Mbps vs MB/s: the difference between megabits and megabytes

By The SpeedFibre team, updated . 3 min read.

Mbps vs MB/s: the difference between megabits and megabytes
In short 1 byte = 8 bits. Divide Mbps by 8 to get MB/s: a 100 Mbps fibre line downloads at about 12 MB/s at most. It is not a trick, just two different units.
Contents
  1. Bits and bytes: the basics
  2. Conversion table
  3. Why providers use bits
  4. Working out a download time
  5. Other unit traps
  6. Bottom line

It is one of the most common questions: "I have 100 Mbps fibre, but my downloads top out at 12 MB/s. Am I being cheated?" No. Your provider and your browser simply use different units.

Bits and bytes: the basics

A bit (lower-case b) is the smallest unit of information: a 0 or a 1. A byte (capital B) groups 8 bits.

  • Internet speeds are expressed in bits per second: kbps, Mbps, Gbps.
  • File sizes and browser download speeds are expressed in bytes: KB, MB, GB and MB/s.

To convert: MB/s = Mbps ÷ 8.

Conversion table

Plan speedMax download speed1 GB file5 GB film
8 Mbps1 MB/sabout 17 minabout 1 h 25
20 Mbps2.5 MB/sabout 7 minabout 34 min
50 Mbps6.25 MB/sabout 2 min 45about 14 min
100 Mbps12.5 MB/sabout 1 min 20about 7 min
200 Mbps25 MB/sabout 40 sabout 3 min 30
500 Mbps62.5 MB/sabout 16 sabout 1 min 25
1 Gbps125 MB/sabout 8 sabout 42 s

These are theoretical times. In practice, subtract 5 to 10% for network headers, and more if the server you download from is slow.

Why providers use bits

Telecoms have always measured lines in bits per second: it is the natural unit of transmission, used by every provider worldwide. It is not a marketing trick to show a bigger number, even if it has that effect.

Working out a download time

The formula: time (s) = size (MB) × 8 ÷ speed (Mbps).

Example: a 50 GB game (50,000 MB) on 200 Mbps fibre: 50,000 × 8 ÷ 200 = 2,000 seconds, about 33 minutes in ideal conditions.

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Other unit traps

  • MB vs MiB: some software shows mebibytes (1 MiB = 1,048,576 bytes). The difference is about 5%.
  • Mobile data GB: your data plan is counted in bytes. A speed test on a fast connection can use several hundred MB, see testing speed on a smartphone.
  • Sync rate: on ADSL, the modem interface shows the line rate, always slightly higher than the usable speed a test measures.

Bottom line

To check whether you get your speed, compare Mbps with Mbps: exactly what our test does. To see what that speed allows in practice, read what is a good internet speed.

#SpeedTest#FiberSpeedTest#FiberInternet

Sources and further reading

Frequently asked questions

How much is 100 Mbps in MB/s?

About 12.5 MB/s at most, and around 11 to 12 MB/s in practice.

Why do Steam or Chrome show a lower speed than the test?

They show megabytes per second (MB/s), not megabits. Multiply by 8 to compare with the test.

What does 1 Gbps mean?

One gigabit per second, i.e. 1,000 Mbps, or about 125 MB/s at most.

The SpeedFibre team Guides written and reviewed by a telecom engineer specialised in fibre optics (FTTH), with more than 8 years of hands-on network maintenance and measurement. Editorial policy.

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