What are Mbit/s (Mbps), kbit/s (kbps), MB/s, kB/s?

When you're describing the speed of your connection, the bit scale should be used. If you're describing how fast a file transferred, use bytes. Note, when you write these abbreviations they are case sensitive.

The bit scale - Connection Speed / Throughput

The smallest amount of information on a computer is called a bit. 1 kilobit (kbit) is 1000 bits, as "kilo" in the word suggests. 1 Megabit (Mbit) is 1000 kilobits or 1,000,000 bits. 1 Gigabit (Gbit) is 1000 Megabits. Far beyond your purpose in testing here, using the same scale you'll arrive at Terabit (Tbit), Petabit (Pbit), Exabit (Ebit), Zettabit (Zbit) and Yottabit (Ybit). e.g. When TestMy.net says your connection is 512 kbit/s, that's 512 kilobits per second. When you see 52 Mbit/s, that's 52 Megabits per second.

The byte scale - Binary File Transfer Speed

Here is where is can get a little confusing. Another scale is the byte scale. Bytes are used when talking about binary data, like files. The reason there's a difference is because in binary data the information is stored using coded symbols, each consisting of 8 bits of information. Early computer systems used 4-bit, 6-bit and even 7-bit binary coded decimal representations. By the 70's 8-bit was the standard architecture. The byte scale is different than the bit scale, each step up is by a factor of 1024 not 1000. Although, a common misconception is that this also applies to networking, it doesn't. In the context of networking speeds the factor is 1000 not 1024. This has more to do with sales and marketing and less to do with true binary law. Where the number of memory locations is always a power of 2 (e.g. 2¹⁰ is 1024). Manufactures started selling their products using a factor of 1000 because it's easier for the consumer to understand. For instance, it can be pretty confusing to the lay person when you tell them that doubling 32K memory results in 65K. To alleviate the confusion new abbreviations were created to represent a conversion with a factor of 1024 (e.g. KiB, MiB, GiB). But this has not been adopted by everyone. So 1 kB can mean 1000 bytes or 1024 bytes depending on who you ask. A good rule of thumb is that if you're talking about memory allocation or storage use a factor of 1024. In the context of networking a factor of 1000 is industry standard. This confuses even the most knowledgeable computer technicians and there can be a lot of debate that surrounds the issue.

So now you should know that 1 byte is equal to 8 bits. 1 kilobyte (kB) is 1024 bytes. 1 Megabyte (MB) is 1024 kilobytes. 1 Gigabyte (GB) is 1024 Megabytes. But that 1 kB/s is equal to 1000 bps. 1 MB/s is 1000 kB/s. 1 GB/s is 1000 MB/s. Using the same scales you then arrive at Terabyte (TB) / Terabytes per second (TB/s), Petabyte (PB) / Petabytes per second (PB/s), Exabyte (EB) / Exabytes per second (EB/s), Zettabyte (ZB) / Zettabytes per second (ZB/s) and Yottabyte (YB) / Yottabytes per second (YB/s). e.g. When TestMy.net says your speed is 465 kB/s, that's 465 kilobytes per second. When you see 2.8 MB/s, that's 2.8 Megabytes per second. Because TMN is in the context of networking, conversion using a factor of 1000 bytes per kilobyte is used.

Your TestMy Speed Test Results

TestMy.net displays your results in both formats because some people want to know their connection speed (shown in kbit/s and Mbit/s). Others want to know the speed of their binary (file) transfer (shown in kB/s and MB/s). Some other websites may confuse you by displaying speeds in bps (bits per second, for instance) and people from other countries may abbreviate differently. TestMy.net will display your speeds using the correct up conversions and English abbreviations. Nobody ever says, "My connection is 50,000,000 bps" or "... 50,000 kbit/s"... you'd instead say 50 Mbit/s. Right? So, TestMy saves you time by converting your speeds automatically when necessary.

Reference Binary Prefix on Wikipedia