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Standard Base64 (RFC 4648)

Published

In this guide

This standards reference on Base64Encoding explains main in plain language for developers and everyday users.

Everything important stays private: try the matching converter or tool in your browser — files and tokens are not uploaded.

Read the full English article below for code samples and edge cases, or switch language anytime from the menu. We keep expanding translations from real user feedback.

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RFC 4648 section 4 defines the “base64” encoding that most programming languages implement by default: btoa(), PHP’s base64_encode(), Python’s b64encode(), Java’s Base64.getEncoder(), and so on.

Rules

  • Alphabet: A–Z a–z 0–9 + / (see Base64 characters).
  • Padding with = so the length is always a multiple of 4.
  • No line breaks. RFC 4648 says implementations must not add line feeds unless a referring specification (like MIME) explicitly requires them.
  • Decoders must reject characters outside the alphabet unless the referring specification says otherwise.
  • Unused bits in the last character should be zero (canonical encoding); strict decoders may reject non-zero bits.

Validation regex

^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$

This matches every valid padded value, including the empty string. Check your value interactively with the Base64 validator.

  • Base64URL for URLs and tokens
  • MIME for e-mail with line wrapping

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Open the homepage Base64 converter or the related tools linked on this page and paste a sample.

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