Transfer Time

Transfer time is how long a file takes to travel across a network from one device to another. If you know the size of the file and the transmission rate of the connection, you can work that time out.

Working out the time

Divide the file size in bits by the transmission rate in bits per second:

time in seconds = file size in bits ÷ transmission rate in bits per second

For example, you upload a 36 MB folder of photos over a 9 Mbps connection. Both values need changing into bits first: × 8 turns the bytes into bits (1 byte = 8 bits), and × 1,000,000 turns megabits per second into bits per second (1 Mbps = 1,000,000 bits per second). Here 1 MB is taken as 1,000,000 bytes, so the whole calculation fits in one expression:

(36 × 8 × 1,000,000) ÷ (9 × 1,000,000) = 288,000,000 ÷ 9,000,000 = 32 seconds

Rearranging to find the file size

If you know the rate and how long the transfer lasted, multiply them to get the size in bits:

file size in bits = transmission rate in bits per second × time in seconds

For example, a connection running at 8 Mbps for 45 seconds carries 8,000,000 × 45 = 360,000,000 bits. Dividing by 8 gives 45,000,000 bytes, which is 45 MB.

Why the answer changes

The time depends on the other two values. Raise the transmission rate and a fixed file arrives sooner, because more bits are delivered every second. Make the file larger and it takes longer at the same rate, because there are simply more bits to deliver. The two scale neatly: doubling the file size doubles the time, while doubling the rate halves it.

Any time you work out this way is theoretical — the quickest it could possibly be. Real network conditions such as congestion, interference and the number of users sharing the connection all slow the traffic down, so the file usually arrives later than your answer suggests.