When you buy a 1TB hard drive or SSD, do you actually get 1TB (1,000,000,000,000 bytes) of storage space? The short answer is no. There are several reasons why 1TB drives provide less than their advertised capacity.
What is a Terabyte?
A terabyte (TB) is a unit of digital data storage capacity equal to 1,000,000,000,000 bytes. The prefix tera- refers to the fourth power of 1000, so 1TB is equivalent to:
1,000 gigabytes (GB)
1,000,000 megabytes (MB)
1,000,000,000 kilobytes (KB)
This definition of 1TB is based on the metric system. Computers, however, use binary numbering systems to store data. This means the actual value of 1TB in binary format is different than the metric value.
Binary vs. Metric Measurements
Computers use the binary numbering system which has a base of 2 rather than a base of 10 like the metric system. In binary, each place value is a power of 2 rather than a power of 10.
The basic units used to measure data capacity in binary are:
- 1 bit = 1 binary digit (either 0 or 1)
- 1 byte = 8 bits
- 1 KB = 1,024 bytes
- 1 MB = 1,024 KB
- 1 GB = 1,024 MB
- 1 TB = 1,024 GB
As you can see, each unit is 1024 times larger than the previous one rather than 1000 times larger like in metric. This means a terabyte in binary is actually:
1,099,511,627,776 bytes
Rather than:
1,000,000,000,000 bytes
This 7% difference between the metric and binary definitions of 1TB accounts for some of the missing capacity when you purchase a 1TB drive.
Hard Drive Manufacturers Use Decimal Units
Hard drive manufacturers advertise the capacity of their products using the metric system rather than binary units. So that 1TB drive is sold as having a capacity of 1,000,000,000,000 bytes even though its true formatted capacity is less.
This practice dates back to when hard drive capacities were small and the metric and binary units were closer in size. Using the metric system allowed manufacturers to advertise slightly larger capacities than the binary reality.
The practice has continued even as drive sizes have grown to the multi-terabyte range. So consumers see “1TB” and assume it is 1,000,000,000,000 bytes when in fact it is 1,099,511,627,776 bytes.
Formatting and Partitioning
When you first use a new hard drive or SSD, the device needs to be formatted and partitioned before it can store files. This process of setting up the file system results in some additional missing bytes.
For example, formatting an internal 1TB hard drive with the NTFS file system will use up about 32GB of space for system files. That leaves only around 931GB of usable space for the end user. Likewise, partitioning and formatting an external 1TB drive may use 2-10GB depending on the file system and number of partitions.
So both the binary vs. metric capacity issue and the space used for formatting contribute to 1TB drives having less than 1,000,000,000,000 bytes available for file storage once initialized.
Overhead from File System Structures
The file system manages how and where data is stored on a drive. To do this efficiently, it uses metadata such as file tables, inodes, journaling files, and directories. This administrative overhead takes up space on the drive.
On average, the file system structures utilize about 5-15% of a drive’s raw capacity. So a 1TB hard drive formatted with NTFS may have up to 150GB reserved for just the NTFS file system management.
This overhead that supports the fundamental operation of the drive is another factor that reduces the usable space below 1,000,000,000,000 bytes.
Manufacturer Reserve Space
Hard drive and SSD manufacturers often reserve a small percentage of the raw capacity for internal use. This reserved space can only be accessed by the manufacturer, not by the end user.
The reserve space allows for areas that may go bad over time to be remapped transparently in the background. It provides a pool of spare blocks to replace any bad blocks that start to develop.
The reserved capacity is generally small, ranging from less than 1% up to about 3%. But for a 1TB drive that still works out to a few gigabytes of space held by the manufacturer rather than usable by the consumer.
Summary of Factors
Here are the key factors that contribute to 1TB drives having less than 1TB of actual usable space:
- Binary vs. metric capacity measurements
- Manufacturer use of decimal units
- Formatting and partitioning overhead
- File system structures using up space
- Manufacturer reserved space
Combining all these factors, a typical consumer 1TB hard drive or SSD may have a formatted capacity around 930GB. Of that, only about 850-890GB may be available for storing files after accounting for the used space from the operating system and applications.
Actual User Capacity Examples
To demonstrate the usable size range you can expect from 1TB drives, here are some real-world examples for common drive types and file systems:
| Drive Type | File System | Total Capacity | Available Space |
|---|---|---|---|
| 2.5″ laptop hard drive | NTFS | 931GB | 898GB |
| 3.5″ desktop hard drive | NTFS | 931GB | 925GB |
| External portable hard drive | exFAT | 931GB | 923GB |
| External desktop hard drive | NTFS | 931GB | 899GB |
| Internal SATA SSD | NTFS | 931GB | 890GB |
| mSATA SSD | NTFS | 931GB | 892GB |
| M.2 NVMe SSD | NTFS | 931GB | 889GB |
As you can see, the available free space on a “1TB” drive after formatting ranges from 890GB to 925GB for common hard drives and SSDs. This is 90-92% of the advertised 1TB capacity.
Does Drive Size Matter for Capacity?
Larger capacity hard drives and SSDs will have a higher percentage of usable space vs. raw capacity. This is because the overhead of file systems and reserved space becomes a smaller fraction of the total capacity.
For example, a 4TB hard drive may have 3.6TB (90%) of usable capacity while an 8TB drive could have 7.3TB (91.5%) available. The unused space takes up a smaller portion of the total capacity.
However, the same factors apply equally to small and large drives. So proportionally, a 250GB SSD will deliver around 225GB (90%) of usable space following the same reasoning as a 1TB drive delivering 900GB.
Maximizing Available Space
If you want to optimize the usable capacity from your hard drives and SSDs, here are some tips:
- Use exFAT rather than NTFS if you don’t need NTFS features. exFAT has less overhead.
- Reduce the number of partitions. Each partition requires file tables and other metadata.
- Leave some unallocated space if possible. The file system won’t use it.
- Get larger drives. As the drive size increases, efficiency improves.
- Store large media files rather than small files. More data relative to file system data.
- Use compression or deduplication software to effectively increase capacity.
With optimal formatting and file storage methods, you may be able to gain back 1-3% more usable space from a 1TB drive.
Is Capacity Misleading?
Given all the above factors, does that mean marketed drive capacities are misleading? It’s a complex topic.
On the one hand, hard drive and SSD manufacturers are correctly reporting the raw binary capacities of their products. The labeling itself is not deceptive.
However, most consumers are unaware of the binary vs. metric capacity distinction. They see “1TB” and quite reasonably expect to get 1,000,000,000,000 bytes of storage space.
For this reason, some argue that storage manufacturers should switch to marketing capacities using the IEC binary prefix convention. This prefixes binary units with “KiB”, “MiB”, “GiB”, “TiB”, etc. to distinguish them from metric units.
On the other hand, metric SI prefixes have been used for decades and are ubiquitous. Introducing new binary prefixes could cause more confusion rather than less among consumers.
Given the complexity, storage manufacturers are unlikely to change how they market capacities anytime soon. So it remains up to consumers to understand they are not getting quite as much space as the label may imply on a new drive.
Conclusion
While 1TB means 1,000,000,000,000 bytes, this metric definition differs from how capacity is measured and formatted on hard drives and SSDs.
Due to binary vs. metric units, drive formatting, file system overhead, and reserved space, a 1TB drive actually provides around 90-92% of its advertised capacity for file storage.
So realistically, the usable space on a new 1TB drive ends up being in the neighborhood of 930GB overall and 890-925GB free for applications and files. Now you know why your “1TB” drive falls short of a full terabyte of storage!