Drive storage capacity refers to the maximum amount of data that can be stored on a drive, such as a hard disk drive (HDD), solid-state drive (SSD), or flash drive. When looking at drive storage, capacity is one of the most important factors to consider along with speed, durability, form factor, and cost. Drive capacity is measured in gigabytes (GB) or terabytes (TB).
Measuring Drive Capacity
Drive capacity is determined by two main factors – the physical size of the storage medium and the density of the storage technology used.
For mechanical hard drives, capacity is largely determined by the number of platters inside the drive and the density of data storage on each platter surface. More platters and higher density equals greater capacity. For example, a 3.5″ desktop HDD with two 1TB platters has a total capacity of 2TB.
Flash storage like SSDs and USB drives do not have platters. Their capacity is determined by the number of memory chips on the circuit board and density of each chip. Higher density NAND flash memory allows SSDs and flash drives to store more data in the same physical space. A 120GB SSD may have only 4-8 NAND chips while a 4TB SSD may have over 100 chips.
Hard Disk Drive Capacity
Common HDD sizes today range from 120GB to 14TB for desktop drives and 500GB to 5TB for laptop drives. Enterprise and data center HDDs can go even higher, currently up to 20TB.
Some key factors for HDD capacity:
| Drive Height | Common capacities |
| 2.5 inch (laptop) | 500GB – 5TB |
| 3.5 inch (desktop) | 1TB – 14TB |
Larger form factors allow for more platters which enables higher capacities.
| Platter Density | Max Capacity |
| 500 Gb/in2 | 1 – 2 TB |
| 1 Tb/in2 | 3 – 6 TB |
| 1.33 Tb/in2 | 8 – 10 TB |
| 2 Tb/in2 | 12 – 16 TB |
HDD capacity growth relies on increasing platter densities through technologies like shingled magnetic recording (SMR) and heat-assisted magnetic recording (HAMR) which allow bits to be placed closer together.
SSD Capacity
SSD capacities have grown enormously in recent years from 64GB to now commonly 1TB to 4TB. High-capacity enterprise SSDs are available up to 64TB.
| Form Factor | Common capacities |
| M.2 | 128GB – 2TB |
| 2.5 inch | 250GB – 16TB |
| PCIe Add-in Card | 800GB – 64TB (enterprise) |
As NAND flash density increases, SSDs can store more data in the same physical hardware. Higher density NAND also allows for more bits per cell (SLC to MLC to TLC to QLC) which also expands capacities. Some key generational NAND flash density milestones:
| NAND Generation | Density |
| SLC | 256 Gb per die |
| MLC | 512 Gb per die |
| TLC | 1 Tb per die |
| QLC | 4 Tb per die |
Factors Affecting Drive Capacity
Some additional factors affect the usable capacity of a drive versus its advertised maximum capacity:
Overprovisioning
SSDs reserve a portion of their raw NAND flash (e.g. 7% extra) that is hidden from the operating system. This overprovisioning improves performance and endurance as the drive has spare area to do activities like garbage collection. But it reduces usable capacity below the advertised amount.
RAID Configurations
Using multiple drives in a RAID array can increase total capacity but also reduces usable capacity. RAID 1 mirrors drives so only 50% of total capacity is usable. RAID 5 with parity uses the equivalent of one drive for parity data, reducing usable space.
Formatting
Formatting the drive requires allocation of space for file tables, directories, and other file system metadata. For example, NTFS takes approximately 6-7% of total drive space. Therefore, the usable capacity after formatting will be below the advertised raw capacity.
Operating System
The operating system and software also consume a small portion of drive capacity. For example, Windows may take up 15-20GB on a 1TB drive. This offsets the usable space compared to the unformatted capacity.
Drive Capacity Requirements
The capacity required for a drive depends on the intended applications and workload.
Desktop Computers
For desktop PCs, 1-6TB offers adequate capacity for everyday office work, entertainment, and moderate gaming needs. Higher workloads like video production and gaming may need 6-10TB internal storage.
Laptops
Laptop HDDs and SSDs range from 128GB to 2TB. 256-512GB is common for everyday usage. 1-2TB is required for power users with large photo, video, or gaming storage needs.
External Storage
External hard drives and SSDs now offer capacities from 500GB to 64TB. Capacity requirements vary widely based on need. 1-2TB portable drives provide basic extra storage. 4TB+ desktop drives offer room for backups and archives. 10TB+ enterprise drives enable large media libraries.
Data Centers
Data center and server applications require enormous drive capacities to store business information, websites, applications, and big data. Enterprise HDDs up to 20TB allow building petabyte-scale storage on hundreds of drives. High capacity enterprise SSDs are also used for faster performance.
The Future of Drive Capacities
Both HDDs and SSDs continue to grow in capacity thanks to new technologies on the horizon:
HAMR and MAMR
Heat and microwave assisted magnetic recording allow for higher platter densities up to 4 Tb/in2, enabling HDD capacities over 50TB in the future.
Penta-Level Cell (PLC) NAND
QLC NAND maxes out at 4 bits per cell. But PLC could store 5 bits per cell, increasing densities by 25%. This could allow SSDs to continue density gains.
3D NAND Stacking
Stacking NAND memory cells in 3D allows greater density in the same footprint. Manufacturers are now at 96-layer NAND but will continue stacking higher, increasing SSD capacities.
New Non-Volatile Memory
New non-volatile memory technologies like ReRAM, MRAM and carbon nanotube RAM offer potential to break through the limits of NAND flash. This could open the door to much higher capacity SSDs.
Conclusion
Drive capacity is determined by the physical size and storage density of the technology used. Both HDDs and SSDs have grown enormously in recent years into the multi-terabyte ranges. Emerging technologies promise to continue expanding capacities to keep up with the world’s exploding data storage needs. Carefully considering capacity requirements for use cases like PCs, laptops, external storage, servers and data centers allows selecting the optimal drive technology and size.
