What is percent-encoding?

Percent-encoding, also called URL encoding, replaces characters that aren't allowed in a URL with a percent sign followed by two hexadecimal digits, such as %20 for a space.

Before and after:

coffee beans & tea   →   coffee%20beans%20%26%20tea
кафе                 →   %D0%BA%D0%B0%D1%84%D0%B5

Characters

What gets encoded.

Space

%20, or + inside a query string.

Ampersand &

%26, because it separates parameters.

Equals =

%3D, because it separates name and value.

Question mark ?

%3F inside a value, because it starts the query.

Hash #

%23, because it starts the fragment.

Non-ASCII letters

Encoded as their UTF-8 bytes, one %XX for each byte.

When

Encode values, not the whole URL.

Encode each parameter name and value before joining them. Encoding a whole URL also encodes the colons and slashes that make it a URL. The query string builder and the URL encoder handle this correctly.

FAQ

Common questions

Is a space %20 or +? +

Both work inside a query string. %20 works everywhere in a URL, + only in the query.

Why do browsers show some URLs unencoded? +

Address bars decode them for readability. The encoded form is what is sent.

Does encoding change where a URL points? +

No. It only changes how characters are written. Servers decode them before use.

What is double encoding? +

Encoding an already encoded value, so %20 becomes %2520. It is a common bug and usually breaks the link.