# Percentage Calculations – Pharmaceutics – I B. Pharma 1st Semester

Percentage calculations

Objective

At the end of this
lecture, student will be able to:

Calculate and
convert percentage values into amounts that can be weighed or measured

Calculate the
quantities of different ingredients in a prescription when relative percentages
are given

Percentage
calculations

Introduction

• Many prescriptions received in pharmacy have active
ingredients expressed as percentage strength

• Percentage values in a prescription should be changed to
amounts

– That can be weighed

– Those which can be measured

Types of Percent

• W/W percent or Weight/Weight percent

• W/V percent or Weight/Volume percent

• V/V percent or volume/volume percent

• When type of percent is not stated,

– Dry ingredients in a dry preparation are percent W/W

– Dry ingredients in a liquid are percent W/V

– A liquid in a liquid are percent V/V

Examples of pharmaceutical dosage forms in which the active
ingredient is often calculated and expressed on percentage basis

 Percentage basis Examples of dosage forms Weight-in-volume Solutions (ophthalmic, nasal, otic, topical, large-volume parenterals) lotions Volume-in-volume Aromatic waters, topical solutions, emulsions Weight-in-weight Ointment, creams and gels

Solving percentage
problems

IF no. of parts / 100 = THEN Amount of solute needed / Total
volume or weight of the product

Example:

How many grams of zinc oxide are needed to make 240 grams of
a 4% (w/w) zinc oxide ointment?

IF        4 g ZnO /
100 g Oint    = THEN    X g ZnO / 240 g

X = 4 X 240 / 100
=      9.6 g

Practice
problems

• How many milliliters of a 5% (w/v) boric acid solution can
be made from 20 grams of boric acid?

• How many milliliters of paraldehyde are needed to make 20
ml of a 10% (v/v) paraldehyde solution?

• How many grams of ephedrine sulfate are needed to make 120
ml of a 2% (w/v) ephedrine sulphate solution?

• How many grams of zinc oxide are needed to make 120 grams
of 20% zinc oxide paste?

• How many grams of potassium permanganate are required to
compound the following prescription:

Potassium permanganate 0.02%

Distilled water q.s. to 240 ml

• How many grams of holocaine hydrochloride and how many
grams of chlorobutanol are needed to compound this prescription?

Holocaine hydrochloride ½%

Chlorobutanol  1/3%

Distilled water q.s. to 60 ml

• If 5 g of a chemical is dissolved in enough water to make
the preparation measure one litre, what is the percentage strength of the
solution?

• How many milliliters of a 0.02% W/V solution can be made
from 2.5 g of a chemical?

• Normal saline solution contains 0.9% W/V NaCl. How many
grams of sodium chloride should be used to make 1.5 liters of normal saline?

• How many grams of glucose will be required to prepare 10
litres of 2.5% solution?

• An injection contains 50 ml of pentobarbital sodium in
each ml of the solution. What is the percentage of the final solution?

• How many grams of sucrose must be dissolved in 475 ml of
water to form a 65% W/V solution?

• An ear drop formula contains 54 mg of antipyrine and 14 mg
of benzocaine in each ml of the solution. Calculate the percentage strength of
each ingredient?

Summary

• Different percent preparations – %W/W, %W/V, %V/V

• Solving problems related to percentage calculations