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URLs can only contain a limited set of ASCII characters. Any character outside that set — spaces, ampersands, equals signs, accented letters — must be percent-encoded as a % followed by the character's two-digit hex code.
There are two common cases. Query string encoding (encodeURIComponent) encodes everything except letters, digits, and - _ . ! ~ * ' ( ) — it escapes /, :, ?, =, & and #, making it safe to use a value inside a query string without breaking the URL structure. Full URL encoding (encodeURI) preserves those structural characters, so it's suitable for encoding an entire URL that may already contain ? and &.
Space becomes %20, ampersand becomes %26, equals becomes %3D, forward slash becomes %2F, hash becomes %23, and the plus sign becomes %2B. Accented characters like é encode to their UTF-8 byte sequence — %C3%A9. The + sign is sometimes used as a shorthand for space in form submissions, but %20 is the standard percent-encoding form.
Use encode mode when building a URL that contains user-supplied text — a search term, a file name, or any value that might contain spaces or special characters. Use decode mode to read a percent-encoded URL, debug a broken link, or inspect the contents of an encoded query string. The swap button copies the output into the other mode's input and flips the direction, which is useful for round-trip testing.
Percent-encoding makes a string safe to include in a URL by replacing unsafe characters. Base64 encoding converts arbitrary binary data into a text-safe representation using 64 printable characters. They solve different problems: use percent-encoding for URL parameters; use Base64 for embedding images in HTML, encoding binary data in JSON, or transmitting data through text-only channels. Both are reversible and lossless.