Base64 Decoder & Encoder

What is Base64?

// Encode plain text to Base64
const encoded = btoa('Hello, World!');
console.log(encoded); // → 'SGVsbG8sIFdvcmxkIQ=='

// Decode Base64 back to text
const decoded = atob('SGVsbG8sIFdvcmxkIQ==');
console.log(decoded); // → 'Hello, World!'

// UTF-8 safe encode (handles Chinese, emoji, any Unicode)
function encodeBase64(str) {
  const bytes = new TextEncoder().encode(str);  // to UTF-8 bytes
  return btoa(String.fromCharCode(...bytes));
}
console.log(encodeBase64('你好')); // → '5L2g5aW9'

Key Features

Examples

Hello, World!

SGVsbG8sIFdvcmxkIQ==

你好世界 🌍

5L2g5aW95LiW55WMIPCfjI0=

{"user":"alice","role":"admin"}

eyJ1c2VyIjoiYWxpY2UiLCJyb2xlIjoiYWRtaW4ifQ==

Line 1
Line 2
Line 3

TGluZSAxCkxpbmUgMgpMaW5lIDM=

How to Use

1. 1

   Enter Text or Base64

   Type or paste plain text in the left area to encode, or paste a Base64 string in the right area to decode. Conversion is instant and bidirectional.

2. 2

   See Real-Time Results

   The other area updates automatically as you type. Any errors (like invalid Base64) are shown immediately.

3. 3

   Copy the Result

   Click the Copy button on either side to copy the encoded or decoded result to your clipboard.

Common Use Cases

Data URI Generation

Encode images or files to Base64 for embedding directly in HTML or CSS as data URIs.

JWT Debugging

Decode Base64-encoded JWT token headers and payloads to inspect claims and expiration times.

API Development

Encode binary data for transmission in JSON APIs, or decode Base64 responses for inspection.

Email Encoding

Encode or decode MIME-encoded email content and attachments.

Certificate Inspection

Decode PEM-encoded certificates and keys to examine their binary content.

Configuration Embedding

Encode configuration data or secrets for safe embedding in environment variables or config files.

Technical Details

RFC 4648 Compliant

Implements standard Base64 encoding as defined in RFC 4648, using the A-Z, a-z, 0-9, +, / alphabet with = padding.

UTF-8 Via TextEncoder

Uses the Web API TextEncoder to convert Unicode strings to UTF-8 bytes before encoding, ensuring correct handling of all scripts and emoji.

Browser-Based Processing

All encoding uses native btoa()/atob() with TextEncoder/TextDecoder — no external libraries, no server calls.

Best Practices

Don't Use Base64 for Security

Base64 is encoding, not encryption. Never use it to 'protect' passwords, tokens, or sensitive data — always use proper encryption.

Watch the Size Increase

Base64 adds ~33% overhead. For large files, consider binary transfer instead of Base64 encoding.

Use UTF-8 Encoding

Always encode text to UTF-8 bytes before Base64 encoding to ensure non-ASCII characters (Chinese, emoji) are handled correctly.

Frequently Asked Questions

What is Base64 encoding?

Base64 is a binary-to-text encoding scheme that represents binary data as a string of printable ASCII characters. It converts every 3 bytes of input into 4 ASCII characters from the set A-Z, a-z, 0-9, +, and /. The '=' character is used for padding when the input length is not a multiple of 3. Base64 is defined in RFC 4648 and is widely used in email (MIME), data URIs, JSON Web Tokens (JWT), and HTTP authentication.

Is my data safe when using this tool?

Yes, completely. All encoding and decoding happens locally in your browser using JavaScript's native btoa(), atob(), TextEncoder, and TextDecoder APIs. Your data never leaves your device — there are no server requests, no cookies, no analytics on your input, and no data storage of any kind.

How does this tool handle non-ASCII characters like Chinese or emoji?

This tool first converts the input text to UTF-8 bytes using the TextEncoder API, then Base64-encodes those bytes. This ensures characters outside the ASCII range — including Chinese (你好), Japanese (こんにちは), Arabic, and emoji (🎉) — are encoded correctly. When decoding, the tool reverses the process: Base64 → bytes → UTF-8 text using TextDecoder.

Is Base64 encryption?

No. Base64 is an encoding, not encryption. It does not provide any security — anyone can decode a Base64 string back to the original data instantly. Base64 is designed for data transport (making binary data safe for text-only channels), not for protecting secrets. If you need to protect data, use proper encryption (AES, RSA) before optionally Base64-encoding the result.

Where is Base64 commonly used?

