Base64 Size Calculator – Free Online Tool
Calculate exact Base64 length, encoding overhead, data URL size, and decoded bytes.
Calculate Exact Base64 Encoded Size
Enter an original file size in bytes, KB, MB, or MiB to calculate the exact Base64 payload length. The calculator applies Base64 grouping and padding rules rather than relying on a rough one-third estimate.
Measure Base64 Encoding Overhead
Base64 commonly adds about 33 percent to binary data, but the exact percentage varies for small inputs because every partial three-byte group expands to four characters. See the exact source bytes, payload characters, padding, and overhead together.
Include the Data URL Prefix
Choose a MIME type to include prefixes such as data:image/png;base64,. This is useful when estimating inline HTML or CSS images because the complete data URL is longer than the encoded payload alone.
Calculate Decoded Bytes From Base64
Paste a Base64 payload or complete data URL to calculate its decoded byte size. Standard and URL-safe alphabets are accepted, whitespace is ignored, and invalid length or padding is explained before a result is shown.
Private Local Base64 Analysis
Calculations run entirely in your browser. Pasted Base64, data URLs, and size inputs are not uploaded to ToolsMint or included in optional analytics.
Frequently Asked Questions
Everything you need to know about the Base64 Size Calculator.
How much larger is Base64 than the original file?
A Base64 payload is usually about 33 percent larger than the binary input. Exact overhead depends on the input byte count because Base64 encodes groups of three bytes as four characters and may add padding.
What is the Base64 length formula?
For an input of n bytes, the padded Base64 payload length is 4 × ceil(n ÷ 3) characters.
Does a data URL add more size?
Yes. A data URL adds a prefix such as data:image/png;base64, before the Base64 payload. The calculator can include that prefix in the total character count.
Can I calculate decoded size from unpadded Base64?
Yes. Valid unpadded standard or URL-safe Base64 is accepted. A remainder of one character is invalid because it cannot represent complete decoded bytes.
Is pasted Base64 uploaded?
No. Validation and size calculation happen locally in your browser, and the Base64 content is not sent to ToolsMint.