How many megabytes in a gigabyte?

I have 13.5 gb in my iphone storage. It says I used 10.5 gb and I only have 3.0 gb left.

This is what it says:
Videos 3.5 gb
Music 1.3 gb
Photos and Camera 206 mb
Draw Free 67.9 mb
Minecraft PE 66.8 mb
Fruit Ninja 56.3 mb
Crunchyroll 44.9 mb
WeChat 40.9 mb
Temple Run 36.6 mb
Instagram 26.1 mb
Tiny Wings 17.2 mb

When I try to add it all up, it doesn't end up as much as 10.5 gb. How come its so much? Where are the other gigabytes coming from?
I always clear my safari history and I don't bookmark or favorite things on safari either. I barely have any notes. I have no alarms. I have a few birthdays in my calendar but its not much. I have I area of weather. My mail is always empty cause I check it everyday and I also always clear my phone calls. Where's it all from? When I added it up I got about 6 gb all together but where does 4.5 other gigabytes come from?

The programing(ios) of the iphone

1000 megabytes in a gigabyte
iphones have space taken up by internal stuff from updates, the hardware, and all that other stuff so thats probably where all your memory is being taken up. Also videos, music, and pictures can take up data.

The other is in your system files where the apps need to run. Just uninstall unwanted application and see how it goes

There are 1, 024MB in 1GB.

The rest of the space will be taken up by the iOS software, contacts, text messages, default apps such as safari, etc.

Also, some apps have a bit more than they say for background things as well.