Base64 String Validator

About this calculator

Base64 encoding turns every 3 bytes of binary data into 4 ASCII characters drawn from a 64-character alphabet (A–Z, a–z, 0–9, +, and / in the standard variant), padding the final group with = characters when the input length isn't a multiple of 3. A well-formed Base64 string must therefore have a length that's a multiple of 4, use only characters from that alphabet, and follow the correct padding rules — this calculator checks all three and reports the decoded byte length.

Developers use it to debug malformed API payloads, JWT tokens, or data URIs, and when handling MIME-encoded email attachments or binary data embedded in JSON or XML, where a single corrupted character or missing padding character can silently break decoding downstream.