Round a Number to 1 Decimal Place in JavaScript

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Borislav Hadzhiev

Last updated: Jul 27, 2022

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Round a Number to 1 Decimal Place #

To round a number to 1 Decimal place:

  1. Use the Math.round() function to round the result of multiplying the number by 10.
  2. The Math.round function will round the result to the nearest integer.
  3. Divide the integer by 10 to get the number rounded to 1 decimal place.
index.js
const num = 5.566; const result = Math.round(num * 10) / 10; console.log(result); // 👉️ 5.6

We used the Math.round function to round the result of multiplying the number by 10 to the nearest integer.

Here are some examples of how the Math.round function works.

index.js
console.log(Math.round(2.22)); // 👉️ 2 console.log(Math.round(2.5)); // 👉️ 3 console.log(Math.round(2.22 * 10)); // 👉️ 22 console.log(Math.round(2.5 * 10)); // 👉️ 25

The function rounds the number up or down to the nearest integer.

If the number is positive and its fractional part is greater than or equal to 0.5, it gets rounded to the next higher absolute value.

If the number is positive and its fractional portion is less than 0.5, it gets rounded to the lower absolute value.

The last step is to divide the integer by 10 to get the result of rounding the number to 1 decimal place.

index.js
// 👇️ 2.5 console.log(Math.round(2.456789 * 10) / 10); // 👇️ 4.7 console.log(Math.round(4.689 * 10) / 10);

Alternatively, you can use the Number.toFixed method.

Round a Number to 1 Decimal Place using toFixed() #

Use the toFixed() method to round a number to 1 decimal place, e.g. num.toFixed(1). The toFixed method formats a number to a specified number of decimal places and rounds the number if necessary.

index.js
const num1 = 5.566; const result1 = num1.toFixed(1); console.log(result1); // 👉️ 5.6 console.log(typeof result1); // 👉️ string // 👇️ if the value is a string // call parseFloat to convert it to a number first const str1 = '5.666'; const result2 = parseFloat(str1).toFixed(1); console.log(result2); // 👉️ 5.7 console.log(typeof result2); // 👉️ string // 👇️ Convert string back to a number const num2 = 5.05; const result3 = Number(num2.toFixed(1)); console.log(result3); // 👉️ 5 console.log(typeof result3); // 👉️ number

In the first example, we used the Number.toFixed method to round a number to 1 decimal place.

The only parameter the method takes is the number of digits that should appear after the decimal point.

The toFixed method returns a string representation of the number.

In our second example, we have a string that is a valid number. We used the parseFloat function to convert the string to a number, because the toFixed method can only be called on numbers.

index.js
const str1 = '5.666'; const result2 = parseFloat(str1).toFixed(1); console.log(result2); // 👉️ 5.7 console.log(typeof result2); // 👉️ string

In the third example, we used the Number object to convert the string that the toFixed method returned to a number.

index.js
const num2 = 5.05; const result3 = Number(num2.toFixed(1)); console.log(result3); // 👉️ 5 console.log(typeof result3); // 👉️ number
However, notice that the trailing zero was dropped. This is because JavaScript doesn't keep any insignificant, trailing zeros around.

The number 5.00 is the same as 5, so the trailing zeros get dropped when the value is converted to a number.

index.js
console.log(5.00 === 5); // 👉️ true
Floating point numbers don't represent all decimals precisely in binary, which can lead to inconsistent results.
index.js
console.log(0.1 + 0.2 === 0.3); // 👉️ false

The sum of 0.1 and 0.2 is actually equal to 0.30000000000000004 instead of 0.3. This is because the binary floating-point format cannot accurately represent numbers like 0.1 or 0.2.

The code gets rounded to the nearest number, resulting in a rounding error.

Further Reading #

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