Guide
Reading internet speed correctly
Between the advertised bandwidth and the number in the download window there is almost always a factor of eight — and behind that, a piece of reality.
Why the numbers do not match
Providers state bandwidth in Mbit/s, megabits per second. Operating systems, browsers and download tools show MB/s, megabytes per second. A byte consists of eight bits, so the MB/s figure is always a factor of eight smaller. A 250 Mbit/s line therefore equals about 31 MB/s. Both figures are correct; they simply describe different units.
Binary and decimal for data volumes
The same confusion repeats with data volumes. MB and GB are decimal (10⁶ and 10⁹ bytes), MiB and GiB are binary (2²⁰ and 2³⁰ bytes). Operating systems count in binary, packaging counts in decimal. At the gigabyte step the difference is about 7.4 %, at the terabyte step about 10 %. If a drive looks smaller than described, this is usually the explanation.
Why nominal bandwidth is rarely reached
Nominal bandwidth describes the line, not the transfer. In between sit protocol overhead, the remote server, Wi-Fi or mobile radio, server load and parallel connections in the same household. For a realistic estimate it therefore makes sense to calculate with half the nominal bandwidth and to read the result as an order of magnitude, not a promise.
How to estimate download time
First convert the file size to MB, then divide the bandwidth by eight to get MB/s, then divide. For a 4 GB file on a 100 Mbit/s line: 4 GB is 4000 MB, 100 Mbit/s is 12.5 MB/s, which gives 320 seconds or about 5.3 minutes. The batch converter converts several values at once when several lines are to be compared.
Matching conversions
Matching tables
Unit family: Data rate
Common questions
Is a 100 Mbit/s line fast?
It transfers 12.5 MB/s. That is a pure conversion of the same data rate, without any statement about what a line achieves in practice.
Why does my download show less than half?
If the nominal bandwidth is in Mbit/s and the display in MB/s, the factor of eight already explains most of the difference.
Is 250 MBit/s the same as 250 MB/s?
No. 250 Mbit/s equals about 31.25 MB/s.
What is the difference between MB and MiB?
MB is decimal and stands for 10⁶ bytes, MiB is binary and stands for 2²⁰ bytes, i.e. 1048576 bytes.
Sources
Do not use as the sole basis for official, medical, safety-critical or contractual decisions.
Citation note: UnitCloud, guide, sources reviewed: September 2026. Source basis: BIPM SI Brochure · IEC 80000-13.