Percentage strength (w/v, v/v, w/w)
Convert between percentage strength, amount of solute, and total quantity for weight-in-volume, volume-in-volume, and weight-in-weight preparations.
What is this for?
Percentage strength is the workhorse expression of concentration in compounding. Leave any one of the three fields blank and PharmCalc will solve for it — enter a strength and a total quantity to find how much solute to weigh, or enter solute and total to find the resulting strength.
How to use it
- Select whether the preparation is weight-in-volume, volume-in-volume, or weight-in-weight. This determines the units of the other two fields.
- Fill in any two of the three remaining fields and leave the third blank.
- Read the solved value in the result panel; the equivalent ratio strength, ppm, and mg/mL are shown alongside.
Worked example
You need to compound 480 mL of a 2% w/v potassium permanganate soak. How many grams of potassium permanganate are required?
Answer: (2 × 480) ÷ 100 = 9.6 g.
Clinical pearls & pitfalls
- Weight-in-volume is by far the most common expression for solutions, but semi-solids — creams, ointments, gels — are almost always weight-in-weight. Using the wrong denominator changes the answer by the density of the base.
- A useful shortcut for w/v: percentage × 10 = mg/mL. So 2% w/v is 20 mg/mL, and 0.9% sodium chloride is 9 mg/mL.
- Percentage strength says nothing about the salt form. A 1% solution of a hydrochloride salt contains less free base than 1% of the base itself; check whether the prescriber intended salt or base.
Assumptions & limitations
- Weight-in-volume mixes mass and volume units by convention. It is not dimensionally consistent, so w/v and w/w percentages of the same preparation are not interchangeable unless the density is 1 g/mL.
- Assumes the solute dissolves completely with no volume contraction. For concentrated alcohol-water mixtures, volume contraction makes v/v percentages non-additive.
References
- United States Pharmacopeia. General Notices and Requirements, 8.140 Percentage Measurements.
- United States Pharmacopeia. General Chapter <795> Pharmaceutical Compounding — Nonsterile Preparations.