Base64 Size Calculator

How it is calculated

Base64 turns every group of 3 input bytes (24 bits) into 4 output characters of 6 bits each. If the input is not a multiple of 3, the last group is padded with = characters, so the output is always a multiple of 4. The tool applies encoded = ceil(n ÷ 3) × 4, where n is the original size in bytes, and overhead = (encoded − n) ÷ n × 100. A fractional input is rounded to a whole number of bytes, a negative number gives no result, and 0 gives 0 bytes and 0% overhead. The figure counts the padded output with no line breaks. It does not add the extra newline characters that some formats such as MIME insert into long Base64 text, and it does not include a data URI prefix.

Common mistakes

• Entering the size in kilobytes or megabytes. The field takes bytes, so a 2 KB file is 2048 bytes (or 2000, depending on how you count a kilobyte).
• Expecting an exact 33% increase on small inputs. Padding dominates: a single byte becomes 4 characters, an overhead of 300%.
• Forgetting that some systems add line breaks (for example every 76 characters) or a data:image/png;base64, prefix, which make the real text longer than this figure.
• Applying the result to the length of the text you typed rather than the size of the binary data before encoding.