Free Base64 Encoder & Decoder Online

What is Base64 encoding?

Base64 is an encoding scheme that converts binary data into a string of 64 printable ASCII characters: A–Z, a–z, 0–9, + and /. It is defined in RFC 4648.

Base64 is not encryption — it is a reversible encoding. Anyone can decode a Base64 string instantly. Its purpose is to safely transmit binary data through text-only channels that can't handle raw bytes — like email (MIME), HTML attributes, or JSON payloads.

The encoded output is approximately 33% larger than the original input, because every 3 bytes of binary become 4 Base64 characters.

How to encode and decode Base64

1. 1
   Select Encode or Decode

   Choose Encode to convert plain text into a Base64 string. Choose Decode to convert a Base64 string back to plain text.

2. 2
   Enter your input

   Paste or type your text in the input field. The tool supports UTF-8 text including accented characters, emoji, and non-Latin scripts.

3. 3
   Use Swap to chain operations

   Click Swap to instantly flip the mode and move the output to the input. Useful when you want to encode, check the result, then decode it back.

4. 4
   Copy the output

   Click Copy to grab the result. Everything runs locally — your text never leaves the browser.

Common uses of Base64

How Base64 encoding works

Base64 works by taking 3 bytes of binary input (24 bits) and splitting them into four 6-bit groups. Each 6-bit group maps to one of 64 characters in the Base64 alphabet. Because 3 input bytes produce 4 output characters, the encoded output is always exactly 33% larger than the original.

When the input length is not a multiple of 3, padding characters (=) are appended to make the output length a multiple of 4. One byte of remainder adds two characters plus ==; two bytes of remainder add three characters plus =.

This tool handles UTF-8 text correctly — multi-byte characters like accented letters, Chinese characters, or emoji are first encoded to their UTF-8 byte sequence, then Base64-encoded. This is the correct behavior for web applications and matches how btoa(unescape(encodeURIComponent(str))) works in JavaScript.

Base64 variants — Standard, URL-safe, and MIME

Not all Base64 is identical. Different contexts require different alphabets and line-break rules:

Base64 in web development — practical examples

These are the most common patterns you will encounter as a web developer:

// Encode "username:password" in Base64
const credentials = btoa('user:password');
fetch(url, { headers: { Authorization: `Basic ${credentials}` } });

/* In your stylesheet */
.icon { background-image: url("data:image/png;base64,iVBORw0KGgo..."); }

// JWT header.payload.signature — split on '.' and decode the middle part
const payload = JSON.parse(atob(token.split('.')[1]));
console.log(payload.sub, payload.exp);

Base64 in web development — data URIs and email

Base64 encoding appears in several specific contexts in web development. Understanding each one helps you use it correctly and avoid common mistakes:

Data URIs for inline assets. A data URI embeds file content directly in a URL string: data:image/png;base64,iVBORw0KGgo.... This lets you embed small images, fonts, or SVGs directly in CSS or HTML without an extra HTTP request. The tradeoff is file size: Base64 increases the asset size by ~33%, and data URIs cannot be cached separately by the browser. Use them only for very small assets (icons under 1 KB) where the saved HTTP request outweighs the size penalty.

Email attachments and MIME encoding. SMTP (email) was originally designed for ASCII-only text. To send binary files (images, PDFs, documents) over email, MIME encodes them as Base64 within a multipart message body. The Content-Transfer-Encoding: base64 header signals this encoding. Email libraries like Nodemailer, SendGrid, and Mailgun handle this automatically — you rarely need to encode attachments yourself.

HTTP Basic Authentication. The HTTP Authorization header for Basic Auth carries credentials as Base64: Authorization: Basic dXNlcjpwYXNzd29yZA==. This is the Base64 encoding of user:password. This provides no security beyond HTTPS — anyone can decode it. Always use Basic Auth over HTTPS only, and prefer token-based authentication for production APIs.

JSON Web Tokens (JWT) headers and payloads. Both the header and payload sections of a JWT are Base64url-encoded JSON objects (URL-safe Base64 without padding). This means the contents are readable by anyone — paste the token on jwt.io to decode it instantly. The security comes entirely from the signature section, not from the encoding. Never put sensitive data (passwords, SSNs, payment info) in a JWT payload.

Base64 encoding at the command line

Most operating systems include a built-in Base64 utility. These commands are useful for quick encoding in scripts, CI/CD pipelines, and server-side tasks:

Note: the -n flag in the echo command prevents a trailing newline from being included in the encoded output. Without it, the result will include the newline character and produce unexpected padding.