How To Encode Text To Base64

Base64 is a way of writing binary data using only printable characters. It takes groups of three bytes, which is twenty-four bits, and splits them into four groups of six bits. Each six-bit value maps to one of sixty-four symbols.

The sixty-four symbols are the twenty-six upper-case letters, the twenty-six lower-case letters, the ten digits, and the plus and slash characters. That is exactly sixty-four, which is why each one can be chosen by a six-bit number.

The encoding grows the data by about a third. Three bytes become four characters, so the output is four thirds of the input size. A one-megabyte file becomes about 1.33 megabytes once encoded.

Three bytes is the natural group size because it divides evenly into six-bit chunks. When the input is not a multiple of three bytes, the final group is padded and the output is completed with one or two equals signs so its length is always a multiple of four.

Those equals signs are padding, not data. One equals sign means the last group held two bytes, and two equals signs mean it held one. A decoder can strip and recreate the padding, which is why the URL-safe variant drops it.

The URL-safe variant replaces plus with hyphen and slash with underscore, and usually removes the padding. Plus and slash are meaningful in URLs and in file names, so the standard alphabet is awkward in those contexts. This variant is often called base64url.

Text is encoded to bytes before it is encoded to Base64. The text is first written out as UTF-8, producing a byte sequence, and only then is that sequence turned into Base64. This is why a short piece of text with an accent can produce more Base64 characters than a plain one of the same length.

Base64 is not encryption. There is no key, the transformation is reversible by anyone, and it hides nothing. It is an encoding, and the distinction matters because treating it as security is a real and common mistake.

The encoding exists because many systems carry text reliably but corrupt raw binary. Email is the classic case: the MIME standard uses Base64 so that attachments can travel through a text-only protocol without their bytes being altered.

Base64 appears in many other places. Data URIs embed small images and fonts directly in a stylesheet, JSON Web Tokens carry their header and payload as base64url, HTTP basic authentication sends a user name and password in Base64, and many APIs accept binary payloads as a Base64 string in a JSON field.

Decoding can fail. A string that is not a multiple of four characters long, or that contains a character outside the alphabet, is not valid Base64 and a strict decoder will reject it. Lenient decoders strip whitespace and repair the padding, which is why some strings decode in one tool and not another.

The line breaks in older Base64 output are a historical artefact. The MIME standard required a line break every seventy-six characters, and many encoders still insert them. Whitespace is not part of the encoding and should be removed before decoding.

The calculator encodes text to Base64 or decodes it back, with a toggle for the URL-safe alphabet. It reports the UTF-8 byte length of the input and the character length of the output, so the size overhead is visible rather than implied.