Base64 Size Calculator

Base64 turns every 3 bytes into 4 characters and pads the last group with “=”, so the output is always a multiple of 4: encoded = ceil(bytes ÷ 3) × 4. That is about 33 % larger — 1,000 bytes become 1,336, and even 1 byte becomes 4.

Base64 represents arbitrary binary data using only printable ASCII characters, which is why it turns up in data URIs, MIME email attachments, JSON Web Tokens and embedded images. It works by splitting the input into groups of three bytes — 24 bits — and re-encoding each group as four characters of six bits. That is the whole story behind the size: encoding n input bytes produces ceil(n / 3) × 4 output bytes. The ceiling is the padding. A partial final group is still rounded up to a full four characters, with = filling the unused positions, so the encoded length is always a multiple of four and a short tail never makes it smaller. For a 1000-byte file that means 1000 ÷ 3 = 333.33, rounded up to 334 groups, times four is 1336 bytes. Because four characters always carry three bytes, the result sits close to four thirds of the original — an overhead of about 33 percent, whatever the size. That predictability is the trade: text-only protocols get to carry binary data, and the price is a third more space. The caveat that matters most is what this figure leaves out. It counts raw Base64 output only: no data-URI prefix such as data:image/png;base64, and no line breaks, though classic MIME wraps every 76 characters. Add those bytes yourself.