ASCII Character Codes Explained
The 128-code ASCII table
ASCII defines 128 codes from 0 to 127. Codes 0-31 and 127 are control characters, and codes 32-126 are printable characters including the space.
The table on this site shows each code in decimal, hex, octal, and binary so you can use the format your tool needs.
Reading A and a
Latin capital letter A is decimal 65, hex 41, octal 101, and binary 01000001. Lowercase a is decimal 97, hex 61, octal 141, and binary 01100001.
The lowercase letters start 32 codes after their uppercase counterparts, which is why case conversions in ASCII are simple arithmetic.
ASCII is the start of Unicode
Unicode assigns the same values to ASCII characters, so U+0041 is also ASCII 65. UTF-8 preserves ASCII bytes as single bytes.
Codes 128-255 are extended ASCII and vary by code page, so this reference treats 0-127 as the authoritative standard table.