Base64 is used extensively in: (1) Data URIs — embedding images directly in HTML/CSS as 'data:image/png;base64,...', (2) Email — MIME encoding for attachments and non-ASCII content, (3) JWT — JSON Web Tokens encode header and payload as Base64URL, (4) HTTP Basic Auth — credentials are sent as Base64-encoded 'username:password', (5) APIs — transmitting binary data in JSON payloads, (6) Certificates — PEM format wraps DER-encoded certificates in Base64.

What is the difference between Base64 and URL-safe Base64?

Standard Base64 uses '+' and '/' characters, which have special meaning in URLs — + represents a space and / is a path separator. URL-safe Base64 (also defined in RFC 4648) replaces '+' with '-' and '/' with '_', making the output safe for use in URLs and filenames without additional percent encoding. If you need to use standard Base64 in a URL, you can percent-encode it with our URL Encoder. This tool uses standard Base64. To convert to URL-safe, simply replace + with - and / with _ in the output.

Why does Base64 increase data size?

Base64 encodes 3 bytes of input into 4 characters of output, resulting in approximately 33% size increase. This is because Base64 uses only 64 printable ASCII characters (6 bits each) to represent 8-bit bytes. For example, the 13-character string 'Hello, World!' becomes the 20-character Base64 string 'SGVsbG8sIFdvcmxkIQ=='. This trade-off is acceptable because the encoded data is safe to transmit through text-only protocols.

How do I encode a file to Base64?

On macOS or Linux, use the command line: base64 < myfile.png > myfile.b64. On Windows, use PowerShell: [Convert]::ToBase64String([IO.File]::ReadAllBytes('myfile.png')). In JavaScript (Node.js), use fs.readFileSync('myfile.png').toString('base64'). In Python, use import base64; base64.b64encode(open('myfile.png','rb').read()). This browser tool handles text input; for large binary files, command-line tools are more efficient.

Can I use Base64 in HTML and CSS?

Yes. Base64 is commonly used in data URIs to embed small assets directly in HTML or CSS, eliminating extra HTTP requests. In HTML: . In CSS: background-image: url('data:image/svg+xml;base64,PHN2Zy...'). This is ideal for small icons and SVGs (under ~5 KB). For larger files, separate file references are more efficient because Base64 adds 33% size overhead and bypasses browser caching.

What is the maximum input size?

This browser-based tool efficiently handles text up to about 5 MB. For very large files or binary data, consider using command-line tools like 'base64' (available on macOS and Linux) or 'openssl base64'. The Base64 standard itself has no size limit.

What characters are in the Base64 alphabet?

The standard Base64 alphabet (RFC 4648) consists of 64 characters: uppercase letters A-Z (values 0-25), lowercase letters a-z (values 26-51), digits 0-9 (values 52-61), plus sign + (value 62), and forward slash / (value 63). The equals sign = is used for padding. The URL-safe variant (Base64URL) replaces + with - and / with _ to avoid conflicts with URL-reserved characters.

I need to embed a small image in my HTML email template — should I use Base64 data URIs or host the image externally?

For HTML email, Base64 data URIs are actually the recommended approach for small images like logos and icons. Many email clients (Outlook, Gmail) aggressively block externally hosted images by default, requiring users to click "Display images" to see them. Embedding images as Base64 data URIs (data:image/png;base64,...) bypasses this problem entirely — the image is part of the email itself. The tradeoff is email file size: Base64 adds ~33% overhead, so a 10 KB PNG becomes ~13 KB in the email. Keep embedded images under 20 KB for best compatibility. For larger images or backgrounds, hosting externally is more practical. Use this tool to encode your image file's bytes to Base64 for embedding.

Why does my Base64 encoded string have + and / characters that break my URL parameters?

Standard Base64 uses + and / as two of its 64 characters, and both have special meaning in URLs (+ means space, / is a path separator). When you include a standard Base64 string in a URL query parameter without encoding, these characters corrupt the value. The solution is to use URL-safe Base64 (also called Base64URL, defined in RFC 4648), which replaces + with - and / with _. This variant is used in JWT tokens, OAuth flows, and any Base64 value that appears in URLs. To convert standard Base64 to URL-safe, simply replace all + with - and / with _ in the output, and optionally remove the = padding characters.

I'm trying to decode a JWT token — how does Base64URL decoding work and how is it different from standard Base64?

A JWT (JSON Web Token) consists of three parts separated by dots: header.payload.signature. The header and payload are each encoded with Base64URL — not standard Base64. Base64URL differs from standard Base64 in two ways: it uses - instead of + and _ instead of /, and it omits the = padding characters. To decode a JWT manually, split the token by dots, take the first or second segment, replace - with + and _ with /, add = padding if needed to make the length a multiple of 4, then Base64-decode. Most JWT debugging is easier with a dedicated JWT decoder, but understanding the Base64URL encoding helps when implementing token handling in code or debugging raw token values.