Why your speed test says one thing and your download says another
A connection rated at 100 can deliver a file at 12 and both numbers can be correct. The gap comes from four things, and only one of them is your provider.
You run a speed test and it reports a large, reassuring number. Then you download a file and the progress bar reports something roughly one-eighth the size. Neither figure is wrong. They are measuring different things, and three of the four reasons have nothing to do with your internet provider.
1. The units are different — and this alone explains most of it
Internet connections are sold in megabits per second. Files are measured in megabytes.
There are eight bits in a byte. So a connection advertised at 100 megabits per second tops out at roughly 12.5 megabytes per second in a download window — before anything else is taken into account.
If your speed test says 100 and your download says 12, nothing is broken. That is the expected result.
The tell: the lower-case b means bits, the upper-case B means bytes. Mbps and MB/s are different by a factor of eight, and the distinction is one character wide.
2. Speed tests are built to find your ceiling
A speed test picks a server close to you, opens several connections at once, and measures the best result it can produce. That is a legitimate way to measure the capacity of your line.
A real download is different in every one of those respects:
- It comes from wherever the file happens to live, which may be far away
- It often uses a single connection rather than several
- It shares the line with everything else happening at that moment
- The server at the other end may be limiting how fast it will send to any one person
A speed test answers “how fast can this connection go under favourable conditions.” A download answers “how fast is this particular transfer going right now.” Those are not the same question.
3. The slowest link decides, and it is often inside your home
Your connection is a chain. The speed you get is set by its narrowest point, and for most households that point is not the line coming in.
Common bottlenecks, roughly in order of how often they are the culprit:
| Where | What happens |
|---|---|
| Wi-Fi | Distance, walls and interference reduce throughput, often severely |
| The device | Older hardware may not support current Wi-Fi standards |
| Other activity | Video calls, backups and updates all compete for the same line |
| The router | Older equipment can cap what it passes through |
| Storage | On a very fast connection, a slow disk can become the limit |
A useful diagnostic: run the test once over Wi-Fi, then once with a cable. If the cabled figure is much higher, the problem is inside your home and no amount of talking to your provider will fix it.
4. The other end has its own limits
A download cannot be faster than the server is willing to send. Many services deliberately cap per-user transfer speed — to serve more people at once, to control costs, or because free-tier access is intentionally slower than paid.
When one site downloads slowly and everything else is fine, the problem is usually at the far end, not yours.
How to work out which one you have
- Compare like with like. Convert first. Divide your advertised megabits by eight to get the realistic megabytes-per-second ceiling
- Test wired, then wireless. A large gap points inside your home
- Test at different times. Consistently slower in the evening suggests congestion
- Try a second source. One slow site is that site’s limit; everything slow is yours
When it is actually worth contacting your provider
- The wired test is well below what you pay for, repeatedly, at different times of day
- The connection drops out rather than merely slowing
- Speeds fell sharply with no change at your end
Everything else is usually Wi-Fi, the device, or the server on the other side — and those you can act on yourself today.
Common questions
- Why is my download about eight times slower than my plan?
- Connections are sold in megabits per second and files are measured in megabytes. There are eight bits in a byte, so a 100 Mbps connection tops out at roughly 12.5 MB/s before anything else is taken into account.
- Does a slow download mean my provider is throttling me?
- Usually not. Most of the gap comes from Wi-Fi, the age of the device, other activity competing for the line, or a limit set by the server sending the file.
- How do I find out whether the problem is inside my home?
- Run a speed test over Wi-Fi, then run it again with a cable. If the cabled result is much higher, the bottleneck is inside your home and your provider cannot fix it.