In order to count pills accurately the average piece weight of pills must be accurately established. The dosage weight, which is what the physician prescribes, should not be confused with the actual pill weight. The dosage weight, the weight of the active ingredient in the pill, is only a portion (and sometimes a very small portion) of the overall pill weight. The difference between the two is the weight of the excipients. These are the buffers, color additives, fillers, etc. that are required to make the pill acceptable to the patient and uniform in size (not weight). In each manufacturing lot the total pill weight is tightly controlled to assure uniformity in the dosage weight throughout the lot. However, lot to lot variations, caused by variation in the density of the excipients, may be larger and could lead to counting errors if the average piece weight is not derive from the current lot.

There are a variety of scales offered that are NTEP approved and can be used for pill counting. While they can all perform the basic pill counting function their capabilities vary widely. The simplest do not use a scanner, and do not read the NDC code on the supply bottle, and, therefore, are unable to perform verification (comparing the bar codes on the supply bottle and the Script to be sure they are the same) unless they are hooked to a Pharmacy Management System with a scanner.

At the other end of the spectrum is a balance that performs verification, stores 10,000 NDC codes along with the drug name, the average piece weight, and the lot number from which the average piece weight was derived. This balance can remind the pharmacist that the average piece weight has not been updated for some period of time. This balance also has automatic internal calibration based upon temperature change and the passage of time.