Base64 Size Calculator

Calculate the encoded size and overhead of Base64 data.

What the Base64 Size Calculator does

Base64 represents binary data using 64 printable characters, encoding three bytes as four characters. That ratio is fixed, so the encoded form is always about a third larger — the price of surviving transport channels that only handle text.

Formula

Encoded bytes = 4 × ⌈raw bytes ÷ 3⌉
Raw bytes ≈ encoded bytes × 3 ÷ 4
Overhead = 33.33% before padding
Padding = (3 − raw mod 3) mod 3 "=" characters

Inputs explained

Input	Unit	Required	Notes
Calculate	one of 2 options	Yes	—
Size	number	Yes	Accepts more than 0.
Size unit	one of 9 options	Yes	—
Data URI prefix length	number	Optional	Characters such as "data:image/png;base64," — that one is 22. Accepts 0 or more.

How to use it

Choose Calculate and Size unit.
Enter Size.
Optionally add Data URI prefix length.
Select Calculate.

Worked example

Encoding a 1 MB file as Base64.

Direction: Encode
Size: 1 MB

1,000,000 bytes is 333,334 three-byte groups, so 1,333,336 encoded characters — 333,336 extra bytes, a 33.33% overhead with two padding characters.

Reading the result

"Size ratio" converges toward exactly 4/3 only for large inputs — the "encoded size for common raw sizes" table shows small files running higher due to padding, so the fixed 33.3% rule of thumb is really an asymptote, not a constant.
"Total data URI length" (once a prefix is entered) is the figure that matters for a real size budget, since the MIME prefix adds on top of the already-inflated Base64 payload.

Assumptions and limitations

This assumes standard Base64 with "=" padding — URL-safe or unpadded variants change the padding character count and sometimes the exact length, which this does not distinguish.

Common mistakes

Assuming Base64 overhead is "roughly a third" for very small payloads — a 1-byte input still produces 4 characters, which is 300% overhead, not 33%; the fixed ratio only holds once the file is large enough that padding rounding becomes negligible.

Frequently asked questions

Why is Base64 exactly a third bigger?

It maps every 3 bytes (24 bits) to 4 six-bit characters. 4 ÷ 3 = 1.333, so the overhead is fixed at 33.3% regardless of content.

Should I inline images as data URIs?

Only for very small assets. You save a request but pay 33% more bytes, lose separate caching, and block the parent file from streaming.