URL Decoder

Overview

Decodes percent-encoded strings — %20, %C3%A9 and friends — back into readable text, and percent-encodes in the other direction. Handles the two conventions that coexist on the web: strict RFC-style decoding where + is a literal plus, and the HTML form convention where + means a space, switchable with one checkbox. Malformed input produces a clear error instead of silent garbage.

How It Works

Choose Encode or Decode at the top of the panel; the tool pre-selects Decode when you arrive at this page. Paste into the left box and the result appears live on the right. In decode mode a checkbox appears: Treat + as space (form-urlencoded). Leave it off for strict percent-decoding, where a plus stays a plus; switch it on when the input is a query string or form body built by the application/x-www-form-urlencoded convention. Invalid escapes do not fail silently — the output shows an explicit error naming the cause.

Step-by-Step Usage Guide

1. Paste the encoded string into the input box; decode mode is pre-selected.
2. If the input comes from an HTML form or a query string with + characters, tick Treat + as space.
3. Read the output; an explicit error means the input was malformed — look for a lone % or a truncated escape.
4. If the result still contains % sequences, it was encoded twice; run the decode again.

Technical Specifications & Standards

Percent-encoding is a byte-level convention: characters that are illegal or ambiguous in a URL component are replaced by % followed by the two-digit hexadecimal value of each UTF-8 byte. É is two bytes (0xC3 0xA9), so it becomes %C3%A9 — verified here — and a space becomes %20, never +, in this convention. The encode side uses encodeURIComponent, which leaves unreserved characters (A–Z, a–z, 0–9, hyphen, underscore, period) plus a small allowed set — !, ', (, ), ~ and * — bare and escapes everything else; it encodes space as %20, so an encoded value round-trips byte-exactly. Decoding is where tools betray their users, in three distinct ways. First, strictness: decodeURIComponent throws a URIError on malformed input — a trailing lone percent in 100%, a truncated %2, or % followed by non-hexadecimal %zz all throw, and this tool surfaces that as an error message rather than passing the damage through, because a failed decode almost always means the string was double-encoded or truncated upstream. Second, the plus: RFC-style percent-encoding treats + as a reserved, literal plus sign, but the HTML form convention — what a browser does when it submits a form and what URLSearchParams implements — encodes a space as +. Feeding form output to a strict decoder leaves the plus signs in: verified, a+b decodes to a+b strictly and to a b with the form convention. The checkbox exists because both behaviours are correct in their own context and wrong in the other's. Third, layering: encoding twice is a classic bug — a space encoded to %20 and then encoded again becomes %2520, because the percent itself is a character that must be encoded. Strict decoding of %2520 yields %20, revealing the extra layer; if your decoded output still contains percent escapes, decode again.

Targeted Use Cases

• Reading the q= parameter of a URL someone pasted into a bug report.
• Decoding log lines and webhook payloads where values arrive percent-encoded.
• Debugging double-encoded links — the %25 signature — before fixing the encoder that produced them.
• Checking whether a + in a query value means a space (form convention) or a literal plus (strict convention).

Notes & Gotchas

• Decode exactly once per encoding layer; if output still shows %XX sequences, there was another layer.
• Never guess the plus convention from the data — decide by source: form bodies and query strings use + for space; path segments use %20.
• Treat a URIError as information: a lone % or truncated escape means the string was cut or corrupted before it reached you.
• For values you will re-encode, decode first, edit the plain text, then encode once — editing encoded text is how double-encoding starts.

Frequently Asked Questions