Arduino How to Convert String to Int Easily

In Arduino, you can convert a string to an integer using the toInt() method like this: int number = myString.toInt();.

Examples

Input"123"

Output123

Input"007"

Output7

Input"abc"

Output0

How to Think About It

To convert a string to an integer in Arduino, think of the string as a number written in text form. You want to read that text and get the actual number it represents. Arduino's String class has a built-in method called toInt() that reads the string from left to right and stops when it finds a non-digit, returning the number it found or zero if none.

Algorithm

1

Get the input string that contains digits.

2

Use the <code>toInt()</code> method on the string to convert it.

3

Store the result as an integer variable.

4

Use the integer as needed in your program.

Code

arduino

void setup() {
  Serial.begin(9600);
  String myString = "1234";
  int number = myString.toInt();
  Serial.println(number);
}

void loop() {
  // Nothing here
}

Output

1234

Dry Run

Let's trace converting the string "1234" to an integer.

1

Initialize string

myString = "1234"

2

Convert string to int

number = myString.toInt(); // number becomes 1234

3

Print result

Serial prints 1234

Step	String	Integer
1	"1234"	N/A
2	"1234"	1234
3	N/A	1234

Why This Works

Step 1: Using toInt() method

The toInt() method reads the string characters and converts them to a number until it finds a non-digit.

Step 2: Returns integer value

It returns the integer value of the digits found or zero if no digits are present.

Step 3: Works with Arduino String class

This method is built into Arduino's String class, making conversion simple and direct.

Alternative Approaches

Using atoi() with c-string

arduino

void setup() {
  Serial.begin(9600);
  char myCharArray[] = "5678";
  int number = atoi(myCharArray);
  Serial.println(number);
}

void loop() {}

This method requires converting Arduino String to a char array and is useful if you prefer C-style strings.

Manual parsing

arduino

void setup() {
  Serial.begin(9600);
  String s = "42";
  int number = 0;
  for (int i = 0; i < s.length(); i++) {
    number = number * 10 + (s.charAt(i) - '0');
  }
  Serial.println(number);
}

void loop() {}

This method manually converts each character to a digit and builds the number, useful for learning but less efficient.

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

Time Complexity

The toInt() method scans each character once, so it takes time proportional to the string length.

Space Complexity

No extra memory is needed besides the integer variable, so space is constant.

Which Approach is Fastest?

Using toInt() is fastest and simplest for Arduino Strings; atoi() is similar but requires char arrays; manual parsing is slower and more complex.

Approach	Time	Space	Best For
toInt()	O(n)	O(1)	Simple Arduino String conversion
atoi() with char array	O(n)	O(1)	C-style strings or legacy code
Manual parsing	O(n)	O(1)	Learning or custom parsing needs