Megabyte (MB) versus Megabit (Mb), What's The Difference?

Really, it's an issue of the Big B vs. little b!

Okay, okay, that just adds to the confusion, right?

All through our Web site we remind you to check carefully when making memory purchases to insure that you are, indeed, getting what you pay for. In late 1999 when we began writing our summaries about EDO and SDRAM, there were many references to Mb and MB, and it never occurred to us that we should explain the differences until several people sent us emails asking what the difference was.

If you dig into the technical details of a DRAM data sheet for Micron/Crucial, Samsung, Kingston and a bunch of others, you'll see just what we're talking about. It becomes and adventure trying to decipher the difference between megabits and megabytes. Okay, so you purchase a 128 megabyte module and you decide you want to know what all those little numbers on the memory chips mean. In this case, we'll use an example from Micron/Crucial and note that your module is marked MT48LC8M8A2.

Now, you log on to the Micron Web site and find the appropriate data sheets, and after a little reading, you see a comment that says you have a module with 64 megabit parts. Wait, you bought a 128MB module, right? Relax, you have one, but let's understand why.

Bits and Bytes
A bit is a single character of data (0 zero or 1 one). A byte is eight characters of data. Therefore, eight bits make a byte. Your computer processes information in a series of eight bits, or, one byte. Here is where the numbers could get a bit (no pun intended) confusing. To make a 128 megabyte memory module, you need eight 128 megabit parts. Remember, it takes eight bits to make a single byte, so multiply 128 megabits by eight parts and you get a total of 128 megabytes.

When your computer reads data from memory, it reads one bit from each of the eight memory chips to make one byte. That means all of the chips on your module are always working together, rather than a single chip working at one time. If the module has ECC, (Error Correcting Code) it will have nine chips instead of eight. The ninth chip would also work in conjunction with the other chips, except it would do the error checking and correction.

Big "B" versus little "b"
Now, if the similar names aren't confusing enough, the abbreviations are worse. Megabit is abbreviated with a lower case b (Mb) and megabyte is abbreviated with a capital B (MB), and memory modules are almost always referred to in terms of megabytes (MB).

Some things don't always match - different densities

Another thing to consider is that the number of megabits in each component don't always match the total number of megabytes in your module. As noted above, the 128 megabyte module was made with eight 128 megabit parts.

There are several ways of making 128 megabyte modules. You can use sixteen 64 megabit parts or four 256 megabit parts. The total number of bits adds up to 128 megabytes. This gives the module assembler flexibility to use larger RAM densities as they become available and to take advantage of DRAM pricing changes to build modules using the most cost effective method.

In many systems, it doesn't matter what configuration you use. When it does matter, we will always be here to help you with that information for you. All you need to know is the total number of megabytes you want to add to your system. Unlike some things, when it comes to memory, more is better.