All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
In principle, assessing the Nisaab (Minimum amount liable for Zakah) for gold is based on the following narrations: Ibn 'Umar and 'Aishah narrated: "that from every twenty Dinar or more. The Prophet used to take half a Dinar and from forty Dinar, one Dinar." {Related by Ibn Majah}. Narrated Ali ibn AbuTalib: The Prophet (PBUH) said: "When you possess two hundred dirhams and one year passes on them, five dirhams are payable. Nothing is incumbent on you, that is, on gold, till it reaches twenty dinars. When you possess twenty dinars and one year passes on them, half a dinar is payable. Whatever exceeds, that will be reckoned properly." {Related by Abu Dawud}.
Muslim scholars have unanimously agreed on this opinion and their consensus was reported by Ibn Al-Monther. Al-Imam An-Nawawi said: " In our Madhab, Shafie school of thought, the Nisaab for gold is twenty Mithqals and whatever exceeds will be reckoned properly, whether it was little or much, and this is the view of the majority of the Muslim scholars." {Al-Majmou`vol.5/pp.503}.
Modern scholars have resorted to Ijtihad (Independent reasoning) to assess the weight of the "Mithqal" or "Dinar" according to modern weights: Those who estimated the weight of the old gold dinar at (4.25 grams) said: The Nisaab for gold is the product of multiplying twenty by (4.25 grams), and that is (85 grams). This is the view of most modern scholars and it is also adopted by the General Iftaa` Department of Jordan.
On the other hand, those who estimated the weight of the old gold dinar at (4.8 grams) said: The Nisaab for gold is the product of multiplying twenty by (4.8 grams), and that is (96 grams). This is the Nisaab adopted in the Shafie book [Al-Fiqh al-Manhaji] Moreover, there are other views, but we opted not to dwell on them. And Allah Knows Best.
Note: 20*4.25 = 85 grams 20*4.8 = 96 grams