Storage is billed in terms of megabyte-days. A megabyte-day represents the use of one megabyte of space for a one day period. This means using 31 megabytes for one day is the same as using one megabyte for 31 days. The "Unbilled Storage" numbers displayed on the Account, Site, and MySQL panels are in byte-days. 31 megabyte-days is equal to 32,505,856 byte-days.
The amount of space that your sites and MySQL processes use is measured periodically based on the number of disk blocks they utilize. These blocks are the system default block size, and represent the smallest possible allocation of disk space; even very small files and empty directories require at least one block each. The system then takes the average of that measurement and the prior measurement. The number of megabyte-days is then calculated on a pro-rata basis using the interval between the two measurements.