There are many situations in programming where we need to calculate percentages such as analysing the differnce between one number with respect to another.

In this post, we will learn how to calculate percentage between two number in JavaScript. In addition, we will calculate percentage increase or decrease between two numbers.

## How to calculate percentage?

When calculating the percentage between two numbers in JavaScript, the formula is as follows:

`Percentage = (First Number/Second Number) * 100`

This formula will give you the percentage of the first number out of the second number.

Letβs look at a few examples of calculating the percentage between two numbers.

**1. Calculate the percentage of 10 out of 20**

```
const firstNumber = 10;
const secondNumber = 20;
const percentage = (firstNumber/secondNumber)*100;
console.log(percentage); //π 50
```

**2. Calculate the percentage of 5 out of 10**

```
const firstNumber = 5;
const secondNumber = 10;
const percentage = (firstNumber/secondNumber)*100;
console.log(percentage); //π 50
```

**3. Calculate 20 out of 100**

```
const firstNumber = 20;
const secondNumber = 100;
const percentage = (firstNumber/secondNumber)*100;
console.log(percentage); //π 20
```

If you want to **round the percentage to the nearest integer**, use the `Math.round()`

method.

```
const firstNumber = 15.2;
const secondNumber = 100;
const percentage = (firstNumber/secondNumber)*100;
console.log(percentage); //π 15.2
const roundedPercentage = Math.round(percentage);
console.log(roundedPercentage); //π 15
```

As you can see, it returns `15`

as its nearest integer from `15.2`

, which is the calculated percentage.

Same way, use the `toFixed()`

method to round the decimal places.

The `toFixed()`

method takes an integer argument that specifies the number of decimal places to use. The number is rounded to this decimal place.

```
const percentage = (40 / 90) * 100;
console.log(percentage); // ποΈ 44.444...
// ποΈ 2 decimals
const fixed = percentage.toFixed(2);
console.log(fixed); // ποΈ "44.44"
```

β οΈ Note that the `toFixed`

method **returns a string**, not a number. And If the number has **no decimal points**, it gets **padded with zeros**.

## How to calculate percentage increase or decrease between two numbers?

To calculate percentage increase or decrease between two numbers we can use math formula like we used to calculate percentage between two numbers.

The formula is to subtract `newNumber`

from `oldNumber`

, divide the result by `oldNumber`

, and multiply this result by `100`

.

`Percentage = ((New Number - Old Number) / Old Number) * 100`

For example, calculate the percentage increase from 10 to 20.

```
const oldNumber = 10;
const newNumber = 20;
const percentage = ((newNumber - oldNumber) / oldNumber) * 100;
console.log(percentage); // π 100
```

It returns a **positive number if the percentages increase**; otherwise, it returns a **negative number**.

For example, calculate the percentage decrease from 20 to 10.

```
const oldNumber = 20;
const newNumber = 10;
const percentage = ((newNumber - oldNumber) / oldNumber) * 100;
console.log(percentage); // π -50
```

It returns negative number means the percentage decrease from `20`

to `10`

is `50%`

.

## Conclusion

In conclusion, calculating the percentage between two numbers in JavaScript is simple. With the right formula, we can quickly and easily calculate the percentage.

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