Base64 Encoder & Decoder
Encode text to Base64 or decode Base64 back to plain text — all in your browser, nothing sent to a server.
Frequently Asked Questions
+ and /, which have special meanings in URLs. URL-safe Base64 replaces + with - and / with _, and strips padding = signs. It is used in JWT tokens and OAuth parameters.A–Z, a–z, 0–9, +, /), making it safe to embed in any text format. Common uses include email attachments (MIME), embedding images in CSS/HTML via data URIs, and transmitting binary tokens in HTTP headers.
=) are added to make the output a multiple of 4 characters. The exact formula is: ⌈n / 3⌉ × 4 output characters for n input bytes.
data:[mediatype];base64,[base64data]. For a PNG image in an <img> tag: <img src="data:image/png;base64,iVBORw0KGgo…">. In CSS: background-image: url('data:image/svg+xml;base64,…');. This technique is useful for small icons and inline SVGs but adds payload size and prevents browser caching of the asset separately.
base64 utility: encode with echo -n "hello" | base64, decode with echo "aGVsbG8=" | base64 --decode (use -d on some systems). On Windows PowerShell: [Convert]::FromBase64String("aGVsbG8=") | ForEach-Object { [char]$_ }. With Python: python3 -c "import base64; print(base64.b64decode('aGVsbG8=').decode())". For binary files, pipe the decoded output directly to a file rather than printing to the terminal.
0), uppercase O (O), lowercase l (l), and uppercase I (I) to eliminate confusion when written by hand, while also dropping + and / for URL safety — it adds ~37% overhead.
= characters are appended to make the output length a multiple of 4. One trailing = means the last group had 2 bytes; two == means it had 1 byte. Padding is required by strict decoders but is often stripped in URL-safe Base64 (used in JWTs) since the decoder can infer the correct length. Omitting padding in standard Base64 may cause decoding errors in some libraries.
+ and /, which have special meaning in URLs — + is a space in query strings and / is a path separator. URL-safe Base64 replaces + with - and / with _, and typically omits padding =. It is used in JWT tokens (all three parts are URL-safe Base64 encoded), OAuth 2.0 PKCE code challenges, and anywhere Base64 data appears in URLs, cookies, or HTTP headers where the standard characters would need percent-encoding.
About This Base64 Encoder / Decoder
This free Base64 tool encodes plain text or binary data to Base64 and decodes Base64 strings back to their original form. It supports standard Base64 (RFC 4648) and URL-safe Base64 variants, and all processing happens in your browser.
Base64 is widely used to embed binary data (images, files) in text-based formats such as JSON, XML, HTML data URIs, and email attachments. Decoding Base64 is a common task when inspecting API responses, JWT payloads, and email headers.
When to use this tool
- Encoding images as data URIs for CSS or HTML
- Decoding Base64-encoded API tokens or credentials
- Inspecting JWT header and payload sections
- Embedding binary content in a JSON or XML payload
Standards & References
How Base64 Encoding Works
Base64 converts arbitrary binary data into a stream of printable ASCII characters using a 64-character alphabet.
Group Bytes
The input is read as raw bytes and grouped into chunks of 3 bytes (24 bits). Each group is split into four 6-bit values.
Map to Characters
Each 6-bit value (0–63) maps to a character in the Base64 alphabet: A–Z (0–25), a–z (26–51), 0–9 (52–61), + (62), / (63).
Pad Output
If the input length isn't divisible by 3, padding characters (=) are added to make the output a multiple of 4 characters. URL-safe mode replaces + with - and / with _.
Common Use Cases
HTTP Basic Authentication
The Authorization: Basic header encodes username:password as Base64. Paste the header value here to decode credentials during API debugging (over HTTPS only).
Data URIs
Small images, fonts, and SVGs are embedded directly in HTML/CSS as Base64 data URIs (data:image/png;base64,…), eliminating extra HTTP requests for small assets.
JWT Token Inspection
JSON Web Tokens are Base64URL-encoded. Paste the second segment (payload) to decode it and read the claims — useful for debugging auth flows without a dedicated JWT tool.
Email MIME Attachments
Email attachments in MIME format are Base64-encoded to transport binary files as text. If you've ever seen long strings in raw email source, they're often Base64-encoded file data.
Binary Data in JSON APIs
JSON has no native binary type. Base64 is the standard workaround: images, PDFs, and byte arrays are serialized as Base64 strings before being sent in JSON request or response bodies.
Cryptographic Key Transport
PEM-formatted certificates and private keys are Base64-encoded DER structures wrapped in -----BEGIN…----- headers. Understanding the encoding helps when working with TLS/SSL certificates.
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Base64 vs Base64url: Why `+`, `/`, and `=` Break URLs, and Why JWTs Use a Different Alphabet
Standard Base64's `+`, `/`, and `=` characters all have special meaning in URLs — which is why a Base64-encoded JWT pasted directly into a URL can silently corrupt, and why Base64url exists as a separate, URL-safe alphabet. Here's exactly which characters differ, why JWTs require Base64url specifically, how padding works and why Base64url commonly omits it, and how to tell the two variants apart.
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