To calculate how long it will take to download or upload any file, we have to divide the size of the file by the data transfer rate, which is usually expressed in bits per second (bps) or a multiple of it.
For multiples of a bit, the metric convention is used for prefixes (i.e. with kilo denoting 1,000, mega denoting 1,000,000, etc) so that 1 kilobit per second (Kbps) is equal to 1,000 bits per second (bps), 1 megabit per second is equal to 1,000,000 bits per second, etc.
| Name | Symbol | $10^n$10n | bits |
|---|---|---|---|
| kilobit | kb | $10^3$103 | $1000$1000 |
| megabit | Mb | $10^6$106 | $1000000$1000000 |
| gigabit | Gb | $10^9$109 | $1000000000$1000000000 |
| terabit | Tb | $10^{12}$1012 | $1000000000000$1000000000000 |
| petabit | Pb | $10^{15}$1015 | $1000000000000000$1000000000000000 |
$\text{Download time }$Download time = $\frac{\text{1 Gb }}{\text{1 Mbps }}$1 Gb 1 Mbps . So if we want to calculate how long it will take to download a 1 gigabit (Gb) file when the transfer rate of our internet connection is 1Mbps, we first have to convert 1Gb into megabits and then divide by the transfer rate:
| $\frac{\text{1 Gb }}{\text{1 Mbps }}$1 Gb 1 Mbps | $=$= | $\frac{\text{1,000 Mb }}{\text{1 Mbps }}$1,000 Mb 1 Mbps |
| $=$= | $\text{1,000 seconds}$1,000 seconds |
We could also have calculated the download time by first converting the transfer rate into gigabits per second and then dividing the file size by this new number:
| $\text{Download time }$Download time | $=$= | $\frac{\text{1 Gb}}{\text{0.001 Gbps}}$1 Gb0.001 Gbps |
| $=$= | $\text{1,000 seconds}$1,000 seconds | |
| $=$= | $\text{16 minutes and 40 seconds}$16 minutes and 40 seconds |
If the file size is expressed not in bits but in bytes, then we have to first convert it into bits by multiplying it by 8 (since 1 byte = 8 bits). So if we wanted to calculate the time it will take to download a 2 gigabyte file when the transfer rate is 1Mbps, we first have to convert 2GB into bits:
| $\text{2 GB}$2 GB | $=$= | $\text{2 x 8 Gb}$2 x 8 Gb |
| $=$= | $\text{16 Gb}$16 Gb |
Now, to calculate the download time, we simply convert the quantity into megabits and divide by the transfer rate:
| $\text{Download time }$Download time | $=$= | $\frac{\text{16,000 Mb}}{\text{1 Mbps }}$16,000 Mb1 Mbps |
| $=$= | $\text{16,000 seconds}$16,000 seconds | |
| $=$= | $\text{4 hours 26 minutes and 40 seconds}$4 hours 26 minutes and 40 seconds |