What is Hexadecimal? How to Read Hex Numbers

Hexadecimal — hex for short — is a base-16 number system: it counts using sixteen symbols, the familiar 0–9 followed by the letters A through F. It's the standard way programmers write bytes, because one byte fits perfectly into exactly two hex digits. The color #FF6600, the hash a94a8fe5, the memory address 0x7FFF5C — all hex, all readable in a minute once you know the trick.

Why does a color picker hand you letters mixed in with your numbers? That question is really asking how base-16 counting works, and the answer is friendlier than it looks. Let's build it from something you already understand: an odometer.

Counting past 9: where A–F come from

Ordinary decimal counting is an odometer with ten symbols per wheel. You count 0 through 9, run out of symbols, roll the wheel back to 0, and tick the next wheel up: 10. Every number system works exactly this way — the only choice is how many symbols each wheel holds.

Hexadecimal's odometer has sixteen positions per wheel. After 9 you don't roll over yet — you keep going through six more symbols, and since we ran out of digits, the alphabet lends a hand: A is 10, B is 11, C is 12, D is 13, E is 14, F is 15. Then the wheel rolls: after F comes 10, which in hex means sixteen, not ten.

So the hex sequence runs: ... 8, 9, A, B, C, D, E, F, 10, 11 ... and each wheel is worth sixteen times the wheel to its right. That's the entire system. If the odometer picture clicked, everything below is just reading practice.

Two hex digits = one byte (the reason hex exists)

Computers store everything in bytes of 8 bits, and writing bits out longhand gets old fast — 11111111 is one byte, and a hash digest is 32 of those. Hex fixes this with a mapping that isn't approximate but exact: one hex digit holds precisely 4 bits (a nibble), so one byte is always exactly two hex digits.

This is why hex, and not decimal, became the byte notation: converting between hex and binary needs no arithmetic at all — you translate nibble by nibble, in either direction, digit for digit. Decimal can't do that; 255 tells you nothing about its bits at a glance, but FF tells you every bit is on. If you read our guide to reading binary, hex is the shorthand for everything you decoded there: the byte 01001000 (the letter H) compresses to 48.

Reading hex in the wild

Once you know the two-digits-per-byte rule, hex starts announcing itself everywhere:

• CSS colors. #FF6600 is three bytes: FF red (255, full), 66 green (102), 00 blue (none) — a strong orange. Decode any color with the Hex to RGB converter.
• Hash digests. A SHA-256 hash shown as 64 hex characters is really 32 bytes, two characters each.
• Memory addresses and error codes. The 0x prefix — 0x7FFF5C, 0xC0000005 — is code for "this number is hex."
• Unicode code points. U+00E9 is é; the number after U+ is hexadecimal, which is also what numeric HTML entities like é use.
• Hex dumps. When a debugger shows you file contents as 48 65 6C 6C 6F, that's bytes in hex — paste them into a Hex to Text converter and it reads "Hello".

Convert hex by hand: two worked examples

Hex → decimal. Each position is a power of 16, rightmost first. Take 2A: the 2 sits in the sixteens place and A is ten, so 2×16 + 10 = 42. Three digits work the same way with 256s: 1F4 is 1×256 + 15×16 + 4×1 = 500.

Hex → text. Take the dump 48 69. Each pair is one byte: 48 is 4×16 + 8 = 72, and 72 in ASCII is H. 69 is 6×16 + 9 = 105 — the letter i. The message says "Hi", and you just read a hex dump by hand.

Here's a self-check before you move on: what's FF in decimal? Fifteen sixteens plus fifteen — 255. If you got there without a calculator, you can read hex.

When the letters trip people up

Two honest warnings from the wild. First, case is meaningless — a9 and A9 are identical — but mixed conventions across tools make people think it matters. Second, a hex string that happens to contain only digits, like 100, is ambiguous without context: decimal one hundred or hex 256? That's the entire job of the 0x prefix and the # in CSS — read them as "hex ahead" and the ambiguity disappears.

Try it yourself

All browser-based, nothing uploaded:

• Hex to Text — decode hex dumps into readable text instantly.
• Text to Hex — see any string as the bytes it really is.
• Hex to RGB — split color codes into their red, green, and blue bytes.
• Binary to Text — the longhand version, for when you want to see every bit.

Conclusion

Hexadecimal is an odometer with sixteen positions per wheel: 0–9, then A–F, then roll over. One digit is four bits, two digits are a byte, and that exact fit is why programmers write bytes in hex everywhere from CSS colors to hash digests. Learn to read pairs — FF is 255, 48 is H — and hex dumps, color codes, and 0x-prefixed mysteries all become plain text you happen to know how to skim.