What is the difference between standard base64 and URL-safe base64?

In standard base64, '+' (62) and '/' (63) can break URLs and filename queries. RFC 4648 defines URL-safe base64 by replacing '+' with '-' (minus) and '/' with '_' (underscore), and optionally omitting padding '='.

Why does base64 encoding increase data size by 33%?

Base64 takes 3 bytes of binary data (24 bits) and divides them into 4 groups of 6 bits. Each 6-bit chunk maps to one ASCII character. Because 3 bytes become 4 bytes, the encoded data is exactly 33.3% larger.

RFC 4648 Specification & Live Converter

Base64 Char Set & Encoding Table

Q: What characters make up the standard base64 char set?

The standard base64 char set consists of 64 distinct characters: uppercase letters A-Z (indices 0-25), lowercase letters a-z (indices 26-51), numeric digits 0-9 (indices 52-61), and two punctuation symbols '+' (index 62) and '/' (index 63). The equals sign '=' serves as padding.

Interactive exploration of the 64-character index table. Convert text, analyze bitwise 6-bit representations, compare base64 standard vs URL-safe alphabets, and inspect padding mechanics.

Live Base64 Studio & Bitwise Inspector

URL-Safe (- / _) Omit '=' Padding

Raw Text / Plaintext (UTF-8) 0 bytes

Base64 Encoded String 0 chars

Bitwise 24-bit to 4×6-bit Conversion Flow (First 3 Bytes)

3 Bytes (24 Bits) $\rightarrow$ 4 Base64 Chars (4 × 6 Bits)

Complete Base64 Character Set (Indices 0–63)

Standard RFC 4648 index, binary 6-bit pattern, ASCII value, and URL-safe substitution.

Padding Character ('='): The equals sign is not part of the 64-index value table. It is exclusively an alignment padding character appended when input byte streams are not evenly divisible by 3. If 1 byte remains, 2 padding chars ('==') are added; if 2 bytes remain, 1 padding char ('=') is added.

How Base64 Character Set Encoding Works (RFC 4648)

Base64 is a binary-to-text encoding scheme that translates raw arbitrary 8-bit bytes into a restricted 64-character subset of ASCII printable characters.

Group 3 Bytes (24 Bits)

Input stream is processed in 3-byte blocks ($3 \times 8 = 24$ bits). For example, "Man" converts to ASCII [77, 97, 110] or 24 contiguous binary bits: 01001101 01100001 01101110.

Split into 4x6 Bits

The 24 bits are divided into 4 groups of 6 bits each ($2^6 = 64$ possibilities). "Man" yields 010011 (19), 010110 (22), 000101 (5), and 101110 (46).

Char Set Lookup

Each 6-bit integer indexes into the base64 char set: 19 = 'T', 22 = 'W', 5 = 'F', 46 = 'u'. Resulting base64 string: TWFu.

Feature	Standard Base64 (RFC 4648 §4)	URL & Filename Safe (RFC 4648 §5)
Index 62	+ (Plus)	- (Minus / Dash)
Index 63	/ (Forward Slash)	_ (Underscore)
Padding '='	Mandatory for standard conformity	Optional / frequently omitted in web JWTs
Common Applications	MIME emails, Data URIs, Basic Auth	OAuth tokens, JWT tokens, Web safe query parameters

Frequently Asked Questions

Common technical questions regarding the base64 character set and encoding specifications.

What characters make up the standard base64 char set?

The standard base64 char set consists of 64 characters: A-Z (indices 0 to 25), a-z (indices 26 to 51), 0-9 (indices 52 to 61), '+' (index 62), and '/' (index 63). The '=' character is reserved for padding at the end of streams.

Why can't I use standard base64 directly inside URLs?

Inside standard base64, '+' often represents a space in URL query strings, and '/' is the universal URL path delimiter. If passed unescaped, web servers will misinterpret the URL. URL-safe base64 solves this by replacing '+' with '-' and '/' with '_'.

Is base64 a form of encryption?

No. Base64 is an encoding format, not encryption. It provides zero confidentiality because anyone can decode base64 without a secret key or password. It is intended solely to transmit binary data over media designed to handle text.