Pharmacists or pharmacy technicians may have to reduce or enlarge formulas for pharmaceutical preparations in the course of their professional practice or manufacturing activities. This article is intendent to assist pharmacists and support personnel in performing the necessary calculations for reducing and enlarging formulas.
- Reducing formula used to calculate the amounts to be used in a pharmaceutical formulation to make a smaller amount than the original formulation.
- Enlarging formula used to calculate the proper amounts to be used in a pharmaceutical formulation to make a larger amount than the original formulation.
Reducing or enlarging formulas can be done by a two-step process:
1. Determine the factor that defines the multiple or the fraction of the amount of formula to be prepared.
- Factor = Quantity of formula desired/ quantity of formula given
2. Multiply the quantity of each ingredient in the formula by the factor to determine the amount of each ingredient required in the reduced or enlarged formula.
Examples
1.
If a formula for 1000 mL contains 5 g of a drug, how many grams of drug are needed to prepare 50 mL of the formula?
Step-1: 50 mL/1000 mL= 0.05 (factor)
Step-2: 5 gx0.05 = 0.25 g (answer).
2.
Rx
• Zinc oxide 50 g
• Pine tar 120 g
• Petrolatum 284 g
How much pine tar would be required to prepare 1 kg of the ointment?
Step-1: Factor = 1000 g/454 g= 2.22
Step-2: Amount of pine tar required = 120 x2.2 = 264 g (answer).
3
From the following formula, calculate the quantity of each ingredient required to make 250 mL of calamine lotion.
• Calamine = 75 g
• Zinc oxide = 75 g
• Glycerin = 15 g
• Bentonite magma = 250 mL
• Ca(OH)2 solution to make = 1000 mL
Step-1: Factor = 250 mL/1000 mL = 0.25
To make only 25% (because 250 mL is 25% of 1000 mL) of the original quantity. The quantity of each ingredient is calculated as follows:
Step-2: Multiply the quantity of each ingredient in the formula by the factor
• Calamine = 75 g × 0.25 = 18.75 g
• Zinc oxide = 75 g × 0.25 = 18.75 g
• Glycerin = 15 g × 0.25 = 3.75 g
• Bentonite magma = 250 mL × 0.25 = 62.5 mL
• Ca(OH)2 solution to make = 1000 mL × 0.25 = 250 mL
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