One thing that often gives people confusion is the difference between a Megabyte (used for file size) and a Megabit (used for download speeds). People often assume that a download speed of 1 Megabit per second (1 Mbps) will allow them to download a 1 Megabyte file in one second. This is not the case, a Megabit is 1/8 as big as a Megabyte, meaning that to download a 1MB file in 1 second you would need a connection of 8Mbps. The difference between a Gigabyte (GB) and a Gigabit (Gb) is the same, with a Gigabyte being 8 times larger than a Gigabit.
To calculate how long it will take a file to download over your connection, you can use Google's simple MB/Mb converter