Urlpercent Encoding Calculator

Percent-encoding exists because URI syntax reserves a small alphabet for structure and leaves everything else undefined. RFC 3986 fixes an unreserved set of exactly 66 characters that never need escaping: A-Z, a-z, 0-9, hyphen, period, underscore and tilde. It also defines gen-delims (colon, slash, question mark, hash, square brackets, at sign) and sub-delims (exclamation, dollar, ampersand, apostrophe, parentheses, asterisk, plus, comma, semicolon, equals) which carry syntactic meaning and must be escaped whenever they appear as data rather than as delimiters. Everything outside those sets is first converted to UTF-8 bytes, then each byte is written as a percent sign plus two hexadecimal digits. The specification says producers should emit uppercase hex digits, though decoders must accept either case. The size cost follows directly from UTF-8: an ASCII character expands from one byte to three, a Latin accented character such as e-acute yields two triplets (%C3%A9) and grows from two bytes to six, and an emoji outside the Basic Multilingual Plane needs four triplets or twelve characters. That is why the size-increase figure here can exceed 200 percent on non-Latin text. The two JavaScript functions differ only in their safe list: encodeURI leaves the structural delimiters intact so a whole URL survives, while encodeURIComponent escapes them and is the correct choice for a single query parameter value. The classic failure modes are double encoding, where %20 becomes %2520 and decodes back to the literal text, and confusing the plus-for-space convention of application/x-www-form-urlencoded form bodies with generic URI encoding, where a plus sign is just a plus sign.

Uniform Resource Locators were specified by Tim Berners-Lee in RFC 1738 in December 1994, which already carried the escape rule that unsafe characters must be written as a percent sign and two hex digits, largely because early network software and printed documents mangled spaces and characters outside a conservative ASCII core. The syntax was generalized into Uniform Resource Identifiers in RFC 2396 in 1998 by Berners-Lee, Roy Fielding and Larry Masinter, which introduced the reserved and unreserved terminology still used today. The current standard is RFC 3986, published in January 2005 and designated STD 66, which tightened the rules by fixing the unreserved set, splitting reserved characters into gen-delims and sub-delims, and recommending uppercase hex output for normalization. Internationalization was handled separately. RFC 3987, also published in 2005, defined Internationalized Resource Identifiers that permit Unicode directly and specified the mapping back to percent-encoded URIs through UTF-8, replacing an earlier era in which non-ASCII bytes were encoded in whatever local codepage the client happened to use. On the browser side JavaScript originally shipped only escape, which used a non-UTF-8 scheme and produced the nonstandard %uXXXX form; encodeURI and encodeURIComponent were added in ECMAScript 3 in 1999 to give UTF-8-correct behavior, and escape has been deprecated ever since. The WHATWG URL Standard, maintained as a living document since 2012, later re-specified parsing and per-context encode sets to match what browsers actually do rather than what the RFCs describe.