JavaScript Program to Convert Kilometers to Miles

Use the formula miles = kilometers * 0.621371 in JavaScript to convert kilometers to miles, for example: const miles = km * 0.621371;.

Examples

Input1

Output0.621371

Input5

Output3.106855

Input0

Output0

How to Think About It

To convert kilometers to miles, multiply the number of kilometers by the fixed conversion factor 0.621371 because 1 kilometer equals approximately 0.621371 miles. This simple multiplication gives the equivalent distance in miles.

Algorithm

1

Get the input value in kilometers.

2

Multiply the kilometers by 0.621371 to convert to miles.

3

Return or display the result in miles.

Code

javascript

function convertKmToMiles(km) {
  const miles = km * 0.621371;
  return miles;
}

const kmInput = 5;
const milesOutput = convertKmToMiles(kmInput);
console.log(milesOutput);

Output

3.106855

Dry Run

Let's trace converting 5 kilometers to miles through the code

1

Input kilometers

km = 5

2

Multiply by conversion factor

miles = 5 * 0.621371 = 3.106855

3

Return result

return 3.106855

Kilometers	Conversion Factor	Miles
5	0.621371	3.106855

Why This Works

Step 1: Conversion factor

The number 0.621371 is the fixed ratio to convert kilometers to miles because 1 kilometer equals approximately 0.621371 miles.

Step 2: Multiplication

Multiplying the kilometers by this factor scales the distance correctly from kilometers to miles.

Step 3: Return value

The function returns the calculated miles so it can be used or displayed elsewhere.

Alternative Approaches

Using arrow function

javascript

const convertKmToMiles = km => km * 0.621371;
console.log(convertKmToMiles(5));

Shorter syntax, good for simple one-line functions.

Using prompt for user input

javascript

const km = parseFloat(prompt('Enter kilometers:'));
const miles = km * 0.621371;
console.log(`Miles: ${miles}`);

Interactive approach to get input from user in browser.

Rounding result

javascript

function convertKmToMiles(km) {
  return +(km * 0.621371).toFixed(2);
}
console.log(convertKmToMiles(5));

Rounds the miles to 2 decimal places for cleaner output.

Complexity: O(1) time, O(1) space

Time Complexity

The program performs a single multiplication operation, so it runs in constant time O(1).

Space Complexity

It uses a fixed amount of memory for variables, so space complexity is O(1).

Which Approach is Fastest?

All approaches use a simple multiplication, so they have the same time and space complexity; differences are mainly in syntax and usability.

Approach	Time	Space	Best For
Standard function	O(1)	O(1)	Clear and reusable code
Arrow function	O(1)	O(1)	Concise syntax for simple tasks
Prompt input	O(1)	O(1)	Interactive user input in browser
Rounded result	O(1)	O(1)	Cleaner output display