If you need to know how long a download will take, the simplest way is to start the download and let the computer do the math for you. However, this is not a good way to find out that a download will take far longer than is practical. Fortunately, after you get a few confusing computer terms out of the way, it is simple to arrive at an estimate of the download time for yourself.

Look up the speed of your Internet connection as advertised by your Internet Service Provider. It will be measured in "kilobits per second" or "megabits per second."

Divide this number by eight to convert it to the more familiar "kilobytes per second" or "megabytes per second." This is the value reported to you by your browser when you actually begin the download because files are normally measured in "kilobytes" or "megabytes" as opposed to the much smaller "kilobits" or "megabits."

Convert the file size so it is in the same unit as your download speed. For example, if the file you intend to download is measured in gigabytes but your Internet speed is measured in megabytes, you should multiply the file size by 1024 because there are 1024 megabytes in a gigabyte. Similarly, 1024 kilobytes are in a megabyte.

Divide the file size by the Internet speed. This will give you the number of seconds the download will take. Divide this by 60 to get the number of minutes, and 60 again to get the number of hours.
Tips & Warnings
 A "bit" is a computer science term that refers to the memory consumed by a single binary decimal. A "byte" consists on 8 bits. Adding to the confusion, theoretical network speed is normally measured in terms of "bits," while file sizes and download speeds are normally measured in the larger "bytes." This means that an advertised "1024 kilobit per second" (1 megabit) Internet connection will have a maximum download speed of "128 kilobytes per second."
 This process will give you a theoretical download time based on your advertised Internet speed. In practice, a number of factors will prevent you from achieving this speed. If you know a typical download speed for your ISP from experience, you can get more accurate results by skipping the first two steps and using your familiar speed in step 3. For example, if you know from experience that a download in your browser normally reports a speed of "150 kbps," you should use this value in Step 3.