Decode Base64 Encoder / Decoder
Free online Base64 decoder. Paste any Base64 string and instantly get the decoded plain text output. Runs entirely in your browser — your data never leaves your machine.
What is Base64 Decoding?
Base64 decoding is the reverse of Base64 encoding — it converts a Base64-encoded string back into its original binary or text representation. Every 4 Base64 characters decode back to 3 bytes of original data, reversing the encoding process exactly.
You will commonly need to decode Base64 when inspecting JWT tokens (the header and payload are Base64url-encoded JSON), reading HTTP Basic Auth credentials from request headers, decoding API tokens embedded in configuration files, or examining email attachments encoded with MIME Base64.
This decoder runs entirely in your browser. Paste any Base64 string to instantly see the decoded text. If the input contains invalid Base64 characters or has incorrect padding, an error message will indicate the problem.
Frequently asked questions
How do I decode a Base64 string?
Paste the Base64-encoded string into the input field. The decoded output appears instantly. If the Base64 string is invalid, an error message will indicate the problem.
What makes a Base64 string invalid?
A Base64 string is invalid if it contains characters outside the Base64 alphabet (A-Z, a-z, 0-9, +, /, =), if the length is not a multiple of 4 (without proper padding), or if the padding characters are in incorrect positions.
Can I decode Base64 from JWTs?
Yes. JWT tokens are three Base64url-encoded segments separated by dots (header.payload.signature). Paste the header or payload segment here to decode the JSON claims. Note that JWT uses URL-safe Base64 (- instead of +, _ instead of /).
What if the decoded output looks like binary data?
If the original encoded data was binary (an image, PDF, or other file), the decoded output will contain non-printable characters. This tool is best suited for decoding text-based Base64 data. Binary decoding typically requires a dedicated file download.
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