The range of ẓanna

ظَنَّ / يَظُنُّ Verb
" To think "

There are some words in Arabic language which cannot be translated with one blanket translation as they are highly context- sensitive.

Example of this -
verb ظَنَّ (past) / يَظُنُّ (present)

It is a verb covering a range of conviction:

weak : to suppose, assume, guess;
middle : to think, believe, expect;
strong : to be convinced or certain

Depending on the context, it can express anything from mere supposition to firm conviction.

At the weaker end:
إِنْ نَظُنُّ إِلَّا ظَنًّا وَمَا نَحْنُ بِمُسْتَيْقِنِينَ
“ We are only supposing, and we are not certain .”

[Sūrat al-Jāthiyah: 32]

Here, ظَنّ is explicitly contrasted with certainty.

But in other Āyah it carries the opposite strength:
إِنِّي ظَنَنْتُ أَنِّي مُلَاقٍ حِسَابِيَهْ
“ Indeed, I was certain that I would meet my reckoning .”

[Sūrat al-Ḥāqqah: 20]

This is why Qurʾānic vocabulary cannot always be understood through one fixed English translation.
The context determines the precise degree of meaning.