ASCII to Base64 Converter

What Is ASCII to Base64 Conversion?

ASCII (American Standard Code for Information Interchange) assigns 128 numeric values (0-127) to characters including letters, digits, punctuation, and control codes. Base64 encoding converts these ASCII byte values into a 64-character text representation defined in RFC 4648. ASCII text containing only printable characters (code points 32-126) can be encoded directly using the JavaScript btoa() function.

The conversion is straightforward because each ASCII character occupies exactly 1 byte. Three ASCII characters (3 bytes, 24 bits) map to 4 Base64 characters (4 groups of 6 bits). This produces a consistent 33% size increase. The process is fully reversible: decoding the Base64 string with atob() restores the original ASCII text byte-for-byte. For a deeper understanding of how Base64 works, see the encoding guide.

What Is the ASCII Character Set?

The ASCII standard defines 128 characters, each assigned a numeric value from 0 to 127. These characters divide into control characters (0-31, 127) and printable characters (32-126). Every ASCII character occupies 1 byte in memory and is directly compatible with UTF-8 encoding.

All 128 ASCII values can be Base64 encoded. The Base64 character table shows how the 64 output characters map to 6-bit index values during the encoding process.

How Does ASCII Map to Base64?

Each ASCII character occupies 1 byte (8 bits, with only 7 bits used for the value). The Base64 algorithm groups 3 input bytes (24 bits) into 4 output characters (4 groups of 6 bits each). Each 6-bit group maps to one character from the Base64 alphabet (A-Z, a-z, 0-9, +, /).

Example: encoding "ABC"

The 3 ASCII characters "ABC" (3 bytes) become the 4 Base64 characters "QUJD". This 3-to-4 ratio is constant for all input data. For a complete walkthrough of each algorithm step, see the Base64 algorithm explanation.

What Is the Difference Between ASCII, UTF-8, and Base64?

ASCII, UTF-8, and Base64 serve different purposes in text and data representation. ASCII defines a character set. UTF-8 extends that character set to cover all Unicode code points. Base64 converts binary data (including text bytes) into a text-safe format for transport.

When encoding ASCII text to Base64, the bytes are identical whether interpreted as ASCII or UTF-8. The distinction matters only for characters above code point 127. The Base64 text encoder handles both ASCII and UTF-8 input automatically.

How Do You Handle Non-ASCII Characters?

Characters above code point 127, such as accented letters (e, n), CJK characters, and emoji, are not part of the ASCII standard. These characters require UTF-8 encoding, which represents them as multi-byte sequences (2-4 bytes per character). The JavaScript btoa() function only accepts single-byte characters (0-255) and throws an error for multi-byte input.

To Base64 encode non-ASCII text in JavaScript, first convert to UTF-8 bytes, then apply Base64 encoding:

// Encode non-ASCII text to Base64
const encoded = btoa(unescape(encodeURIComponent(text)));

// Decode Base64 back to non-ASCII text
const decoded = decodeURIComponent(escape(atob(encoded)));

The encodeURIComponent() function converts Unicode characters to percent-encoded UTF-8 bytes. The unescape() function converts those percent-encoded bytes to single-byte characters that btoa() can process. For encoding text that may contain non-ASCII characters, use the Base64 text encoder, which handles this conversion automatically. For language-specific implementations, see the JavaScript Base64 guide or Python Base64 guide.

Frequently Asked Questions