EMAIL DIAGNOSTICS
Estimate MIME attachment size and base64 overhead
The size of a file on disk is only one part of an email message. MIME headers, boundaries, transfer encoding and line breaks also consume bytes. Inspect a representative message before extrapolating a monthly data estimate.
Inspect a MIME message locally
The base64 size calculation
Base64 represents three input bytes with four encoded characters, with padding for an incomplete final group. The basic size formula is 4 × ceil(raw bytes ÷ 3), before line wrapping and MIME headers. For very small files, padding makes the percentage overhead larger than the familiar one-third approximation.
| Raw bytes | Base64 characters | Before MIME overhead |
|---|---|---|
| 1 | 4 | 300% larger |
| 2 | 4 | 100% larger |
| 3 | 4 | 33.3% larger |
| 1,024 | 1,368 | 33.6% larger |
| 1,000,000 | 1,333,336 | About 33.3% larger |
Use a representative MIME message
Open a saved .eml file or paste the raw headers and body into the inspector. Start with its built-in sample to see a two-part message. Use the source message rather than a screenshot or rendered email: visible text does not expose all boundaries or encoding. The tool recognizes a bounded subset of MIME and rejects malformed structures rather than promising universal mail-client compatibility.
Read the byte totals carefully
Normalized wire bytes describe the message after line endings have been normalized for inspection. Decoded base64 bytes estimate the original bytes in those encoded parts. Base64 payload wire bytes cover those parts' encoded data, not every byte in the whole message. These measurements answer different questions. Compression, provider transformations and transport details can change an actual bill.
Scale the data without leaking content
Set the number of copies to model aggregate transfer for the same payload. Sending one million copies is an arithmetic scenario, not evidence of your future volume. The exported report omits addresses, filenames, body text and raw headers. Still inspect any artifact before sharing it. The local workspace never fetches attachments or sends a test message.
Connect size to the email budget
If message-data charges apply to your provider, carry a representative encoded-size assumption into the email budget planner. Confirm the chargeable unit and current rate first. A payload estimate does not establish maximum accepted message size, spam placement or delivery success. Use the provider's applicable message limits and a separate sending test for those questions.
Read the measurement cards before the JSON
Normalized message size counts the whole serialized message with CRLF line endings. Decoded base64 content describes the content before base64 encoding; encoded base64 wire size includes its encoded text and line breaks. The overhead card compares these two values, which is why a tiny three-byte example can show 33.3% while a wrapped real payload differs. Base64 measurements may include inline images as well as attachments. The monthly card multiplies the measured encoded base64 bytes by recipient copies and the displayed data-rate assumption. A sub-cent result is shown as less than $0.01 rather than rounded to zero.
Troubleshoot source and parser errors
Paste the original message source rather than the visible message body. The parser needs recognizable headers, a blank line, and the MIME body. A multipart message also needs a boundary parameter and its closing marker. The built-in synthetic example provides a known working structure. If a local file fails UTF-8/ASCII decoding, export a compatible original message; renaming an arbitrary binary file to .eml does not convert it. Messages larger than 2 MB are outside this local tool’s scope. Inspecting an excerpt cannot establish the complete message size.
Reproduce this example
The attachment decodes to ABC: three bytes represented by four base64 characters. Whole-message bytes also include MIME headers, boundaries and line endings.
Load Three-byte synthetic MIME attachment
Download three-byte-attachment.eml — synthetic data you can save or reference. The diagnostic samples contain no real addresses, credentials or hosted streams. Scenario CSVs document inputs; the calculators do not import CSV.
Common questions
Why is my attachment bigger after encoding?
Base64 expands groups of three bytes into four characters and may add padding. MIME framing and line wrapping add more bytes.
Will this send the email or upload the .eml file?
No. File reading and parsing occur in the browser workspace. The tool has no send-email operation.
Can I share the report without exposing recipients?
The report deliberately omits raw content and identifying headers. Review the resulting file before sharing; do not share your original message by mistake.
Try it with your own scenario
Inspect a MIME message locally
The tool is free to use. Product inputs stay in the local browser workspace. Provider links, where present, are separate from the calculation.
References and scope
Updated 2026-09-24. Examples are illustrative calculations, not measured customer outcomes. Provider prices and terms must be checked at purchase time.