-8). What is apparent from these verses is that good deeds will appear heavy on one's scale while evil deeds will appear light.
Therefore the weight of the deeds is relative and not absolute. This is contrary to the idea of weighing both good and evil deeds, and then comparing the two to see which one is heavier—and thus judging whether one should go to paradise or to hell. If that were the case, then it could be that the two weights may equal and the final abode may be undecided.
There is another verse which talks about the nullification of actions, where there will be nothing to be weighed. God says: They are the ones who deny the signs of their Lord and the encounter with Him. So their works have failed. On the Day of Resurrection We will not set for them any scale (18:105). Based on this verse, only those actions, whether good or bad, which remain and are not annulled until the day (of judgment) will be weighed or measured.
Now we come to the question as to how will the deeds be measured or weighed? Notice that when we want to weigh an object, we place a unit of weight as a standard of measurement on one side of the scale and the object to be measured on the other side. Therefore, there must be a standard to be used for weighing the deeds, where the deeds are classified as heavy or light based on how well they match the standard. That standard is what determines the weight (heavy or light) of one's scales. The Qur'an says: The weight [of deeds] on that day is the truth... (7:8). This shows that the standard for measuring deeds on the Day of Judgment will be "the truth." Good deeds will be heavy on the scale because they include the truth while evil deeds will have no weight because they have falsehood and vanity. We can also say that in the case of prayers, the standard unit would be "perfect prayer." The same can be said about other obligatory acts. Likewise, the unit for weighing speech would be "truthfulness" in the speech.
Conclusions (as per tafsir al-Mizan):
(1) The weighing on the Day of Resurrection will consist of matching the deeds with the standard of truth. To the extent that an act comprises of the truth, it will be rewarded. Otherwise, it will result in perdition. This is the just balance.
(2) Every person will have multiple scales that will be set up to weigh their deeds. The touchstone for weighing every deed will be its perfect performance which does justice to that act. The Day of Resurrection is a day when nothing will have any power of influence except the truth. On that day all authority will belong to God, Who is the Truth.
Reference: al-Mizan Eng. Vol. 15 (pub. by Tawheed Inst. 2018)