What Does "0x" Mean in Programming, Crypto, and Everyday Tech?

I've been writing code for a while now, and I still remember the first time I saw 0xFF lurking in someone's JavaScript. I had absolutely no idea what it meant, but I was too embarrassed to ask. Turns out, that little 0x thing trips up way more people than you'd think.

If you've ever copied a hex color code, looked at an Ethereum address, or stumbled across some low-level code and wondered "what the hell does 0x mean?" — you're in good company. Let me break it down.

The Real Answer (Not the Textbook One)

When you see 0x followed by numbers and letters, you're looking at a hexadecimal number. It's basically just a different way to write numbers that computers love and humans can actually read (unlike binary, which looks like digital vomit).

Here's the deal:

• 0xFF = 255 in regular numbers
• 0x10 = 16
• 0xA = 10

The 0x is programmer speak for "this number is in base 16, not base 10."

Hexadecimal in 60 Seconds

Instead of our normal 0-9 digits, hex uses 16 symbols:

0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F

Where A=10, B=11, C=12, D=13, E=14, F=15.

Why? Because each hex digit perfectly represents 4 binary digits. So instead of writing 11111111 in binary, you can write 0xFF in hex. Much cleaner.

Regular Number	Hex	Binary
0	0x0	0000
10	0xA	1010
15	0xF	1111
16	0x10	10000
255	0xFF	11111111

Where You'll Actually See This Stuff

CSS Colors (You've Definitely Seen This)

background: #FF0000; /* That's 0xFF0000 - pure red */
color: #333333;      /* Dark gray */

Memory Addresses (If You Do Low-Level Stuff)

int *ptr = (int *)0x7fff5fbff5ac;  // Pointing to a specific memory spot

Ethereum and Crypto

0x742d35cc6634C0532925a3b8D4C9DB5C72bB04C3  // Your wallet address

JavaScript (And Most Programming Languages)

console.log(0xFF);   // Prints 255
console.log(0x20);   // Prints 32

Assembly Code (For the Brave)

mov eax, 0x42    ; Move 66 into a register

Where This Crazy Convention Came From

The 0x thing started with the C programming language back in the 1970s. Some smart people at Bell Labs decided that 0 would mean "not a regular decimal number" and x would specify "hexadecimal."

It worked so well that basically every programming language since then just copied it. Python, JavaScript, Go, Rust, even Solidity (for Ethereum smart contracts) - they all use 0x.

Try It Right Now

Open up your browser's console (press F12, then click Console) and type:

0xFF
0x10
0xCAFE

Hit enter after each one. You'll see the decimal equivalents pop up.

Or if you're into Python:

print(0xFF)  # 255
print(hex(255))  # '0xff'

The Stuff You'll Probably Encounter

If you're doing web development, you'll mostly see hex in color codes. If you're doing systems programming or working with crypto, you'll see it everywhere - memory addresses, smart contract addresses, transaction hashes, you name it.

Some hex values that show up constantly:

• 0x0 = 0 (null/nothing)
• 0xFF = 255 (max value for 8 bits)
• 0x100 = 256 (common buffer size)
• 0xFFFF = 65535 (max 16-bit value)

Common Questions

Why not just use regular numbers?
Sometimes hex is way more convenient. 0xFF is cleaner than writing 255 when you're thinking about bits and bytes.

Is this the same as CSS colors?
Yep! #FF0000 in CSS is the same as 0xFF0000 - both represent pure red in hex.

Why do crypto addresses use this?
Ethereum addresses are just big hex numbers. Using 0x makes it clear they're not decimal numbers.

The Bottom Line

Every time you see 0x, you're looking at a hex number. It's just a more convenient way to represent certain values, especially when dealing with computer internals, colors, or cryptocurrency.

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