You are on a 100 Mbps plan, you start downloading a game and look at the number: 11.8. The natural reaction is that you are getting a tenth of what you pay for. In fact you are getting almost the full speed. The problem is the unit, not the line.
One letter makes the difference
- Mbps (lower-case b) = megabits per second. This is the unit providers and speed test sites use.
- MB/s (capital B) = megabytes per second. This is the unit browsers, download managers, Steam and so on use.
One byte = 8 bits. So to convert megabits to megabytes, divide by 8.
Quick conversion table
| Plan | Theoretical maximum in a download manager | What you usually see in practice |
|---|---|---|
| 16 Mbps | 2 MB/s | 1.7–1.9 |
| 50 Mbps | 6.25 MB/s | 5.5–6 |
| 100 Mbps | 12.5 MB/s | 11–12 |
| 300 Mbps | 37.5 MB/s | 33–36 |
| 1000 Mbps (1 Gbps) | 125 MB/s | 100–118 |
Why is the real figure a little lower than the theoretical one? Because every data packet carries extra information (addresses, sequence numbers, error checks) that is not counted as part of the file. That takes roughly 5–6% of capacity. It is also why speed test tools, ours included, apply a small compensation factor to estimate the line's full speed.
And at very high speeds (gigabit and above), the server you are downloading from or your device's storage may be the limit rather than the line.
How to work out download time
The formula is simple:
Time in seconds = (file size in GB × 8000) ÷ speed in Mbps
The 8000 comes from: 1 GB = 1000 MB = 8000 megabits.
Examples
| File | At 50 Mbps | At 100 Mbps | At 500 Mbps |
|---|---|---|---|
| 4 GB film | About 11 minutes | About 5.5 minutes | About 1 minute |
| 6 GB phone update | 16 minutes | 8 minutes | 1.6 minutes |
| 100 GB game | 4.4 hours | 2.2 hours | 27 minutes |
These are ideal times with the line running at full speed. In practice add 10–20%, or more on Wi-Fi or at peak time.
The same idea applies to upload
If your upload speed is 10 Mbps, uploading a 2 GB video takes (2 × 8000) ÷ 10 = 1,600 seconds, about 27 minutes. That explains why uploading video to YouTube or Google Drive takes so long on ADSL lines even when downloads are fast.
When the slowness is real
If you are on 100 Mbps and your download manager steadily shows 3 or 4 MB/s (that is 24–32 megabits), something genuinely deserves a look. Start with a wired speed test, then go through the list of common causes of low speed.
Remember too that some sites limit download speed per user on their side, so one site may be slow while others are fast. Test more than one source before judging the line.