Hard drive is a kind of ordinary data storage device, and many people are caring about its capacity when buying it. From this article, you can get some information about the unit and calculation method of hard drive capacity and so on.

Introduction to Hard Drive

As a kind of common data storage device, the hard drive is used to store and retrieve different kinds of digital data by using one or more rigid rapidly rotating disks coated with magnetic material.

The platters paired with magnetic heads are arranged on a moving actuator arm, and the moving actuator arm will read and write data to the platter surfaces.

Tip: If you want to make some changes to your hard drive, then I highly recommend you to use MiniTool software.

image of hard drive

The hard drive is a kind of non-volatile storage, which can retain stored data in powered off situations. And in the early 1960s, for general-purpose PCs, hard drives were the primary secondary storage device. And until now, Toshiba, Western Digital and Seagate are still rolling out new hard drives.

Tip: If you want to know something about the newest HHD of Western Digital, read this post – Western Digital Releases 10 TB Ultrastar DC HC330 Hard Drive.

The prominent features of the hard drive are capacity and performance. The largest hard drive capacity is pretty larger than SSD capacity. So do you know the unit of hard drive size and the calculation method of it? Next, you can get some information about them which can help you make full use of hard drive in your daily life and work.

Hard Drive Capacity Calculation Method

In general, the hard drive capacity size which is reported by an operating system is smaller than the amount stated by manufacturers. But why?

There are some possible reasons: the operating system occupying some space, some space for data redundancy or file system structures. In addition, the difference between SI decimal prefixes units and binary prefixes can also cause the capacity difference.

Multiples of Bytes
Decimal PrefixesBinary Prefixes
KB (Kilobyte)103KiB (Kibibyte)210
MB (Megabyte)106MiB (Mebibyte)220
GB (Gigabyte)109GiB (Gibibyte)230
TB (Terabyte)1012TiB (Tebibyte)240
PB (Petabyte)1015PiB (Pebibyte)250
EB (Exabyte)


EiB (Exbibyte)


Let us take a 120GB hard drive as an example:

The hard drive capacity calculation method of Manufacturer is: 120GB=120,000MB=120,000,000KB=120,000,000,000 bytes; while the hard drive capacity calculation method of is: 120,000,000,000 bytes/1024=117,187,500KB/1024=114, 440.9MB/1024=111.8GB.

Modern HDDs appear as a sequential set of logical blocks in their host controller and the hard drive capacity calculation method is to multiply the number of blocks by the block size. You can get the information from the product specification of manufacture, as well as from the drive itself by using an operating system function that calls a low-level drive command.

The total capacity of the old HDD is calculated as the number of cylinders per recording area, the product of the number of bytes per sector (the most common 512) and the number of zones of the drive.

The total published capacity of the modern HDD excludes the spare capacity for defect management; however, in many early hard drives, there were a certain number of sectors saved as spares, which reduced the capacity for the operating system to use.

The Unit of Hard Drive Capacity

MB (megabytes) was mainly used when the hard disk was produced at the beginning of 1956. As time goes by, MB is too small to meet the demand for calculating the hard drive capacity. Now, the main units of the hard drive are GB (gigabytes) and TB (terabytes).

Usually, manufacturers use these SI-based units to indicate the total capacity of the hard drive. For example, a hard drive can be 100 GB, 1 TB, 12 TB, or even bigger.

The external hard disk is also widely used all over the world which is typically connected to your computer via USB. When you are using the external hard drive, you may encounter an “external hard drive not recognized” issue.

This issue may be caused by different reasons, and you need to use different methods to deal with it. Here, we recommend you to read this post to learn some available solutions: Solved – External Hard Drive Not Recognized.

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