A drug strength or concentration of its active ingredients can be expressed in many different ways. In order to standardize the expression of active ingredients in drug listing Structured Product Labeling (SPL) submitted to FDA, the agency has adopted a series of automated validation rules to allow for certain expressions.

For example, the strength of active ingredient is not allowed to be included as a percentage value but can be a concentration of an amount of solute in an amount of solution:

• As w/w - mass (grams) of solute in mass (100 gm) of solution, like topical creams and ointments
• As w/v - mass (grams) of solute in a volume (100ml) of solution, like oral liquids
• As v/v - volume (milliliters) of solute in a volume (100ml) of solution, like alcohol

The strength data element in a listing SPL is designed to accept submissions mostly in concentrations of w/w or w/v format, when the strength of an active ingredient is expressed as a percentage. Percentages must be converted into ratios of w/w or w/v with a value in the numerator and in the denominator including the correct units of measure in order to pass the SPL validation rules.

Exceptions to this rule are alcohol, medical gases and water. Strength of active ingredients in these products can be included in the SPL as v/v and does not need to be converted into w/w or w/v. In cases like alcohol, the value of density can fluctuate depending on the temperature. Converting the strength into w/w or w/v may lead to an incorrect strength. It is therefore recommended that the strength of active ingredient for these products be included as v/v in the listing SPL. 

In most cases, the strength used is that for a single dose following the conventions in the table below. An example of “weight” is milligram, an example of “volume” is milliliter, an example of “time” is hour, and an example of “each” is tablet.

Note that for powders that are reconstituted, the denominator is the volume following reconstitution if the amount is fixed. For example, if 10 mg of a powder in a vial is mixed with 5 ml of diluent, the strength is 10 mg in 5ml. If the reconstituted amount is not fixed, then the denominator is 1. For example, if 10 mg of a powder in a vial is mixed with 5-10 ml of diluent, the strength is 10 mg in 1 vial.