Bits vs bytes: the 8x mix-up behind most internet speed confusion
Your plan says 300 Mbps but downloads show 37 MB/s. Learn why bits and bytes differ by eight, and how to read every speed number correctly.
If you have ever thought your internet was eight times slower than advertised, you have met the most common speed misunderstanding there is.
Two different units for the same idea
- A bit is the smallest unit of data. Internet providers and network gear measure speed in bits per second: kilobits (Kbps), megabits (Mbps) and gigabits (Gbps).
- A byte is eight bits. File sizes, browsers and download managers mostly use bytes: KB, MB and GB, and speeds in KB/s or MB/s.
The capital letter is the clue. Capital B means bytes. Lowercase b means bits.
The rule
Divide Mbps by 8 to get MB/s.
| Plan | MB/s at best |
|---|---|
| 50 Mbps | 6.25 |
| 100 Mbps | 12.5 |
| 300 Mbps | 37.5 |
| 1,000 Mbps | 125 |
Why real numbers land lower
Even after converting, real downloads usually settle a bit below the maximum. Part of every packet is addressing and control data rather than your file. Wi-Fi quality, other devices and the sending server also play a part.
Where the confusion bites
Comparing a plan to a download manager. A plan in Mbps and a manager in MB/s are different scales. Convert before you complain.
Reading a speed meter. Some tools show bits, others bytes, and a few let you choose. NetSpeedDisplay lets you pick bytes or bits from the Speed unit menu, so you can match your plan or your downloads.
Estimating time. A 6 GB file at 10 MB/s takes about 600 seconds, which is ten minutes. At 10 Mbps it would take eight times longer.
Binary and decimal prefixes
To add one more wrinkle, some tools count a kilobyte as 1,000 bytes and others as 1,024. The difference is small for speeds, so do not worry about it when judging whether a connection is healthy.
A worked example
Suppose your plan is 400 Mbps and you start a 10 GB download. At best that is 50 MB/s, so the download takes about 200 seconds in theory, or a little over three minutes. If the download manager settles at 40 MB/s, you are at about 80 percent of the theoretical maximum. That is a healthy result once overhead and server limits are considered.
Now suppose the same download shows 5 MB/s. That is only a tenth of the theoretical rate, so it is worth investigating: try another server, a cable and a quiet network.
Common questions
Why do providers use bits? It is the long-standing convention in networking, and it produces bigger numbers.
Is Mbps the same as MB/s? No. One MB/s is eight Mbps.
Which unit should my speed meter show? Whichever you compare against. Choose bits to compare with your plan and bytes to compare with downloads.
What about gigabit? One gigabit per second is 1,000 Mbps, or about 125 MB/s at best.
Quick habits that help
- Always check whether the unit has a capital B or a lowercase b.
- Keep one unit in your head. Many people convert everything to MB/s.
- When two numbers differ by about eight, suspect units before you suspect your provider.
For the full background, read KB/s, MB/s and Mbps explained, and keep the glossary handy.