Round a Number to N Decimal Places in Java

Venter C.

—January 15, 2025

The ProblemThe Problem

You don’t know how to round a value to a specific number of decimal places in Java.

The SolutionThe Solution

There are multiple ways to round a number in Java, depending on the precision you require and how you plan to use the result.

• The BigDecimal class is best suited to financial and high-precision applications.
• The DecimalFormat class is better suited to user-facing applications.
• The String.format() method is useful when formatting numbers for display purposes.
• Custom utility methods can be used to create precise and reusable rounding methods.

Use the BigDecimal ClassUse the BigDecimal Class

The Java BigDecimal class gives you precise control over rounding while avoiding the issues caused by floating-point arithmetic. You can specify both the number of decimal places and the rounding mode.

For example, the following code creates a BigDecimal object from the double value and uses the setScale() method to specify two decimal places and the HALF_UP rounding mode:

import java.math.BigDecimal;
import java.math.RoundingMode;

public class Main {
    public static void main(String[] args) {
        double value = 12.34567;
        int decimalPlaces = 2;

        BigDecimal bd = new BigDecimal(Double.toString(value));
        bd = bd.setScale(decimalPlaces, RoundingMode.HALF_UP);

        System.out.println("Rounded value: " + bd);
    }
}

The HALF_UP rounding mode is a common rounding strategy in which values are rounded up when the digit after the rounding place is 5 or greater.

In this case, the output will be:

Rounded value: 12.35

The precision offered by the BigDecimal class makes this approach suitable for financial and high-precision applications.

Use the DecimalFormat ClassUse the DecimalFormat Class

You can use the Java DecimalFormat class to define a format pattern that rounds numbers to a specific number of decimal places for display.

The following example creates a DecimalFormat object with the format pattern #.## (where # represents digits) to round the number to 2 decimal places:

import java.text.DecimalFormat;

public class Main {
    public static void main(String[] args) {
        double value = 12.34567;
        DecimalFormat df = new DecimalFormat("#.##");

        System.out.println("Rounded value: " + df.format(value));
    }
}

The output will be:

Rounded value: 12.35

Because the format() method returns the rounded number as a String, this approach is best used to format numbers for user-facing applications. It is not ideal for calculations, as the string result is less precise than other options.

Use the String.format() MethodUse the String.format() Method

Like the DecimalFormat class, the Java String.format() method allows you to format a value to a specific number of decimal places and outputs it as a string.

Note the different syntax in the following example:

public class Main {
    public static void main(String[] args) {
        double value = 12.34567;

        String roundedValue = String.format("%.2f", value);
        System.out.println("Rounded value: " + roundedValue);
    }
}

The String.format() method is used with the format specifier %.2f, where 2 represents the number of decimal places and f represents a floating-point number. The result is a string with the value rounded to 2 decimal places.

The output will be:

Rounded value: 12.35

The String.format() method is easy and useful when you need to display formatted numbers, but like DecimalFormat, the result is a string and not suitable for further numerical calculations.

Use Custom Utility MethodsUse Custom Utility Methods

To make the rounding process reusable, you can create a custom utility method that encapsulates one of the approaches shown above.

The following example uses the BigDecimal class:

import java.math.BigDecimal;
import java.math.RoundingMode;

public class Main {
    public static void main(String[] args) {
        double value = 12.34567;
        double roundedValue = roundToNDecimalPlaces(value, 2);
        System.out.println("Rounded value: " + roundedValue);
    }

    public static double roundToNDecimalPlaces(double value, int decimalPlaces) {
        BigDecimal bd = new BigDecimal(Double.toString(value));
        bd = bd.setScale(decimalPlaces, RoundingMode.HALF_UP);
        return bd.doubleValue();
    }
}

This custom utility method creates a reusable roundToNDecimalPlaces() method that takes the value and the number of decimal places as parameters, uses BigDecimal to round the number, and returns the result as a double.

Further ReadingFurther Reading

• The Java BigDecimal class documentation
• The Java DecimalFormat class documentation
• The Java String.format() method documentation
• The Java RoundingMode class documentation