A worked answer
A 110 by 95 inch king with 4 inches of overhang needs batting about 118 by 103 inches. A King package around 120 by 120 usually covers it. Measure before you trust the name on the bag.
Quiltler’s king quilt is 110 by 95 inches. Four inches of overhang makes 118 by 103. Unlike twin and queen, a typical King batting package (120 by 120) actually covers that rectangle. If your longarmer wants 8 inches extra, 110 + 16 = 126, and even a 120 inch square is short. Ask before you buy the bag.
Use the batting calculator with your overhang. Backing is where king quilts hurt more than batting — see how much backing for a king quilt. Design the top in the king quilt designer.
With 4 inches of overhang, a 110 by 95 inch top needs about 118 by 103 inches of batting. A King package around 120 by 120 usually fits. California King tops are a different rectangle — type those inches into the calculator.
A 120 inch square package is the usual shop answer. Rolls of 96 inch batting will not cover 118 inches without a join.
Quiltler lists Cal King separately in the sizes guide. Do not use this 110 by 95 card for a 72 by 84 mattress without changing the numbers.