When you look at polyester batting weights, the way it works is that if you were to cut off a yard and weigh it, that would be it's weight.
So if I have a 96" wide poly that's 18 oz. And I have a 48" wide poly that's 9 oz. They are the same thickness because one is half the width of the other.
Here's another example: 96" wide that's 12 oz. is the same thickness as a 120" wide 15 oz. And the 48" wide 6 oz is the same thickness, too.
And... 96" wide 8 oz is the same thickness as 48" wide 4 oz.
18 oz is the thickest poly batting we carry at the store. It comes from Winline.