Base64 Validator

Check if a Base64 string is valid. Get detailed analysis including character set, padding, and MIME type detection.

Last Updated: September 2026

100% Client-Side — Your data never leaves your device

Enter a Base64 string to validate

Paste a Base64 string above to validate it

Validate Base64 in Code

Check if a string is valid Base64 in your application.

JavaScript

// Validate Base64 with regex + decode attempt
function isValidBase64(str: string): boolean {
  // Strip data URI prefix if present
  const clean = str.includes(",") ? str.split(",")[1] : str;
  // Quick character check
  if (!/^[A-Za-z0-9+/]*={0,2}$/.test(clean)) return false;
  // Length must be multiple of 4
  if (clean.length % 4 !== 0) return false;
  // Try decoding
  try {
    atob(clean);
    return true;
  } catch {
    return false;
  }
}

console.log(isValidBase64("SGVsbG8="));   // true
console.log(isValidBase64("!!!bad!!!"));  // false

Frequently Asked Questions

Search below or browse through our most commonly asked questions.

A Base64 string can be invalid if it contains characters outside the allowed alphabet (A-Z, a-z, 0-9, +, /), has incorrect padding (=), or has a length that isn't a multiple of 4 (for standard Base64).

The Validator will tell you exactly what is wrong. If you need it automatically fixed, use our 'Base64 Repair Tool' which can add missing padding and strip illegal characters.