Base64 Encoder & Decoder

What Is Base64 Encoding?

Base64 is a binary-to-text encoding scheme that converts binary data into a set of 64 printable ASCII characters. It is not encryption or compression — it is a way to safely represent binary data (images, files, audio) using only letters, digits, and two special characters.

The name comes from the fact that it uses 64 possible characters (A-Z, a-z, 0-9, +, /) to represent data. Each Base64 character represents exactly 6 bits of the original binary data, which is why the encoded output is always about 33% larger than the original input.

Base64 is defined in RFC 4648 and has become the standard way to embed binary data in text-based formats like JSON, XML, HTML, email messages, and URL query strings.

The Problem Base64 Solves

Many systems — email protocols, JSON APIs, HTML documents, URL query strings — are designed to handle text, not raw binary. Binary data contains bytes that can be interpreted as control characters, line breaks, or in some encodings, byte values that don't exist.

For example, the byte value 0x00 (null) would terminate a C string. The byte 0x0A (newline) embedded in a JSON value would break the format. SMTP (email) was historically designed for 7-bit ASCII, making any byte with the 8th bit set unreliably transmitted.

Base64 solves this by mapping every 3 bytes of binary data into 4 safe, printable ASCII characters that survive any text-based transport without corruption. When you send an image attachment via email, embed a PNG in an HTML data URI, or transmit a file through a JSON API, Base64 is working behind the scenes.

How Base64 Encoding Works — Algorithm Walkthrough

The encoding algorithm processes the input data in groups of 3 bytes at a time. Each 3-byte group (24 bits) is split into four 6-bit values, and each 6-bit value is mapped to a character in the Base64 alphabet.

Step-by-Step Example: Encoding “Cat”

Let's trace through the encoding of the word “Cat” (3 bytes, one complete chunk) to see exactly how this works.

The 3-byte input “Cat” became 4 Base64 characters: Q2F0. This 4:3 ratio means Base64 output is always ~33% larger than the raw binary input.

The Base64 Character Table

Each 6-bit value (0–63) maps to exactly one character. Here is the complete mapping:

The = sign is not part of the 64-character alphabet. It is used solely as padding at the end of the encoded string when the input is not evenly divisible by 3 bytes.

Base64 Padding Explained

What happens when your input is not a perfect multiple of 3 bytes? The standard requires the output length to be a multiple of 4 characters, so padding with = fills the gap.

The rule is simple:

• 1 remaining byte → add == (2 padding characters)
• 2 remaining bytes → add = (1 padding character)
• 0 remaining bytes (exact multiple of 3) → no padding needed

Many URL-safe Base64 implementations (like JWT) omit padding entirely since the decoder can infer the original length from the encoded length alone. When padding is removed, you can always restore it: add trailing = until the length is a multiple of 4.

Base64 Variants: Standard vs URL-Safe vs MIME

Not all Base64 is the same. Different contexts use slightly different alphabets and formatting rules:

Where You Encounter Base64 Every Day

Base64 is everywhere in modern computing. Here are the most common places you'll encounter it — often without realizing:

• HTML Data URIs: Embedded images, fonts, and SVGs in CSS and HTML. Example: <img src="data:image/png;base64,iVBORw..." />
• Email Attachments: Every image, PDF, or ZIP you send via email is Base64-encoded by your mail client before transit. The MIME standard requires it.
• JWT Tokens: JSON Web Tokens use Base64URL to encode header and payload parts. The familiar eyJhbGciOi... pattern is Base64URL.
• HTTP Basic Auth: The Authorization: Basic ... header carries Base64-encoded credentials: btoa("user:pass")
• JSON APIs: When you need to transmit binary file content through a text-only JSON REST or GraphQL endpoint, Base64 strings are the go-to format.
• OAuth 2.0 & OpenID Connect: Client credentials, assertion payloads, and state parameters often use Base64URL encoding for safe URL transit.
• Kubernetes Secrets: Values in Kubernetes Secret manifests are Base64-encoded: echo -n "mypassword" | base64
• CSS background-images: Small icons and patterns are often inlined as Base64 data URIs to save HTTP requests.

Quick Reference: Encode and Decode in Any Language

Every modern programming environment includes built-in Base64 support. Here are the one-liners:

// JavaScript (Browser)
const encoded = btoa("Hello");       // "SGVsbG8="
const decoded = atob("SGVsbG8=");    // "Hello"

// Python
import base64
encoded = base64.b64encode(b"Hello").decode()  # SGVsbG8=
decoded = base64.b64decode(encoded)           # b"Hello"

// Java
String encoded = Base64.getEncoder().encodeToString("Hello".getBytes());
String decoded = new String(Base64.getDecoder().decode(encoded));

// C#
string encoded = Convert.ToBase64String(Encoding.UTF8.GetBytes("Hello"));
string decoded = Encoding.UTF8.GetString(Convert.FromBase64String(encoded));

// Go
import "encoding/base64"
encoded := base64.StdEncoding.EncodeToString([]byte("Hello"))
decoded, _ := base64.StdEncoding.DecodeString(encoded)

// Terminal
echo -n "Hello" | base64           # SGVsbG8=
echo -n "SGVsbG8=" | base64 -d     # Hello

Common Base64 Misconceptions

Start Converting Right Now

All our tools run entirely in your browser. Paste a string, upload a file, or type into any tool above — your data is processed locally on your device and never leaves it. Use the tabs at the top to switch between text encoding, image-to-Base64 conversion, and Base64-to-file decoding. Browse the complete collection of 21 tools below for every Base64 task you might need.