If it's full, it will be holding 6 quarts.
First you need to know how much the quart weighs. 1 US fluid ounce - 0.03125 US quarts
2 quarts
7 quarts if just a fluid change.
6 quarts
2.1 quarts
8 quarts
10 1/2 quarts
4 quarts
6 quarts would hold 6 quarts, or 1.5 gallons, or 12 pints or 24 cups or 192 fluid ounces.
13.4 quarts with a filter change
2.5 Quarts
about 1 and 3/4 quarts