Base64 Decode

Related

What is Base64 Decoding?

Use Cases

• Decode Base64-encoded API responses and JWT tokens to view the original JSON or text content
• Restore readable text from Base64 fields found in server logs, error stacks, or database exports
• Recover configuration values and credentials stored in Base64 format during system migrations
• Troubleshoot and fix decoding errors caused by missing padding, URL-Safe characters, or line breaks
• Decode Data URIs to inspect or save embedded images, fonts, or CSS assets
• Decode HTTP Basic Authentication header values (Basic dXNlcjpwYXNz) to inspect the username:password pair

How to Use

1. Paste the Base64 string you want to decode into the input box
2. Pick the charset — UTF-8 (default), ASCII, or ISO-8859-1 — that matches the original encoding
3. The tool auto-detects the variant (Standard, URL-Safe, or MIME) and decodes the string in real time
4. Copy the decoded result to your clipboard, or download it as a text / binary file

Features

• Live decode with debounce: paste a Base64 string and the result appears in 300ms without manually clicking a button
• Multi-charset support: UTF-8 (for CJK / emoji / accented text), ASCII, and ISO-8859-1 cover the vast majority of real-world payloads
• URL-Safe Base64 compatible: automatically normalises - and _ back to + and /, and re-adds missing = padding
• Detailed error localisation: when decoding fails, the tool reports the exact position of the bad character so you can fix the source quickly
• Auto-detect format: instantly recognises MIME line breaks, URL-Safe variants, and missing padding without manual toggling
• Binary file decode: switch the output to binary to download decoded images, PDFs, or other file attachments directly
• One-click copy and download: copy the decoded text to the clipboard, or download it as a .txt / .bin file

Code Examples

// Standard decode (supports UTF-8)
function b64Decode(str) {
  return decodeURIComponent(escape(atob(str)));
}

// URL-Safe decode (for JWT / OAuth)
function b64UrlDecode(str) {
  // Replace URL-safe chars back to standard
  str = str.replace(/-/g, '+').replace(/_/g, '/');
  // Add missing padding
  while (str.length % 4) str += '=';
  return decodeURIComponent(escape(atob(str)));
}

console.log(b64Decode('SGVsbG8g5LiW55WM'));   // Hello 世界
console.log(b64UrlDecode('SGVsbG8g5LiW55WM')); // Hello 世界

// Decode a JWT payload (Base64URL)
function decodeJwtPayload(token) {
  const payload = token.split('.')[1];
  return JSON.parse(b64UrlDecode(payload));
}

import base64

# Standard decode (UTF-8)
decoded = base64.b64decode('SGVsbG8g5LiW55WM').decode('utf-8')
print(decoded)  # Hello 世界

# Standard decode with auto-padding
def b64_decode_auto(s: str) -> bytes:
    s = s.strip()
    # Add missing padding for URL-Safe variants
    s += '=' * (-len(s) % 4)
    return base64.b64decode(s)

print(b64_decode_auto('SGVsbG8g5LiW55WM').decode('utf-8'))  # Hello 世界

# URL-Safe decode (for JWT / OAuth)
url_decoded = base64.urlsafe_b64decode('SGVsbG8g5LiW55WM' + '=' * (-len('SGVsbG8g5LiW55WM') % 4))
print(url_decoded.decode('utf-8'))  # Hello 世界

# Decode a JWT payload
import json
def decode_jwt_payload(token: str) -> dict:
    payload = token.split('.')[1]
    payload += '=' * (-len(payload) % 4)
    return json.loads(base64.urlsafe_b64decode(payload))

print(decode_jwt_payload('eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiIxMjM0In0.abc'))

import java.nio.charset.StandardCharsets;
import java.util.Base64;

public class Base64DecodeExample {
    public static void main(String[] args) {
        String encoded = "SGVsbG8g5LiW55WM";

        // Standard decode
        String decoded = new String(
            Base64.getDecoder().decode(encoded), StandardCharsets.UTF_8);
        System.out.println(decoded); // Hello 世界

        // URL-Safe decode (auto handles missing padding)
        String urlSafe = new String(
            Base64.getUrlDecoder().decode(encoded), StandardCharsets.UTF_8);
        System.out.println(urlSafe);

        // MIME decode (handles 76-char line wrapping)
        String mimeEncoded = "SGVsbG8g5LiW55WM\n";
        String mimeDecoded = new String(
            Base64.getMimeDecoder().decode(mimeEncoded), StandardCharsets.UTF_8);
        System.out.println(mimeDecoded);
    }
}

FAQ

Glossary

Base64

A binary-to-text encoding scheme that uses 64 printable ASCII characters (A-Z, a-z, 0-9, +, /) plus the = padding character. Defined in RFC 4648, it lets binary data travel safely through text-only systems such as JSON, XML, and email.Base64 Encoder

RFC 4648

The current IETF standard for Base16, Base32, and Base64 encodings. Section 4 defines Standard Base64, and Section 5 defines the URL-Safe variant. It supersedes the older RFC 2045 and RFC 3548.URL-Safe Base64 Tool

Padding

The = character used at the end of a Base64 string to make the output length a multiple of 4. Standard Base64 always pads; Base64URL typically omits padding. The decoder auto-fills missing = when needed.Base64 Padding Tool

atob

The browser's native Base64 decode API (ASCII-to-binary). Returns a binary string where each character holds one byte. Only supports Latin-1 — multibyte UTF-8 text must be transcoded via decodeURIComponent / TextDecoder first.MDN atob()

btoa

The browser's native Base64 encode API (binary-to-ASCII), the counterpart to atob. Same Latin-1 limitation: encoding UTF-8 text requires pre-encoding with encodeURIComponent / TextEncoder.MDN btoa()

MIME

Multipurpose Internet Mail Extensions — the standard (RFC 2045) that originally introduced Base64 as a content transfer encoding for email. MIME Base64 wraps output to 76 characters per line (CRLF).MIME Base64 Tool

Base64URL (URL-Safe Base64)

The URL-friendly Base64 variant defined in RFC 4648 §5. Replaces + with - and / with _, and usually drops = padding. Used in JWT, OAuth PKCE, URL query parameters, and filenames.JWT Tool

InvalidCharacterError

The error thrown when a Base64 string contains characters outside the allowed alphabet (for example spaces, newlines, quotes, or -/_ in Standard mode). The decoder strips whitespace and offers URL-Safe mode to handle these cases automatically.URL-Safe Base64 Tool

Base64 Variant Comparison

Common Base64 Decode Errors

Authoritative References

• ReferenceRFC 4648 - The Base16, Base32, and Base64 Data Encodings
• ReferenceRFC 2045 - MIME Part One: Format of Internet Message Bodies
• ReferenceRFC 7519 - JSON Web Token (JWT)
• ReferenceMDN Web Docs - atob()
• ReferenceMDN Web Docs - btoa()
• ReferenceMDN Web Docs - TextDecoder
• ReferenceWikipedia - Base64