Why the two numbers never match
Broadband is sold in megabits per second and file managers show megabytes per second, so a 100 Mbps line tops out at 12.5 MB per second before any overhead.
Protocol overhead, Wi-Fi, contention in the street cabinet and the server's own upload limit all reduce what you measure. A result within about 10 per cent of the plan on a wired connection is normal.
Worked example
A plan promising 100 Mbps that measures 72 Mbps is delivering 9 MB per second, which is 72 per cent of the advertised speed, a shortfall of 28 Mbps. A 1 GB file takes about 111 seconds at that rate.
The same file at the full 100 Mbps would take 80 seconds.
Getting a fair measurement
Test on a wired connection if you can, with nothing else using the line, and repeat at different times of day. A single Wi-Fi test measures your Wi-Fi as much as your line.
Compare like with like: a test server far away measures the route as well as the connection.
Limits of this calculator
It converts and compares the figures you enter. It does not measure your connection, and it does not know your line's sync speed or the contention ratio on your cabinet.
It does not model upload speeds, latency or packet loss, which matter more than raw throughput for calls and games.
For the time a specific file takes, use the download time calculator.
Inputs for the internet speed calculator
Speed you measured
The download figure from a speed test, in megabits per second.
Speed your plan promises
The advertised download speed, in megabits per second.
File size to time
A file size you care about, in decimal gigabytes, for the transfer estimate.
CALCULATOR QUESTIONS
My internet speed calculator questions
Does this run a speed test?
No. It compares a figure you already measured. A real test needs a server and a download stream.
Why is my result only 70 per cent of the plan?
Wi-Fi, congestion, overhead and the test server all reduce the figure. Test wired at a quiet time before concluding the line is at fault.
Is 100 Mbps the same as 100 MB/s?
No. 100 Mbps is 12.5 MB per second, eight times smaller.
Should I complain to my provider?
Only after a wired test repeated at several times. Keep the results, because a single Wi-Fi reading is not evidence about the line.