How Percent Encoding Works

Percent encoding (also called URL encoding) is the mechanism defined by RFC 3986 for representing characters in a URI that are not allowed in their raw form. Each encoded character is replaced by a triplet consisting of a percent sign (%) followed by two hexadecimal digits representing the character's byte value.

The encoding process:

1. Determine if the character needs encoding (is it outside the unreserved set?)
2. Convert the character to its byte representation using UTF-8
3. For each byte, output % followed by the byte's value in uppercase hexadecimal

Example: The space character (ASCII 32):

• Decimal: 32
• Hexadecimal: 20
• Percent-encoded: %20

Character sets defined by RFC 3986:

Unreserved characters (never need encoding): A-Z a-z 0-9 - _ . ~

Reserved characters (have special meaning, encode when used as data): : / ? # [ ] @ ! $ & ' ( ) * + , ; =

All other characters (must always be encoded): Spaces, control characters, non-ASCII characters, and characters like { } | \ ^ < >`

How multi-byte characters work: Characters outside ASCII are first converted to UTF-8 bytes, then each byte is percent-encoded:

é (e-acute) → UTF-8: 0xC3 0xA9 → %C3%A9
✓ (checkmark) → UTF-8: 0xE2 0x9C 0x93 → %E2%9C%93

Case sensitivity: RFC 3986 states that hexadecimal digits in percent-encoded triplets are case-insensitive (%2f and %2F are equivalent), but uppercase is recommended for consistency and is what JavaScript's encoding functions produce.

Decoding: Percent decoding reverses the process. The % is found, the following two hex digits are read, converted to a byte, and the bytes are assembled into characters using UTF-8. Invalid sequences (like %GG or a truncated %2) should be treated as errors.

Pitfall: A common misconception is that percent encoding is like HTML entity encoding or Base64 encoding. It is not. Percent encoding is URL-specific and only encodes individual bytes. Confusing it with other encoding schemes leads to double-encoding bugs or corrupted data. Always use the URL-specific encoding functions provided by your programming language.