R.I. Gen. Laws § 44-49-8: Measurement.

For the purpose of calculating this tax, a quantity of a controlled substance is measured by the weight of the substance whether pure or impure or dilute, or by dosage units when the substance is not sold by weight, in the dealer’s possession. A quantity of a controlled substance is dilute if it consists of a detectable quantity of pure controlled substance and any excipients or fillers.