Mbps vs MB/s: Why Your Download Is Slower Than Your Internet Plan
You pay for a 100 Mbps internet plan, start a large download, and the browser shows about 11 MB/s. It looks like you are getting a tenth of what you pay for. In most cases nothing is wrong: the two numbers use different units. This guide explains the difference, what else slows real downloads, and how to check your speed fairly.
Bits and bytes
Internet providers advertise speed in megabits per second (Mbps). Browsers, download managers and game launchers usually show megabytes per second (MB/s). One byte is eight bits, so:
MB/s = Mbps ÷ 8
| Plan | Maximum download speed |
|---|---|
| 25 Mbps | 3.1 MB/s |
| 50 Mbps | 6.25 MB/s |
| 100 Mbps | 12.5 MB/s |
| 300 Mbps | 37.5 MB/s |
| 1 Gbps | 125 MB/s |
So 11 MB/s on a 100 Mbps plan is 88 Mbps, which is actually a good result. Convert any value with the Mbps to MB/s converter. The lowercase "b" (bits) versus uppercase "B" (bytes) is the clue in the abbreviations.
Where the remaining speed goes
- Protocol overhead: every packet carries headers for TCP, IP and often encryption. Typically 3–7% of the line speed is used for this, so even a perfect connection rarely shows more than about 94% of the plan as file data.
- Wi-Fi: distance, walls, interference from neighbours and older devices often cut real speed by a third or more. A cable to the router is the fairest test.
- The server: many download servers limit how fast each user can download, especially at busy times.
- Sharing: streaming, cloud backups and updates on other devices use the same connection.
- Peak hours: some networks slow down in the evening when many customers are online.
Gigabytes and gibibytes
A second unit trap affects file sizes. Storage makers and most phone plans use decimal units: 1 GB = 1,000,000,000 bytes. Windows and some apps calculate in binary units (1 GiB = 1,073,741,824 bytes) but label them "GB". That is why a "1 TB" drive shows about 931 GB in Windows, and why a file can appear about 7% larger in one program than in another. See the GB to GiB and TB to TiB converters.
How long will my download take?
Seconds = file size in bytes × 8 ÷ speed in bits per second
A 10 GB file over 100 Mbps takes 10 × 8,000 ÷ 100 = 800 seconds, or 13 minutes 20 seconds at full speed; at a realistic 90% efficiency, about 15 minutes. The download time calculator works it out for any size and speed, including uploads.
Upload is often much slower
Many cable and DSL plans are asymmetric: a 100 Mbps download plan might upload at only 10–20 Mbps. Uploading a large video or an initial cloud backup can therefore take ten times longer than downloading it. Fibre plans are more often symmetric.
How to test your speed properly
- Connect a computer to the router with an Ethernet cable.
- Pause other downloads, streaming and backups on all devices.
- Run a speed test to a nearby server, three times at different times of day.
- Compare the result in Mbps with your plan. Results of 85–95% of the plan on a cable are normal.
- If wired results are good but Wi-Fi is slow, the issue is the wireless setup, not the provider: try moving the router, using the 5 GHz band or adding a mesh access point.
If wired tests are consistently far below the plan, contact your provider with the results.
Summary
Divide Mbps by 8 to get MB/s, expect 5–15% less for overhead, and test over a cable before blaming the provider. Watch out for GB versus GiB when comparing file sizes, and remember that uploads are often much slower than downloads.