Bits and bytes: the most common mix-up
Providers advertise speed in megabits per second (Mb or Mbps) while browsers and download tools show megabytes per second (MB/s). One byte is 8 bits, so divide by 8: 600 Mbps is 75 MB/s, 300 Mbps is 37.5 MB/s and 1 Gbps is 125 MB/s.
| Fibre plan | Theoretical maximum | Typical real (90%) | Download 10 GB |
|---|---|---|---|
| 100 Mbps | 12.5 MB/s | 11.25 MB/s | 14 min 49 s |
| 300 Mbps | 37.5 MB/s | 33.75 MB/s | 4 min 56 s |
| 600 Mbps | 75 MB/s | 67.5 MB/s | 2 min 28 s |
| 1,000 Mbps | 125 MB/s | 112.5 MB/s | 1 min 29 s |
Calculated examples; rounded values.
Why you never reach 100%
Between 5% and 10% of the speed goes to TCP/IP headers and connection handling, so nobody gets exactly the advertised figure. If you get around 90% of your plan over a cable in a reliable test, your connection is fine; if you regularly get half, it is worth investigating.
WiFi is usually the bottleneck
A phone or laptop on WiFi 5 a few metres from the router may get 200-400 Mbps in practice, and much less through walls. With WiFi 6 and good coverage you can go beyond 600 Mbps, but the reliable way to use 1 Gbps is a cable.
If a WiFi test comes out low, repeat it with a network cable plugged straight into the router before blaming your provider.
Other reasons for slow downloads
The server you download from has its own limit too: many websites, game stores and cloud services give no more than 20-50 MB/s per connection. Programs that download over several connections at once make better use of the fibre.
Check the cable as well: old category 5 or damaged cables can cap you at 100 Mbps. Plans above 100 Mb need a Cat 5e or Cat 6 cable and a Gigabit port on the computer. At peak times, congestion on the provider or server network can also lower speed.
How to measure speed properly
Connect the computer to the router with a cable, close downloads and programs that use the internet, and run several tests on different servers at different times. Note the average, not the best result.
If the router shows its sync speed or has a built-in test, use it: that rules out the computer and WiFi.
Fibre, cable and 5G do not behave the same
Fibre to the home (FTTH) gets closest to the advertised speed and is the most stable. Coaxial cable, 4G or 5G connections can vary a lot by time of day and the number of users nearby, and their upload is usually far lower than download.
If your provider offers fibre at your address, it is usually the best option even when the advertised speed is the same.
How long a file takes to download
Divide the file size in megabytes by the real speed in MB/s. A 100 GB game on a 600 Mb fibre running at 65 MB/s takes about 26 minutes; on 300 Mb, about 50 minutes. The transfer time calculator does the maths with the efficiency you choose.
Many platforms cap speed per user, so real times can be longer.
When to complain
Check your contract for the minimum guaranteed speed. If you repeatedly fall below it over a cable, keep the test results and complain to your provider in writing. If they do not answer or you disagree, contact the telecoms regulator or consumer body in your country.
Frequently asked questions
Why does the speed test show 600 but downloads run at 50 MB/s?
Because 50 MB/s is 400 Mbps: the test uses several connections and nearby servers, while a download depends on a single server. Your fibre is fine; the limit is at the other end.
Is a 1 Gb plan worth it?
Only if you connect devices by cable or have WiFi 6 or newer and several people use a lot of bandwidth at once. For browsing and streaming in an average home, 300-600 Mb is usually more than enough.
What about upload speed?
On symmetric fibre upload equals download. It matters for cloud backups, video calls and sending large files.