Hex to Decimal

Each digit is a power of 16

Reading right to left, positions are worth 1, 16, 256, 4096. So 0x1F is 1×16 + 15 = 31, and 0xFF is 255 — the largest value a single byte holds, which is why FF appears everywhere a byte is at its maximum. A–F represent 10 to 15.

Two hex digits are exactly one byte

This is why hex survived while octal faded. A byte maps to exactly two hex characters with no remainder, so a memory dump, a hash or a colour value reads cleanly in pairs. Binary is exact but unreadable at length; decimal does not align with byte boundaries at all.

Where you meet it

CSS colours, where #FF5733 is three bytes of red, green and blue. Memory addresses in debuggers and crash reports. MAC addresses. Unicode code points written U+1F600. HTTP status extensions and driver error codes such as 0x80070005, where the leading bits encode severity and facility rather than being an arbitrary number.

Prefixes signal the base

0x is the C-family convention and the most common. Some assemblers use a trailing h, HTML and CSS use #, and Unicode uses U+. A bare string of digits and letters is ambiguous — 100 is 256 in hex and 100 in decimal, and nothing in the characters themselves says which.

Case never matters

0xFF and 0xff are identical. Conventions differ by ecosystem — lowercase in C and Unix tooling, uppercase in Windows error codes and MAC addresses — but no parser distinguishes them.