How can I format bytes a cell in Excel as KB, MB, GB etc?

I have a value in a cell that's in bytes. But nobody can read 728398112238. I'd rather it say 678.37GB

To write a formula to format it relatively easy (here's one: http://www.yonahruss.com/2007/02/format-excel-numbers-as-gb-mb-kb-b.html)

But is there any way to do this as a 'format'? I'd like to have the big number in the cell, but have it display as the human-readable format.

You can't really do calculations in the formatting features of Excel. You can use something like the following to do a rough estimation though:

[<500000]#,##0" B";[<500000000]#,##0,," MB";#,##0,,," GB"

Here is one that I have been using: -

[<1000000]0.00," KB";[<1000000000]0.00,," MB";0.00,,," GB"

Seems to work fine.