Why Is the Size of My Email Larger Than Its Attached Files?

If you are using a locally installed mail client, you may have noticed that the size of an email seems to be a bit larger than the files you have attached to it. Why is that and is it a cause for concern? Today’s SuperUser Q&A post has the answer to a confused reader’s question.

Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.

The Question

SuperUser reader arc_lupus wants to know why the size of his email is larger than its attached files:

When attaching files to email, I have noticed that Mozilla Thunderbird calculates the total size of the resulting email as much larger than the files I have attached.

Here is a recent example: two images, one at 13 MB and the other at 3.6 MB, approximately 17 MB in total, and there were four lines of text. Mozilla Thunderbird then asked me if I really wanted to send an email with a total size of 22 MB.

Where is that difference in size coming from? Having 5 MB of text sounds like a bit much.

Why would the size of an email be larger than its attached files?

The Answer

SuperUser contributor David Schwartz has the answer for us:

Your data was 17 MiB, there are 1024 KiB in a MiB, 1024 B in a KiB, and 8 bits in a byte. So that is a total of 142,606,336 bits.

Base 64 encoding encodes every 6 bits as a separate byte, so we need about 23,767,722 bytes. Dividing by 1024 twice gives us a total of 22.67 MiB. So that is where the 22 MiB comes from.

Email is a pretty old technology and does not assume an 8-bit clean pipe.


Have something to add to the explanation? Sound off in the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.