What is application/x-www-form-urlencoded?

It is the default content type for HTML form submissions over POST. Each field becomes key=value, pairs are joined with ampersands, spaces become plus signs, and any reserved or non-ASCII character is percent-encoded as a UTF-8 byte sequence.

Why are spaces encoded as plus signs and not %20?

The form-urlencoded rules predate the modern URL spec and define space as the plus sign. A correct decoder must turn plus back into a space first, then percent-decode. In a path segment, by contrast, a space is %20, which is why form encoding differs from generic URL encoding.

Which characters are left unescaped?

Letters, digits, and the characters star, minus, period, and underscore pass through unchanged. Everything else, including the literal plus sign, equals sign, and ampersand inside a value, is percent-encoded so it cannot be confused with a separator.

How is non-ASCII text handled?

Non-ASCII characters are first converted to UTF-8 bytes, then each byte is percent-encoded. For example an accented e becomes %C3%A9. This matches what browsers send for UTF-8 forms.

Is my data uploaded anywhere?

No. All encoding and decoding runs locally in your browser, so form fields and captured request bodies never leave your device.

HTML Form URL Encoding

application/x-www-form-urlencoded is the wire format browsers use when you submit an HTML form. This tool converts key/value pairs into that body and parses a body back into readable pairs, in your browser.

How it works

Form encoding builds key=value&key2=value2:

Each key and value is encoded independently. Letters, digits, and * - . _ pass through.
A space becomes + (this is the historic form-urlencoded rule, not %20).
Every other character is converted to UTF-8 bytes and each byte is percent-encoded as %XX, so =, &, +, and non-ASCII are all safe.
Pairs are joined with & and each pair is key=value.

Decoding reverses this: split on &, then on the first =, turn every + back into a space, then percent-decode each %XX byte and UTF-8 decode the result.

Tips and examples

Given the pairs:

name=Ada Lovelace
note=1 + 1 = 2

the encoded body is name=Ada+Lovelace&note=1+%2B+1+%3D+2. Note how the literal + and = inside the note value are escaped as %2B and %3D so they are not mistaken for a space or a separator. When decoding, the + between words is restored to a space before any percent-decoding happens — getting that order wrong is the classic form-decoding bug.