It is easy because ASCII is already a code representing decimal numbers against letters. The original ASCII is 128 decimal characters. The decimal number from 0 to 127 is allocated to each one of those, including letters of the numbers, punctuation marks, and the English alphabet. To manually convert ASCII to decimal, identify the character in the ASCII table, and pick the decimal number against each character in your ASCII...

In the ASCII code, each of these characters are assigned a decimal number from 0 to 127. For example, the ASCII representation of upper case A is 65 and the lower case a is 97. The decimal numeral system is the most commonly used and the standard system in daily life. It uses the number 10 as its base (radix).