Base64 Decode Online

What Is Base64 Decoding?

Base64 decoding is the reverse process of Base64 encoding: it converts a Base64-encoded ASCII string back into the original binary data or text. Every 4 Base64 characters decode to 3 bytes of original data. The decoder looks up each character in the Base64 alphabet, reconstructs the original 6-bit groups, and reassembles them into 8-bit bytes.

Base64-encoded data is recognisable by its use of uppercase and lowercase letters, digits, and either +/ (standard) or -_ (URL-safe), often ending with one or two = padding characters. It appears frequently in JWT tokens, email attachments, data URIs, API responses, and configuration files — anywhere binary or structured data must be embedded in a text-only context.

Why Use This Tool?

This decoder handles both standard and URL-safe Base64, automatically fixes missing padding, and decodes entirely in your browser with no data sent to servers.

How to Use This Base64 Decoder Online

No account, no upload, no configuration — paste a Base64 string and the decoded output appears instantly.

1. 1
   Paste Your Base64 String

   Click the input field and paste any Base64-encoded string — standard (+/) or URL-safe (-_) format. The decoder accepts strings with or without padding (= characters) and handles both variants automatically.
2. 2
   Auto-Detection and Repair

   The decoder automatically detects whether the input is standard or URL-safe Base64 and restores missing padding before decoding. No manual adjustment needed.
3. 3
   Inspect the Output

   The decoded result appears on the right. If the input was valid UTF-8 text, you will see the original string. If it was binary data, the output will show the raw bytes as text — some characters may appear as replacement characters.
4. 4
   Copy or Use the Result

   Click Copy to send the decoded text to your clipboard. Use it to inspect JWT payloads, recover encoded configuration values, or verify what a Base64 string actually contains.

How Decoding Works

Each Base64 character maps to a 6-bit value (0–63). Four consecutive characters provide 24 bits, which decode to 3 bytes of original data. The example below shows how "TWFu" decodes back to "Man":

Understanding Padding

Base64 encoding groups input bytes in sets of 3. When the input length is not divisible by 3, padding (=) characters are added to complete the final group. When decoding, these = characters are stripped and the decoder knows to discard the extra zero-bits that were added during encoding.

Common Use Cases

Common Pitfalls

These are the most frequent causes of Base64 decoding errors in practice:

Code Examples

How to Base64-decode a string in popular languages and environments:

// Standard Base64
const decoded = decodeURIComponent(escape(atob(encoded)))

// URL-safe Base64 (restore padding first)
function decodeUrlSafe(str) {
  const padded = str.replace(/-/g, '+').replace(/_/g, '/')
  const pad = padded.length % 4
  return decodeURIComponent(escape(atob(padded + '='.repeat(pad ? 4 - pad : 0))))
}

// Standard
const decoded = Buffer.from(encoded, 'base64').toString('utf8')

// URL-safe
const decoded = Buffer.from(encoded, 'base64url').toString('utf8')

import base64

# Standard
decoded = base64.b64decode(encoded).decode('utf-8')

# URL-safe (add padding if missing)
padding = '=' * (-len(encoded) % 4)
decoded = base64.urlsafe_b64decode(encoded + padding).decode('utf-8')

# Standard
echo "SGVsbG8sIFdvcmxkIQ==" | base64 -d

# URL-safe (restore + and / first)
echo "SGVsbG8sIFdvcmxkIQ" | tr '-_' '+/' | base64 -d

Base64 Decoder Online vs. Other Tools

Several Base64 decoders exist online. The key differences for daily developer use are privacy, auto-repair, and Unicode handling.

Frequently Asked Questions