Base64 Encode / Decode
Encode text to Base64 and decode it back, with URL-safe variant support.
About Base64 Encode / Decode
Base64 encodes arbitrary binary data as printable ASCII characters, making it safe to embed in text protocols like JSON, HTML attributes, email (MIME) and HTTP headers. It maps every 3 bytes of input to 4 characters from a 64-character alphabet, resulting in a ≈33% size increase. Standard Base64 (RFC 4648 §4) uses +, / and = padding; URL-safe Base64 (§5) replaces + with -, / with _, and drops padding, making it safe to embed in URLs and JWTs without percent-encoding. This calculator always encodes text as UTF-8 bytes first, which is the only correct way to handle non-ASCII characters like accented letters and emoji.
Formulas
Base64 output length
Output length = ⌈(4/3) × input_bytes⌉ (padded to nearest multiple of 4)
- L_in
- Input length in bytes (UTF-8 encoded)
- L_out
- Output length in Base64 characters (including padding)
Size overhead
Every 3 bytes of input become 4 Base64 characters — a 33% size increase.
Worked example
With the default values below, Base64 Encode / Decode returns Output of SGVsbG8sIFdvcmxkIQ==. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Modeone of Encode (text → Base64), Decode (Base64 → text)
- Encode (text → Base64)
- Alphabetone of Standard (RFC 4648 §4, uses + and /), URL-safe (RFC 4648 §5, uses - and _, no padding)
- Standard (RFC 4648 §4, uses + and /)
- Input Text
- Hello, World!
- Base64 Input
- SGVsbG8sIFdvcmxkIQ==
Results
- OutputEncoded or decoded result.
- SGVsbG8sIFdvcmxkIQ==
- Input Length (bytes / chars)
- 13
- Output Length (bytes / chars)
- 20
- Base64 Size Overhead (%)Base64 adds ~33% to the original size (4 output chars per 3 input bytes).
- 54
Step by step
- 1Encode input to UTF-8 bytes= 13 bytes
- 2Encode each 3-byte group as 4 Base64 charactersAlphabet: Standard (RFC 4648 §4)
- 3Result= SGVsbG8sIFdvcmxkIQ==
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
Why can't I just use btoa() for Unicode strings?
btoa() is a legacy function that only accepts characters in the Latin-1 range (U+0000–U+00FF). Passing a string containing emoji, Chinese characters or any code point above U+00FF causes it to throw or produce wrong output. The correct approach is to first encode the string to UTF-8 bytes (via TextEncoder), then Base64-encode those bytes.
When should I use URL-safe Base64?
Whenever the Base64 output will appear in a URL query parameter, a URL path segment, or a JWT. Standard Base64 uses + and /, which are special characters in URLs and must be percent-encoded. URL-safe Base64 replaces them with - and _ (neither is special in a URL), so no additional encoding is needed.
Can Base64 detect corruption or tampering?
No — it has no checksums or error detection. If a Base64 string has been altered, the decoder will either produce garbage output or throw a 'malformed input' error. For integrity protection, use a hash (SHA-256) or MAC in addition to Base64.
Is Base64 a form of encryption?
No. Base64 is an encoding, not an encryption. It is trivially reversible and provides no confidentiality whatsoever. Anyone who can see the Base64 string can decode it instantly. For confidentiality, encrypt the data with AES or similar before (optionally) Base64-encoding it.