Why your download is 8× slower than your speed test
You run a speed test. It says 100 Mbps. You start a download and your browser says 12 MB/s. It looks like you are getting an eighth of what you pay for.
You are not. Those two numbers describe the same connection in different units — and the gap between them is almost exactly the number 8, every time, for everyone.
There are 8 bits in a byte. Internet plans are sold in bits per second; files are measured in bytes. That is the entire discrepancy.
One letter, eight times the number
The two units differ by a single capital letter, which is why nobody notices.
Lowercase b. What your ISP advertises and what almost every speed test reports. Measures the wire.
Uppercase B. What your browser, download manager and operating system show. Measures the file.
What common plan speeds actually mean
The right-hand column is the fastest a single download can realistically show on that plan.
| ADVERTISED | TYPICAL PLAN | REAL DOWNLOAD |
|---|---|---|
| 10 Mbps | Entry-level DSL | 1.25 MB/s |
| 25 Mbps | Basic broadband | 3.13 MB/s |
| 50 Mbps | Standard cable | 6.25 MB/s |
| 100 Mbps | Common fibre tier | 12.5 MB/s |
| 300 Mbps | Mid fibre tier | 37.5 MB/s |
| 1000 Mbps | Gigabit | 125 MB/s |
How long a download should take
Best-case times on a 100 Mbps line — that is 12.5 MB/s, assuming nothing else is competing for the connection.
When it is a unit mix-up, and when it is a real problem
- Your download sits near one eighth of your speed test result.
- It lands within 10–15% of the converted figure.
- It starts slow, then climbs and holds steady.
- Downloads run at half the converted figure or less, consistently.
- The speed test itself is far below your plan.
- Speeds collapse only over Wi-Fi, but hold over cable.
Even a healthy connection rarely hits the converted number exactly. Protocol overhead costs a few percent, the server on the other end has its own limits, and older Wi-Fi hardware caps out well below a modern fibre line. Close is normal. Half is not.
Skip the conversion entirely
SwiftCheck reports in MB/s from the start, so the number it gives you is the number your download will show. One click from the toolbar, under 30 seconds, no tracking.
Add to Chrome — FreeCommon questions
Why is my download speed lower than my speed test?
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Usually it is not — the two are measuring the same thing in different units. Speed tests and internet plans are sold in megabits per second (Mbps). Browsers and download managers report megabytes per second (MB/s). There are 8 bits in a byte, so a 100 Mbps connection downloads at about 12.5 MB/s at best. If your download shows roughly one eighth of your speed test result, your connection is working exactly as advertised.
What is the difference between Mbps and MB/s?
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Mbps is megabits per second, with a lowercase b. MB/s is megabytes per second, with an uppercase B. One byte is 8 bits, so MB/s is always the smaller number: divide Mbps by 8 to get MB/s, and multiply MB/s by 8 to get Mbps. The units are not interchangeable, and the capitalisation is the only thing distinguishing them in writing.
How do I convert Mbps to MB/s?
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Divide by 8. A 50 Mbps connection is 6.25 MB/s, 100 Mbps is 12.5 MB/s, and 1000 Mbps (gigabit) is 125 MB/s. To go the other way, multiply: a download running at 5 MB/s is using 40 Mbps of your line.
Why do internet providers advertise in Mbps instead of MB/s?
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Because the number is eight times larger. Bits per second is also the genuine convention for measuring data on a wire, so the unit is not dishonest in itself — but given a choice between advertising 100 Mbps and 12.5 MB/s for the same connection, the larger number wins every time.
Should my download hit the full converted speed?
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Rarely, and not for long. Protocol overhead costs a few percent, the server you are downloading from has its own limits, Wi-Fi and older routers cap out below the line speed, and other devices share the connection. Landing within about 10-15% of the converted figure means the connection is healthy. Consistently landing at half of it or less points at a real problem worth investigating.
Does SwiftCheck show Mbps or MB/s?
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MB/s, deliberately. The whole point of the extension is that the number it reports is directly comparable to the number your browser shows while a file downloads, so there is no mental arithmetic and no false alarm.