Alasir Enterprises
Main Page >  Motley >  MicroHouse PC Hardware Library Volume I: Hard Drives  

Previous Table of Contents Next


Four figures commonly are used in advertising drive capacity:

  Unformatted capacity, in millions of bytes (M)
  Formatted capacity, in millions of bytes (M)
  Unformatted capacity, in megabytes (Meg)
  Formatted capacity, in megabytes (Meg)

Most manufacturers of IDE and SCSI drives now report only the formatted capacities because these drives are delivered preformatted. Most of the time, advertisements refer to the unformatted or formatted capacity in millions of bytes because these figures are larger than the same capacity expressed in megabytes. This situation generates a great deal of confusion when the user runs FDISK (which reports total drive capacity in megabytes) and wonders where the missing space is. This question ranks as one of the most common questions that I hear during my seminars. Fortunately, the answer is easy. It only involves a little math to figure it out.

Perhaps the most common questions I get are concerning "missing" drive capacity. Consider the following example: "I just installed a new Western Digital AC2200 drive, billed as 212M. When I entered the drive parameters (989 cylinders, 12 heads, 35 sectors per track), both the BIOS Setup routine and FDISK report the drive as only 203MB! What happened to the other 9MB?"

The answer is only a few calculations away. By multiplying the drive specification parameters, you get this result:

Cylinders: 989
Heads: 12
Sectors per track: 35
Bytes per sector: 512
Total bytes: 212.67M
Total megabytes: 202.82MB

The result figures to a capacity of 212.67M or 202.82MB. Drive manufacturers usually report drive capacity in millions of bytes, whereas your BIOS and FDISK usually report the capacity in megabytes. One megabyte equals 1,048,576 bytes (or 1,024KB, wherein each kilobyte is 1,024 bytes). So the bottom line is that this 212.67M drive also is a 202.82MB drive! What is really confusing is that there is no industry-wide accepted way of differentiating binary megabytes from decimal ones. Usually drive manufacturers will always report metric megabytes, since they result in larger, more impressive sounding numbers! One additional item to note about this particular drive is that it is a zoned recording drive and that the actual physical parameters are different. Physically, this drive has 1,971 cylinders and four heads; however, the total number of sectors on the drive (and, therefore, the capacity) is the same no matter how you translate the parameters.

Although Western Digital does not report the unformatted capacity of this particular drive, unformatted capacity usually works out to be about 19% larger than a drive’s formatted capacity. The Seagate ST-12550N Barracuda 2GB drive, for example, is advertised as having the following capacities:

Unformatted capacity: 2,572.00M
Unformatted capacity: 2,452.85MB
Formatted capacity: 2,139.00M
Formatted capacity: 2,039.91MB

Each of these four figures is a correct answer to the question "What is the storage capacity of the drive?" As you can see, however, the numbers are very different. In fact, yet another number could be used. Divide the 2,039.91MB by 1,024, and the drive's capacity is 1.99GB! So when you are comparing or discussing drive capacities, make sure that you are working with a consistent unit of measure, or your comparisons will be meaningless.

The true industry standard abbreviations for these figures are shown in Table 1.4.

Table 1.4  Standard Abbreviations and Meanings.

Abbreviation Description Decimal Meaning Binary Meaning

Kbit Kilobit 1,000 1,024
K Kilobyte 1,000 1,024
Mbit Megabit 1,000,000 1,048,576
M Megabyte 1,000,000 1,048,576
Gbit Gigabit 1,000,000,000 1,073,741,824
G Gigabyte 1,000,000,000 1,073,741,824
Tbit Terabit 1,000,000,000,000 1,099,511,627,776
T Terabyte 1,000,000,000,000 1,099,511,627,776

Unfortunately, there are no differences in the abbreviations when used to indicate metric verses binary values. In other words, M can be used to indicate both "millions of bytes" and megabytes. In general, memory values are always computed using the binary derived meanings, while disk capacity goes either way. Unfortunately, this often leads to confusion in reporting disk capacities. Note that bits and bytes are distinguished by a lower- or uppercase B. For example, millions of bits are indicated by using a lowercase b, resulting in the abbreviation of Mbps for million bits per second, while MBps indicates million bytes per second.

Previous Table of Contents Next