Reading the contents of a JWT token, inspecting an encoded HTTP header or decoding an image embedded in CSS: Base64 decoding is a daily operation for developers. An online tool makes this operation instantaneous and code-free.
Base64 decoding is the reverse of encoding. The algorithm takes an ASCII string containing only Base64 alphabet characters (A-Z, a-z, 0-9, +, /) and = padding characters, then reconstructs the original bytes. If the string contains invalid characters or the padding is incorrect, decoding fails. Note that Base64url (used in JWTs) replaces + with - and / with _ for URL compatibility.
📐 Formula
Decoded size = (Base64 length × 3/4) - padding
📊 Reference table
| Base64 encoded | Decoded text |
|---|---|
| SGVsbG8= | Hello |
| SGVsbG8gV29ybGQ= | Hello World |
| dXNlcjpwYXNzd29yZA== | user:password |
| eyJhbGciOiJIUzI1NiJ9 | {"alg":"HS256"} |
| VG9vbFNtYXJ0bHk= | ToolSmartly |
💡 Practical examples
Example 1: decode a JWT token
A JWT consists of 3 parts separated by dots. Copy the 2nd part (payload), paste into the Base64 decoder and read the JSON content: sub, iat, exp, roles…
Example 2: read an Authorization header
If you see Authorization: Basic dXNlcjpwYXNzd29yZA==, decode the part after 'Basic' to get the clear-text credentials: user:password.
Example 3: extract an embedded image
A src="data:image/png;base64,iVBORw..." contains an encoded image. Copy the part after the comma, decode to binary and save as a .png file.