How Base64 works and what decoding gives you back
Base64 is a way of writing arbitrary bytes using only 64 safe, printable characters: the letters A to Z and a to z, the digits 0 to 9, plus and slash. Every three bytes of data are regrouped into four characters, which is why encoded text is about a third longer than the original. The equals signs you sometimes see at the end are padding that fills the last group.
Decoding reverses this. The tool reads the characters, rebuilds the bytes, and then interprets those bytes as UTF-8 text so that accented letters and symbols come back correctly. For example, the string SGVsbG8sIHdvcmxkIQ== decodes to "Hello, world!". Base64 is an encoding, not encryption: anyone can decode it, and it offers no secrecy.
When you need to decode a Base64 string
Base64 appears wherever binary or awkward text has to travel through a text-only channel: email attachments, data URIs, basic authentication headers, configuration files, and API responses.
This tool is meant for the text case: you have a string and want to know what it says. If the Base64 stands for a file such as an image or a PDF, the decoded bytes are not readable text and this page will not show them usefully. When the string is one part of a signed token, the JWT Decoder is a better fit, and to produce Base64 yourself use the Base64 Encoder.
How to decode Base64 with this tool
- The "Base64 string" box starts with a sample value so you can see a result straight away.
- Replace the sample with your own Base64 text. Paste it exactly as you received it.
- The "Decoded text" box fills in immediately as you type. There is no Decode button.
- Click Copy to copy the decoded text.
Decoding uses your browser's built-in functions and nothing is submitted by the page.
Why decoding sometimes fails
If the input is not valid, the result box shows "That doesn't look like valid Base64." and Copy is disabled. The most common causes are characters that are not part of the standard alphabet and text that was cut off when copied.
The tool expects the standard alphabet with plus and slash. URL-safe Base64, which uses a hyphen and an underscore instead, is not converted automatically, so swap those characters back (hyphen to plus, underscore to slash) before pasting.
A string can be valid Base64 and still fail here if the underlying bytes are not UTF-8 text, which is typical for images, archives and encrypted data. In that case the page reports the same error even though the Base64 itself is fine.