What is a qullah?
First of all you should know that a qullah is basically a big vessel (search via google القلة). One could ask himself whether these vessels have been fabricated everywhere with the same volume....
The relatad ahadith about qullatayn (two qullahs) are linked in my answer on Levels of obligatoriness of rulings within schools of law. Here just one out of them to make clear what is meant:
"The Messenger of Allah (ﷺ) was asked about water and how some animals and carnivorous beasts might drink from it. He said: 'If the water is more than two Qullahs, it will not become filthy.'" (Sunan an-Nasa-i)
Qullah is a rather (slightly) varying measure as many other measures in the Arabic and Islamic world so you may find the quantity of one sa'a in Medina different than in 'Iraq etc. which makes things rather difficult.
So each estimation or calculation will depend on a certain definition.
216 l or 160.5 or ...
For example in (Arabic) fatwa #16107 on islamweb the answer relies on the information that two qullahs are equalt to 500 'Iraqi ratl which makes them equal to 160.5 l.
On ahlalhadeeth.com one post estimates one 'Iraqi ratl as equal to 406.25 g -> 500*406.25g = 203125 g or 203.125 kg -> 203.125 l
Just as a reference the Egyptian ratl is equal to 450 g
According to the book al-fiqh al-Muyassar الفقه الميسر one Qullah is equal to 93.075 sa'a which is equal to 160.5 l.
Some say al-Mudd is equal to 675 g and 0.688 l and is equalt 1.33 ratl ('Iraqi) in the view of a-Shafi'i and the scholars of al-Hijaz while the hanafi's consider 1 mudd = 2 ratl which means it is 3800 g.
as-Sa'a = 4 mudd is equal to 2176 g and 2.75 l (shafi'i view and the view of scholars of al-Hijaz) or 3800 g according the scholars of al-Iraq and the hanfis.
islamqa #154588 offers a measure of 1 qullah being equal to 102 kg according to sheikh Abudllah ibn Sulayman al-Manee' عبد الله بن سليمان المنيع based on the calculation: that one qullah is equal to 250 ratl and one ratl is 408 g
Feqh.web somebody has heard of the following equal measures: 161 l, 191 l, 206 l, 213 l and 306 l.
all or most of these might have a background, but are hard to verify.
Maybe a good Conclusion
Sheikh ibn Amin in his answer post on ahlalhadeeth.com writes:
عموماً الصواب أنه لا يوجد مقدار معين للقلتين، وإنما المقصود من الحديث الماء الكثير
Generally the most correct is that there's no specific measure (volume) for the two qullahs, but what the hadith means is that the amount of water is big.
So maybe the calculation quoted by @goldpseudo:
If you take qullatayn as the volume of a 1.25 cubit cube where a cubit is about 48 cm, you get 216 litres. But I don't know how agreed-upon that calculation is.
is an answer to your question, while mine tries to explain the issue with the ancient Arabic measures.