Why is it hard for people to know how to abbreviate megabytes? The SI standard is MB, but continually people insist on using Mb (megabits), mB (millibytes), and mb (millibits) to mean MB or 1,000,000 bytes. In addition, don’t confuse MB with MiB which corresponds to 2^20 or 1048576 bytes. Memory is typically measured in MiB while hard disk drives are measured in MB.