Base64 Size Calculator | Data URI Overhead

Calculate the exact Base64-encoded size of any file or string, or work backward from Base64 length to bytes. Includes data URI overhead. Free, no signup. Designed specifically for developers and professionals, this free online calculator runs entirely in your browser to process tasks quickly and securely. Enjoy unlimited usage with no data tracking and no registration required.

Also known as: base64 size calculator · base64 length calculator · calculate base64 encoded size · base64 to bytes calculator · base64 overhead calculator · data uri size calculator · base64 encoding size increase · file size after base64 encoding

math calculators

encoding and decoding tools

calculators

Input Size

Paste text to measure its exact UTF-8 byte size, or enter a byte count directly.

Text (optional, measures exact byte size)

Or enter byte count directly

Data URI mime type (optional)

Results

Base64 Length

1,368 chars

from 1.00 KB

Overhead

+33.6%

Padding chars

Calculation

1. Base64 groups 3 bytes into 4 characters: ceil(1024 / 3) x 4 = 1368

2. Overhead: (1368 - 1024) / 1024 x 100 = 33.6%

About Base64 Size Calculator

Work out exactly how large a file or string becomes once Base64-encoded, or how many bytes a Base64 blob decodes to, including the overhead a data URI prefix adds.

Key Features

Exact UTF-8 byte counting for pasted text, not a rough character-count guess
Real Base64 encoding math: 3 bytes become 4 characters, with correct padding
Data URI overhead for common mime types (image/png, application/json, and more)
Exact decode of a real pasted Base64 string via the browser's own decoder
Runs fully client-side, nothing you paste leaves your browser

Use Cases

Sizing a base64-encoded image before embedding it as a CSS or JSON data URI
Estimating the JSON payload size increase from base64-encoding a file attachment
Checking whether a JWT or API token will fit a header size limit once encoded
Reverse-engineering the original file size from a base64 blob's length

Supported Formats

Any UTF-8 text (byte count via TextEncoder)

Standard and URL-safe Base64 alphabets

Related Keywords

base64 size calculator

base64 length calculator

base64 overhead

data uri size

base64 to bytes

base64 encoding size increase

Why is Base64 always about 33% larger than the original?

Base64 encodes every 3 raw bytes as 4 printable characters, since each character represents 6 bits and 3 bytes is exactly 24 bits. That 3-to-4 ratio is a fixed 33.3% size increase before any padding, which is why an image or file always grows by roughly a third once base64-encoded into a data URI or a JSON string.

Privacy & Security

All processing happens locally in your browser. Your data never leaves your computer, ensuring complete privacy and security.

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Frequently asked questions

What is Base64 Size Calculator used for?

Calculate the exact Base64-encoded size of any file or string, or work backward from Base64 length to bytes. Includes data URI overhead. Free, no signup.

Is Base64 Size Calculator free to use?

Yes. Base64 Size Calculator is completely free with no usage limits, and there is no paid tier required to use it.

Do I need an account to use Base64 Size Calculator?

No. Base64 Size Calculator works immediately in your browser with no signup, login, or email address required.

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