Email attachment size calculator
Attachments get about a third bigger when they are sent, so a 20 MB file can be too big for a 25 MB limit. Enter your file sizes to see the real email size.
Why attachments get bigger
Email was designed for plain text. To send a file, your email app converts it to text with Base64 encoding, which turns every 3 bytes into 4 characters. That adds about 33%, plus a little more for line breaks. So:
| Your files | Size as an email |
|---|---|
| 5 MB | about 6.9 MB |
| 10 MB | about 13.7 MB |
| 15 MB | about 20.6 MB |
| 18 MB | about 24.7 MB |
| 20 MB | about 27.4 MB |
What to do when it's too big
- Send a link instead. Gmail offers Google Drive, Outlook offers OneDrive, and Apple Mail offers Mail Drop for large files.
- Zip the files, or resize photos before attaching them.
- Split them across several emails.
Remember that the receiver's server has a limit too. A message that leaves your account can still bounce with error 552 5.3.4.
What the email attachment size calculator does
This calculator shows how big your email will really be once your attachments are added. Email apps convert files to text before sending, which makes them about a third larger, so a file that looks small enough can still be rejected. Type in your file sizes and you will see the real size and whether it fits the limits of the major email providers.
Benefits
- See the real size before you hit sendThe tool adds the encoding overhead for you, so you know the size the mail server will actually see, not just the size on your computer.
- Add up several files at onceEnter as many file sizes as you like, separated by commas, and the calculator totals them and tells you how many files you entered.
- Work in MB, KB or GBPick the unit that matches what your computer shows, so you don't have to convert anything by hand.
- Check five providers in one viewA table shows whether your email fits the limits for Gmail, Outlook.com, Microsoft 365 (default setting), Yahoo Mail and iCloud Mail.
- Results update as you typeThe answer changes the moment you edit a number, so you can try removing a file and see if the rest will fit.
- Clear advice when it is too bigIf the email is over 20 MB, the tool suggests sending a link from Google Drive, OneDrive, Dropbox or iCloud, or zipping and resizing the files.
- Avoid surprise bouncesKnowing the true size ahead of time helps you avoid a 552 5.3.4 "message too large" bounce from the person you are writing to.
Who it is for
- Office staff who email contracts, scans and reports and want to know if a batch of files will go through.
- Photographers and designers checking whether a set of images will fit before sending to a client.
- Small business owners sending invoices or quotes with several PDFs attached.
- Students sending assignments or project files to a teacher with a strict email provider.
- IT helpers explaining to a coworker why a 20 MB file keeps bouncing.
How to use it
- Type the size of each file you want to attach, separated by commas.
- Choose the unit: MB, KB or GB.
- Read the estimated email size and check the provider table to see where it fits.
- If it is too big, remove a file, zip or resize them, or send a cloud link instead.
Your privacy
The calculation runs entirely in your browser. You only type numbers, never upload files, and nothing is sent to ListaFix or anyone else.
Questions people ask
- What is the Gmail attachment size limit?
- Gmail lets you send up to 25 MB of attachments. Bigger files are sent as a Google Drive link.
- Why does a 20 MB file not fit in a 25 MB email?
- Encoding makes it about 27 MB, which is over the limit.
- Does zipping make files smaller?
- It helps a lot for documents and spreadsheets, but very little for photos, videos and PDFs, which are already compressed.