Explained: HDD Areal Density

Explained: HDD Areal Density

Have you ever wondered how hard drives manage to store vast amounts of data in such a small space? In part, this is made possible by areal density—the driving factor in HDD capacity.  

This is areal density, explained. 

What is areal density?

Areal density is the amount of data that can be stored per square inch on a platter’s surface.  

With increased areal density, more data can be stored in a smaller physical space.  

How is areal density measured?  

Areal density is typically expressed in gigabits per square inch (Gb/in2), or sometimes in terabits per square inch (Tb/in2) for higher-capacity hard drives. 

What is the formula for areal density?  

Areal density is calculated by multiplying the number of bits per inch (BPI) by the number of tracks per inch (TPI).  

What are bits and tracks on a hard drive? 

HDDs utilize small areas on their disk surfaces, where data is stored as either positive or negative magnetization. Each small area represents a “bit” of information and is arranged closely together to form circular “tracks” on the surface of the rotating disk. 

How do you increase areal density? 

There are two paths to increase areal density on a hard drive platter: either add more tracks per inch by squeezing the tracks more closely together or add more bits per inch by reducing the spacing between bits. 

How has Western Digital extended areal density leadership? 

Over the years, Western Digital has utilized a range of technologies to increase areal density including: 

  1. Energy-Assisted Perpendicular Magnetic Recording (ePMR): ePMR technology brings an additional electric current to the writing mechanism, creating a more consistent signal, so that bits can be placed closer together. 
  1. Shingled Magnetic Recording (SMR): Like shingles on a roof, tracks are overlayed on a disk so more tracks and data can be squeezed onto the same space.  
  1. UltraSMR: A new, denser form of SMR technology is an accumulation of hardware, software, and firmware algorithm advances that allow for more aggressive track spacing. 
  1. OptiNAND™: With OptiNAND, a hard drive’s platter can fit more tracks per inch by using expanded metadata and offloading tasks to an embedded iNAND flash device. 
  1. Triple Stage Actuator: Using three pivot points makes it easier to position the head precisely on the track. This precision allows for tracks to be written closer together, increasing TPI. 

What does the future of innovation in hard drive technology mean for areal density? 

Western Digital’s HDD roadmap includes a 2nd Generation ePMR platform, which allows for areal densities more than 1.3 Tb/in2. The company expects UltraSMR gains will continue into the future. 


Artwork by Cat Tervo


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