(10 Megabytes / 0.625 Megabytes per second = 16 seconds to download the file). All you need to remember is this simple formula: File Size In Megabytes / (Download Speed In Megabits / 8) = Time In Seconds . A 15 Megabyte file, downloading at 10 Megabits per second: 15 / (10/8) = 12 seconds.