Disk capacity refers to the amount of data that can be stored on a disk drive. There are a few key factors that determine the total capacity of a disk drive:
1. Physical Capacity
The physical capacity of a disk drive is determined by two main factors:
- Number of platters – Hard disk drives contain round platters coated with magnetic material that store data. More platters = more surface area to store data.
- Recording density – This refers to how tightly data can be packed on the disk surface. Higher density = more data storage per square inch.
So a drive with 2 platters and a density of 100 Gigabits per square inch would have twice the capacity of a drive with 1 platter and 50 Gigabits per square inch density. In general, physical capacity has steadily grown over time as drive technology improves.
2. Formatted Capacity
The physical capacity must be formatted before it can store files in an operating system. Formatting performs these key functions:
- Reserves space for file tables, directories, and other metadata.
- Creates sectors and tracks for organizing data.
- Performs drive error checking and marking of bad sectors.
After formatting, the usable space on the drive (formatted capacity) is always lower than the physical capacity. A 500 GB physical drive may have a formatted capacity of 465 GB for example.
3. Marketing Capacity
Drive manufacturers advertise the capacity with a rounded number for marketing purposes. So a 465 GB formatted drive would be marketed as a 500 GB drive. The marketing capacity is always slightly higher than the formatted capacity.
4. Factors That Reduce Formatted Capacity
There are other factors that can further reduce the usable space on a hard drive from the formatted capacity:
- File system overhead – File systems like NTFS and FAT reserve space for their own metadata and structures.
- Partitions – Dividing the drive into multiple partitions reduces usable space in each partition.
- RAID configurations – Using drives in RAID setups like RAID-5 or RAID-1 duplicates data across drives, reducing usable space.
- Replication and backups – Copying data to other drives as backups reduces available capacity.
Accounting for all these factors, the usable disk space seen in a computer system is often 80-90% of the marketed capacity. Understanding the difference between physical, formatted, and usable capacity is important when purchasing storage drives and planning storage needs.
Measuring Capacity
There are a few key metrics used to measure disk capacity:
- Bytes – Smallest unit of digital storage, either a 0 or 1. Usually used for small storage amounts.
- Kilobytes (KB) – 1,000 bytes.
- Megabytes (MB) – 1,000 KB or 1 million bytes.
- Gigabytes (GB) – 1,000 MB or 1 billion bytes.
- Terabytes (TB) – 1,000 GB or 1 trillion bytes.
- Petabytes (PB) – 1,000 TB or 1 quadrillion bytes.
Consumer hard drives are typically in the range of hundreds of GB to a few TB. Enterprise storage systems can be hundreds of TB to a few PB.
Storage Unit Prefixes
Here are some common decimal and binary storage unit prefixes:
| Decimal | Binary |
|---|---|
| Kilobyte (KB) | Kibibyte (KiB) |
| Megabyte (MB) | Mebibyte (MiB) |
| Gigabyte (GB) | Gibibyte (GiB) |
| Terabyte (TB) | Tebibyte (TiB) |
The decimal prefixes are based on powers of 10, while the binary prefixes are based on powers of 2. So 1 KB = 1000 bytes, but 1 KiB = 1024 bytes.
Hard Disk Drive Capacity Measurements
Here are some key measurements for HDD capacity:
- Disk revolutions per minute (RPM) – Faster RPMs allow faster data access but use more power.
- Cache size – Onboard memory to buffer reads/writes. More cache can improve performance.
- Average seek time – Time taken to position R/W heads over data. Lower is better.
- Interface – Affects data transfer speeds. SATA, SAS, USB are common.
- Form factor – Physical size of the drive. 2.5″ vs 3.5″ are common sizes.
Solid State Drive (SSD) Capacity
SSDs use non-volatile flash memory to store data, so different metrics are used:
- Total bytes – Unlike HDDs, byte capacity translates directly to storage space.
- R/W speeds – Faster than HDDs due to no moving parts and internal parallelism.
- Interface – Nearly all modern SSDs use SATA, SAS, M.2, or PCIe interfaces.
- NAND type – SLC, MLC, TLC, QLC affect performance, endurance, and cost.
- Wear leveling – Improves lifespan by spreading writes across all cells.
Factors Affecting SSD Capacity
Some factors that can reduce usable SSD capacity:
- Over-provisioning – Reserving spare blocks to extend endurance.
- TRIM support – Reclaims unused blocks not currently storing data.
- Compression and deduplication – Can expand logical capacity beyond physical.
- RAID configurations – Like HDDs, RAID setups require capacity overhead.
