Quick answer: megabytes (1 × 106 bytes).
With data caps being all the rage these days you might be wondering how you can check your data usage in iOS. It’s simple. Go to Settings > General > Usage > Cellular Usage.
I couldn’t tell and the docs weren’t clear so I tested it. First I needed a test file. I took a video file and used 7zip to build a file that’s exactly 10 MB in size.
I chose a video file to make sure compression on the network or in the app I use to test this wouldn’t affect my results (video is already compressed so it won’t benefit from additional compression). I zipped it because that’s an easy way to slice off 10 MB of it (and it also further insures that the result is not compressible).
And that blob of 10,485,760 bytes is truly 10 mebibytes (10 × 220).
To see what affect this has, I downloaded the file to my phone with Dropbox and checked the affect on my usage. If iOS uses the binary-based definition of a megabyte (220), it should show “10.0 MB”. If it uses that other definition of a megabyte (106), it should show “10.4 MB” (or “10.5 MB” if there’s any rounding).
10.4 MB. Well there you have it: it uses the base-10 definition.