Disk Density

What is Disk Density?

Disk density, also referred to as areal density, signifies the metric for the amount of data a magnetic disk or a tape disk can store. In simple terms, it is the number of bits stored per square inch of the disk surface.

Typically, disk density is measured with reference to Bits Per Inch (BPI) and the Tracks Per Inch (TPI). The disk density is obtained as the number of BPI times the TPI.

Understanding Disk Density

What is Disk Density

Disk density or areal density is the measurement of the data stored on a specific unit of the physical area of a storage device.

The disk density of a magnetic storage indicates different characteristics that affect the efficacy of the data stored. These physical characteristics include:

Disk density is typically measured in Gbits/in2. However, larger units are used today such as Tbits/in2, where 1 terabyte is equal to 1024 gigabytes.

In general, the formula used for measuring the disk density of a storage medium is Tracks Per Inch (TPI) x Bits Per Inch (BPI).

This means that the density will increase when either of these parameters in the equation increases.

Well, whether it is increased or decreased, in both cases there are several challenges faced by the disk drive as well as the suspension assembly.

The designers have to find the best ways to narrow the tracks but in such a way that it does not affect the ability of the assembly to read the weaker magnetic signals in each bit.

This means that the signal should be decreased especially in order to increase the number of bits. This will ensure that each of the bits is distinguishable from the others.

Therefore, from the point of view of a suspension assembly designer, increasing the disk density will involve any one or all of the following:

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However, with the development in technology and design of the hard disks, the areal density of the disk storage systems has increased significantly as compared to the time when International Business Machines or IBM released the Random Access Method of Accounting and Control (RAMAC) hard disk PC in 1956.

The areal density of the magnetic disk in RAMAC was two thousand bits per square inch.

It is pretty small as compared to the disk density of modern days, which is in excess of one terabyte per square inch in some cases.

On the other hand, the bit density of the magnetic tape has increased by a factor of about 30 over a period of ten years from 6.7 gigabits per square inch in the debut magnetic tape from IBM to 201 GB in the sputtered media tape introduced by IBM and Sony.

This means that the tape cartridge can hold more than 300 terabytes of data, making the magnetic particles on the sputtered media tape a fraction of the size of earlier particles.

Interesting Facts

There are quite a few interesting facts about disk density that are good to know at this point. These are:


Disk density can be of different types, and based on that, the disks are categorized as explained hereunder.

Singe Density:

This typically refers to the 8-inch storage media, and are also known as SD or 1D disks. Usually, these are the first generation of floppy disks that had distinguishable features such as:

Double Density:

Also known as DD or 2D, these disks came with double the capacity of the SD and can be distinguished by their characteristic features.

One of the most notable features of the DD drives is an improved line code in place of the regular FM encoding such as:

High Density:

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The high-density, or HD drives are quite similar to the DD drives. However, the 3½-inch double density storage media uses an iron oxide coating just like the 5¼-inch DD or quad-density disks.

The drives are stronger due to the 670-oersted write heads and the track width is narrower, being 0.115 mm (0.0045 inches supporting a disk density of 135 TPI and 8717 BPI.

On the other hand, the 3½-inch HD disks use cobalt disk coating just like the 5¼-inch HD drives and a 700-oersted write heads to support a higher disk density of 17434 BPI.

Extra High Density:

Also known as ED disks, these drives come with double the capacity of the HD disks. This is achieved since these disks typically come with the following:

Triple Density:

Also referred to as TD, these drives come with triple the track density or capacity of the ED drives. The improved track density allows for improving other parameters of the drives that support longitudinal recording.

Quad Density:

Also referred to as QD or 4D, this is double the capacity of DD up to 96 TPI. It is achieved by contracting the width of the tracks to 0.160 mm (0.0063 inches).

However, there are a few specific manufacturers, such as Teac, Micro Peripherals and Micropolis, that produce 4D drives with a track density of 100 TPI. These quad-density drives are not compatible with the 96 TPI variants.

The capacity of these 4D drives is increased to write 9646 BPI as follows:

Disk Density Formula

The density, or surface density, of a disk can be calculated by using the traditional formula of density (Mass/Area).

However, since the disk in a hard drive is circular, it will largely depend on its radius (say, r) and on the distance from the center. It can be expressed as ρ r = A + B r.

Now, knowing the mass and area will need some basic knowledge of arithmetic.

As for the mass of the ring of radius r, it will be θ = (A + Br) × 2πr dr × r2.

However, in simple terms, mass = volume x density.

Typically, according to traditional math, the area of the circular disk will be equal to (πr^2).

If you divide the mass by this area, you will get the disk density.

However, to know the mass of the disk, you will need to know its volume first. Ideally, a circular disk is another form of a cylinder, just a flat one.

Therefore, according to mensuration, the volume of a circular disk will be equal to (πr^2) *h, where ‘r’ is the radius of the disc, ‘h’ is the thickness, and ‘π’ or pi is a constant equal to 3.14 or 22/7.

The formula for volume is not hard to remember. You can think of a pizza, which is a circular disk in shape. If its radius is denoted by ‘z’ and the height by ‘a’, the formula of its volume will be pi*z*z*a.

Now, you have everything (mass, area, and volume) that you need to find the disk density.

What is a High Density Disk?

Typically, a high-density disk refers to its storage capacity which is much higher in comparison to a double-density disk. In simple terms, a high-density disk can hold more than 720 KB of data.

However, for a better understanding of high-density disks, further explanation is required.

This is because the terms high-density disk and double-density disk are often used by many users interchangeably, resulting in a lot of confusion.

If you consider the floppy disks of the earlier days, a high-quality 5-inch floppy disk could hold as much as 1.2-MB of data.

In comparison, a high-density 3-inch or a 3.5-inch disk could hold 1.44 MB and 1.5 MB of data, respectively.

Typically, this added data storage capacity is availed by writing data on both sides of the disk. Therefore, such disks are usually referred to as double-sided disks.

If the computer today comes with a floppy disk, it will be a high-density disk, and not a double density disk, as it was in the earlier computer models.

However, the term ‘high-density’ is mostly an obsolete term or idea in the IT sector today because newer storage technologies are used now with different metrics such as ‘area density’ and ‘surface density’ in place of high density.

However, in general, the floppy disks are somewhat obsolete now and have been replaced by Universal Serial Bus (USB) connected flash drives and others, that are as such designed as high-density storage media, with a storage density much more than 1 GB per square inch in general.

In fact, it is not uncommon for a drive as small as a thumb to offer a storage capacity of lots of gigabytes. This is one main reason for the term high density disk being phased out, largely.


So, coming to the end of this article, now you are quite familiar with disk density and how to measure it.

You now also know what a HD disk means.

With such higher knowledge, you can now very well guess the capacity of the disk of the hard drive in your PC, how much data it is holding, and how much it should hold.