How to Convert Celsius to Fahrenheit in Java

The Challenge

Convert temperature from Celsius to Fahrenheit using the formula: F = (C × 9/5) + 32

Solution

java

public class Main {
    public static void main(String[] args) {
        double celsius = 25.0;
        double fahrenheit = (celsius * 9/5) + 32;
        
        System.out.println(celsius + "°C = " + fahrenheit + "°F");
        // Output: 25.0°C = 77.0°F
    }
}

Fahrenheit to Celsius

java

double fahrenheit = 77.0;
double celsius = (fahrenheit - 32) * 5/9;

System.out.println(fahrenheit + "°F = " + celsius + "°C");
// Output: 77.0°F = 25.0°C

Using Methods

java

public static double celsiusToFahrenheit(double celsius) {
    return (celsius * 9/5) + 32;
}

public static double fahrenheitToCelsius(double fahrenheit) {
    return (fahrenheit - 32) * 5/9;
}

// Usage
System.out.println(celsiusToFahrenheit(0));    // 32.0
System.out.println(celsiusToFahrenheit(100));  // 212.0
System.out.println(fahrenheitToCelsius(32));   // 0.0
System.out.println(fahrenheitToCelsius(212));  // 100.0

Did You Know?

The Celsius and Fahrenheit scales intersect at -40°! That means -40°C = -40°F. It's the only temperature where both scales show the same number!

Watch Out!

Use 9.0/5.0 instead of 9/5 to avoid integer division:

java

// Wrong - integer division
double f = (25 * 9/5) + 32;  // 77.0 (works but risky)

// Correct - floating-point division
double f = (25 * 9.0/5.0) + 32;  // 77.0 (safer)

Real-World Use Cases

Weather apps: Display temperature in user's preferred unit
Cooking: Convert recipe temperatures
Science: Convert between temperature scales
Travel: Understand local weather forecasts

›Short versionOpen if you only have a minute.

The formula is F = C * 9 / 5 + 32. The division has to see a double, or the fraction disappears.
9 / 5 in ints is 1. 1 * 9 / 5 in ints is 1, then + 32 is 33, not 33.8.
25 hides the bug: 25 * 9 is divisible by 5, so the int path still prints 77.
Format the number when you print it. Do not round it before the formula finishes.

Celsius to Fahrenheit

Convert Celsius to Fahrenheit using F = C * 9 / 5 + 32.

Working approach

Keep the temperature in a double.
Write celsius * 9.0 / 5.0 + 32 so neither factor forces integer division.
Apply the inverse, (fahrenheit - 32) * 5.0 / 9.0, with the same rule.

Walk the sample

Input 25 C with double arithmetic
25 * 9 / 5 is 45
45 + 32 is 77 F

The trap

Left-to-right integer arithmetic truncates. The sample input 25 still prints 77, so the bug shows up on values such as 1.

java

static double toFahrenheit(double celsius) {
    return celsius * 9.0 / 5.0 + 32;
}

// int path: 1 * 9 / 5 + 32 == 33, not 33.8
// 25 * 9 / 5 + 32 == 77 only because 225 is divisible by 5

What does int f = 1 * 9 / 5 + 32 produce, and why does 25 still look correct?

1 * 9 / 5 is 9 / 5, which is 1 in integer arithmetic, then + 32 is 33.

25 * 9 is 225, and 225 / 5 is 45 with no remainder, so the truncated formula happens to match 77. The test case in the sample does not expose the bug.

Time O(1), space O(1).

Easy to get wrong

Writing 9 / 5 with two int literals.
Trusting the result for 25 and not trying a value that is not a multiple of 5.