Comparing HDD and SSD Capacity
HDDs and SSDs take different approaches to measuring capacity:
| HDD | SSD | |
|---|---|---|
| Base unit | Physical platter capacity in bytes | NAND flash capacity in bytes |
| Key metrics | RPM, seek time, cache size | R/W speed, NAND type, interface |
| Usable vs physical | Lower due to formatting overhead | Can be higher with compression |
In general, SSD usable capacity maps closely to physical capacity, while HDD usable capacity is lower. But advanced techniques like deduplication and compression can improve effective capacities for both drive types.
Hard Disk Drive (HDD) Tiers
HDDs are segmented into tiers based on performance and use case:
Tier 1 – Enterprise HDDs
- Highest performance and reliability
- 24/7 operation
- Higher RPM (10K/15K)
- Larger cache size
- Used for enterprise servers and storage arrays
Tier 2 – Desktop HDDs
- Balanced price and performance
- Moderate workloads
- 7,200 RPM typically
- Used for desktop PCs, gaming systems
Tier 3 – Portable HDDs
- Prioritize low power, weight
- 5,400 RPM typically
- Smaller form factors
- Used for laptops, external portable drives
Each tier offers different capacities optimized for their operating environments and use cases.
Solid State Drive (SSD) Tiers
SSD tiers are similarly segmented based on use case:
Tier 1 – Enterprise SSDs
- Highest endurance and reliability
- SLC or MLC NAND flash
- Large capacities up to multi-TB
- Used for mission-critical enterprise workloads
Tier 2 – Client SSDs
- Cost-optimized MLC NAND
- Capacities from 128GB to 2TB
- SATA and M.2 interfaces
- Used for desktop, mobile, gaming systems
Tier 3 – Consumer SSDs
- TLC or QLC NAND for low cost
- Capacities from 32GB to 1TB
- SATA or USB interfaces
- Light consumer workloads
How Operating Systems Display Disk Capacity
Operating systems report disk capacity to users differently from the physical metrics:
- Total size – The marketed capacity, not accounting for formatting, etc.
- Used space – Capacity used to store files, applications, etc.
- Free space – Remaining available capacity for new data.
- Units displayed – Usually GB or TB, using decimal prefixes.
So a 465 GB physical HDD may show as 500 GB total capacity, with 362 GB used and 138 GB free after OS formatting and overhead. OSes hide the complexities of physical metrics from users.
Factors That Influence Disk Capacity Requirements
When determining required disk capacity, consider these factors:
- Current storage needs – Existing drive capacity and fullness.
- Growth requirements – Expected storage needs in 1-3 years.
- Drive purpose – OS, applications, data storage, backups, etc.
- Drive workload – Light consumer use vs. heavy enterprise workloads affect needs.
- Redundancy policies – RAID, replication, retention policies multiply needs.
- Number of users/devices – More users and endpoints increase capacity demands.
Modeling current needs, growth, and redundancies are key to right-sizing disk capacity.
Typical Capacity Ranges by Use Case
Here are some typical HDD/SSD capacity ranges for different use cases:
| Use Case | HDD Capacity | SSD Capacity |
|---|---|---|
| Personal computing | 250GB – 2TB | 128GB – 1TB |
| Gaming systems | 500GB – 6TB | 256GB – 2TB |
| Enterprise servers | 600GB – 10TB | 400GB – 16TB |
| Enterprise SAN/NAS | 2TB – 100TB+ | 800GB – 64TB |
Enterprise and specialized use cases require much larger capacities than personal computing.
Increasing Disk Capacity
If existing storage becomes insufficient, there are a few options to expand capacity:
- Replace with higher capacity drives
- Add additional drives
- Use external USB drives
- Leverage cloud storage
- Delete unused files and applications
- Enable compression or deduplication
Upgrading system RAM can also help by allowing more caching of data.
Key Disk Capacity Terms
Here are some key terms related to disk capacity:
- Physical capacity – Maximum hardware limit determined by physical components.
- Formatted capacity – Usable capacity after low-level formatting.
- Marketing capacity – Rounded number used for sales and marketing.
- Logical capacity – Usable space presented after RAID, partitioning, etc.
- Used capacity – Space currently occupied by user data and applications.
- Free capacity – Remaining available space for additional storage.
Conclusion
Disk capacity is determined by the physical makeup of HDDs and SSDs but many layers of abstraction exist between physical components and usable space. Understanding how physical metrics translate to usable capacity is key when selecting storage solutions and predicting capacity requirements.