A message with two attachments
Entered raw sizes of 2 MiB and 1 MiB model roughly 4.11 MiB after Base64 and line breaks. An entered 0.25 MiB allowance gives about 4.36 MiB planned total against the illustrative 5 MiB budget.
Document control
Free Invoice Attachment Size Planner
Plan a message-size budget from up to three entered file sizes. Raw attachment size and encoded message size differ; verify the actual sender and recipient limits and their units separately.
What you can do here
Estimate Base64 attachment size plus an entered message allowance against a stated MiB budget, without uploading files or assuming an email provider's limits.
Built for real invoice workflows
Check the actual files and delivery rules before sending the approved invoice message.
Preparing your calculator with today’s date…
Calculation notes
How the result is calculated
One MiB is 1,048,576 bytes. Round each entered file size to bytes, then model each Base64 part as 4 × ceil(bytes/3) characters with two-byte CRLF breaks between 76-character lines. Sum the encoded parts and entered allowance. The allowance covers body, headers, boundaries and final separators. Signed headroom = budget bytes − planned bytes. No file inspection, actual provider limit, compression or successful delivery is established.
Formulas and examples reviewed October 2, 2026. Illustrative figures; no tax or contractual determination.
A zero-size slot
Zero in an unused attachment slot adds no padding or line breaks. Keep the separately entered body and header allowance in the total.
Questions about this calculation
Can I enter a limit stated in MB as MiB?
Convert its units first: decimal MB and binary MiB differ. Verify what the sender and recipient actually count.
Does being within the estimate guarantee sending?
No. Actual message composition, file policy and both delivery services can impose other requirements. Inspect the prepared message and relevant limits.
Sources and further reading
How to use it
1
Enter each raw attachment size in MiB, using zero for unused slots.
2
Enter a separate allowance for message body, headers and boundaries.
3
Compare the estimate with your verified MiB budget, then check the actual prepared message.
Why it helps
Account for Base64 padding and modelled line breaks.
Keep MiB units and the allowance explicit.
Identify an estimate above the entered budget before preparing delivery.
Next step
From free tool to live invoice workflow.
Use the result when creating your invoice in InvoiceAgent. Planner setup links carry the settings they describe into a draft. Review your dates, amounts, PDF, and reminder settings before scheduling.
Related workflows
Move from the quick answer into the rest of the invoice system.
Record sizes and limits
Record sizes and limits
Preserve actual unit and policy references.
Review prepared-message size
Review prepared-message size
Keep planning estimates separate from delivery evidence.
Invoice templates
Invoice templates
Use the result inside invoice formats that are ready for clients and easier to automate.
Knowledge base
Knowledge base
Review the billing concepts behind the numbers before they go onto a live invoice.
Profession pages
Profession pages
See how freelancers, consultants, and agencies apply the same calculator output in different workflows.