Speed test Guides
Updated 23 September 2026
Film, video game, backup: with your speed you can estimate in advance how long a transfer will take. Here is the formula, a ready-to-use table and the traps that skew the calculation.
Speeds are given in megabits per second, files in megabytes. One byte is 8 bits, so:
time in seconds = size in MB × 8 ÷ speed in Mbps
Example: a 4 GB film, so 4,000 MB, on a 100 Mbps line: 4,000 × 8 ÷ 100 = 320 seconds, just over 5 minutes.
| File | Size | 10 Mbps | 50 Mbps | 100 Mbps | 300 Mbps | 1 Gbps |
|---|---|---|---|---|---|---|
| Photo | 5 MB | 4 s | under 1 s | under 1 s | instant | instant |
| MP3 song | 8 MB | 6 s | 1 s | under 1 s | instant | instant |
| Mobile app | 200 MB | 2 min 40 s | 32 s | 16 s | 5 s | 2 s |
| HD film | 4 GB | 53 min | 11 min | 5 min 20 s | 1 min 47 s | 32 s |
| Recent video game | 100 GB | 22 h | 4 h 27 min | 2 h 13 min | 44 min | 13 min |
These are theoretical times at a constant speed. In practice, allow 10% to 20% more.
To send a file, it is upload speed that counts, often far below download on cable and copper lines. Sending 1 GB:
| Upload speed | Time |
|---|---|
| 1 Mbps (ADSL) | 2 h 13 min |
| 5 Mbps | 27 min |
| 20 Mbps | 6 min 40 s |
| 100 Mbps | 1 min 20 s |
| 500 Mbps (fibre) | 16 s |
That is why an online backup or sending videos can take all night on ADSL, even though downloads seemed quick.
Manufacturers count 1 GB = 1,000 MB, while some operating systems actually display gibibytes, where 1 GiB = 1,024 MiB. A file your computer shows as 100 "GB" may therefore be 107 GB in decimal terms. The gap is about 7%: it does not explain a download that is twice as slow, but it matters for very large files.
First measure your real speed, ideally over a cable. Then take 80% of that figure to allow for losses and apply the formula. For a big download, start it overnight or when nobody else is using the connection: you often gain far more than by changing plan.
Because 100 Mbps is about 12.5 MB/s: one byte is 8 bits. Your download is running at full speed.
In theory, about 13 minutes at 1 Gbps, 44 minutes at 300 Mbps, just over 2 hours at 100 Mbps and nearly 22 hours at 10 Mbps. The game store may still limit the speed.
Sending uses upload speed, often five to twenty times lower than download on ADSL, VDSL and cable. Fibre to the home usually offers much higher upload.