It is common practice to reduce a recipe for dispensing or enlarge the recipe as required for the purpose of patient. This is required when a manufacturing formula is to be taken from the laboratory scale to a manufacturing floor and a formula for the large scale manufacturing is to be adopted for prescribing to a patient. Such calculations may also involve conversions from one system of weights and measures to another.

The following rules are generally useful in such system

**Rule 1:**

To convert a recipe in imperial system to metric system

"Write grams in place of grains, divide the minims by 1.1 and write the result as milliliters. For large batch, read avoirdupois ounce as grams and fluid ounces as milliliters"

**Rule 2:**

To convert a recipe in metric system to imperial system

"For a small batch write grains in place of grams and multiply the milliliters by 1.1 and write the result as minims. For large batch, write ounce in place of grams and fluid ounce in place of minims"

**Example**

Rx

Sodium hypophosphite gr. Xx

Sulphuric acid fl dr. I

Cinnamon water q.s. ad fl.oz. ii

Dispense 50 ml

**Calculation:**

Convert to metric system

Sodium hypophosphite 20 g. = 20 x 50/872.7 = 1.15 g

Sulphuric acid 60/1.1 = 55.5 ml = 55.45 x 50 / 872.7 = 3.2 ml

Cinnamon water q.s. ad 2 x 480/1.1 = 872.5 ml = 50 ml

Dispense 50 ml

However, the simpler solution of the problem would be to retain the original recipe and prepare 2 fl. oz. of the mixture, which corresponds to about 60 ml. Out of 2fl.oz, 60 ml, could be dispensed and the rest rejected.

**Example**

Rx

Pt. Nitrate

Sod. Chloride

Camphor of each 3 g

Tincture opium 15 ml

Alcohol to make 200 ml

Dispense 4 fl.oz.

**Solution**

4 fl oz of the prescription is to be dispensed hence quantities can be converted to Imperial system.

R

Pt. Nitrate

Sod. Chloride

Camphor each 3 grains

Tincture opium = 15 x 1.1 = 16.5 minims

Alcohol = 200 x l.l = 220 minims

4 x 480= 1920 minims

Quantity of Pt. Nitrate, Sod. Chloride and Camphor = 3 x 1920/220 =26.2 grains

So to avoid this we can prepare 9 times the formulal980 minims and dispense 4 fl.oz and reject 60 minims

**Example**

Calculate the amounts of ingredients required to make 50 ml of calamine lotion.

Rx

Calamine 150 g

Zinc oxide 50 g

Bentonite 30 g

Sodium Citrate 5 g

Liquefied Phenol 5 ml

Purified water, q.s. 1000 ml

Dispense 50 ml.

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