Python Program to Convert Hexadecimal to Decimal

You can convert a hexadecimal string to decimal in Python using int(hex_string, 16), where hex_string is your hexadecimal number as a string.

Examples

Input1A

Output26

InputFF

Output255

Input0

Output0

How to Think About It

To convert a hexadecimal number to decimal, understand that hexadecimal uses base 16 with digits 0-9 and letters A-F. Each digit represents a power of 16. The program reads the hex string and calculates the decimal value by multiplying each digit by 16 raised to its position power, then sums all these values.

Algorithm

1

Get the hexadecimal number as a string input.

2

Use a built-in function or manually convert each hex digit to its decimal equivalent.

3

Multiply each digit by 16 raised to the power of its position from right to left.

4

Sum all these values to get the decimal number.

5

Return or print the decimal number.

Code

python

hex_num = input("Enter a hexadecimal number: ")
dec_num = int(hex_num, 16)
print(f"Decimal value: {dec_num}")

Output

Enter a hexadecimal number: 1A
Decimal value: 26

Dry Run

Let's trace the input '1A' through the code

1

Input hexadecimal

User inputs '1A'

2

Convert using int()

int('1A', 16) converts '1A' to decimal 26

3

Print result

Prints 'Decimal value: 26'

Hex Digit	Decimal Value	Position (power of 16)	Calculation
A	10	16^0 = 1	10 * 1 = 10
1	1	16^1 = 16	1 * 16 = 16

Why This Works

Step 1: Using int() with base 16

The int() function can convert strings in any base to decimal by specifying the base as 16 for hexadecimal.

Step 2: Hexadecimal digits

Hex digits include 0-9 and A-F, where A=10, B=11, ..., F=15, which int() understands automatically.

Step 3: Output decimal value

The converted decimal number is printed as the final result.

Alternative Approaches

Manual conversion using loop

python

hex_num = input("Enter a hexadecimal number: ")
dec_num = 0
hex_digits = '0123456789ABCDEF'
hex_num = hex_num.upper()
for i, digit in enumerate(reversed(hex_num)):
    dec_num += hex_digits.index(digit) * (16 ** i)
print(f"Decimal value: {dec_num}")

This method shows how to convert manually without built-in functions but is longer and less efficient.

Using int() with error handling

python

hex_num = input("Enter a hexadecimal number: ")
try:
    dec_num = int(hex_num, 16)
    print(f"Decimal value: {dec_num}")
except ValueError:
    print("Invalid hexadecimal number")

This approach adds error handling to manage invalid inputs gracefully.

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

Time Complexity

The conversion scans each character once, so time grows linearly with the length of the hexadecimal string.

Space Complexity

Only a few variables are used, so space is constant regardless of input size.

Which Approach is Fastest?

Using Python's built-in int() is fastest and simplest compared to manual conversion.

Approach	Time	Space	Best For
Built-in int() function	O(n)	O(1)	Quick and reliable conversion
Manual loop conversion	O(n)	O(1)	Learning how conversion works
int() with error handling	O(n)	O(1)	Robust user input handling