Percentage calculation is very useful in pharmacy and pharmaceutical calculations to measure the number of grams contained in 100 mL of solution. Which is expressed by Percent Weight in Volume (%w/v).

#### Percentage Calculation Formula

To determine the percentage of X, we have to divide the value (X) by the total value (Y) and then multiply the resultant by 100.

Percentage (%) = X/Y ×100%

#### Percentage Calculation Example

**1. Express 1:2000 as a percentage strength.**

1 part/2000 parts = X parts/100 parts.

So, X = 1 × 100/2000 = 0.05%

**2. Express 0.5% w/w as a ratio strength.**

0.5 g/100 g = 1 part/X parts.

X = 100 × 1/0.5

X = 200

Therefore, the ratio strength = 1:1000

**3. Express 5 ppm as a percentage strength.**

5 ppm = 5 part per million = 5:1000000

So, 5 part/1000000 = X parts/100 parts

X = 5 × 100/1000000 = 0.0005%

**Related:** Ratio and Proportion Calculation in Pharmacy

#### Pharmaceutical Percentage Calculation

**1. Express 1 in 200 w/v solution of potassium permanganate as a percentage.**

1/200 = X / 100

X = 1 × 100/200 = 0.5% w/v

**2. How many grams of a drug should be used to prepare 250 g of a 10% w/w solution?**

Let X be the weight of the drug needed:

So, X/250 = 10 g/100 g

X = 10 × 250/100 = 25 g

**3. Express 1.8 g of sodium chloride, made up to 200 mL with water, as a percentage.**

1.8/200 = X/100

X = 100 × 1.8/200 = 0.9%

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