NAS Drive Sizes: Real Capacity for Every Drive from 1TB to 24TB
Every NAS hard drive is sold in decimal terabytes but your operating system reads them in binary tebibytes (TiB). This reference table shows exactly what each drive size delivers once installed.
See exactly how much usable storage your configuration delivers.
Why Your Drive Shows Less Than Advertised
Drive manufacturers define 1 TB as exactly 1,000,000,000,000 bytes (1012). Operating systems define 1 TiB as 1,099,511,627,776 bytes (240). The difference is 9.95%. And it applies to every drive, every time, with no exceptions.
A "4TB" drive contains exactly 4,000,000,000,000 bytes. Divide that by 1,099,511,627,776 and you get 3.637 TiB. Your NAS will show approximately 3.64 TiB or 3,726 GB depending on how it displays storage.
Complete 1TB-24TB Reference Table
| Advertised | Real (TiB) | Real (GB) | Loss |
|---|---|---|---|
| 1 TB | 0.91 TiB | 931 GB | −6.8% |
| 2 TB | 1.82 TiB | 1,863 GB | −6.8% |
| 3 TB | 2.73 TiB | 2,794 GB | −6.8% |
| 4 TB | 3.64 TiB | 3,726 GB | −6.8% |
| 6 TB | 5.46 TiB | 5,588 GB | −6.8% |
| 8 TB | 7.28 TiB | 7,451 GB | −6.8% |
| 10 TB | 9.09 TiB | 9,313 GB | −6.8% |
| 12 TB | 10.91 TiB | 11,176 GB | −6.8% |
| 14 TB | 12.73 TiB | 13,038 GB | −6.8% |
| 16 TB | 14.55 TiB | 14,901 GB | −6.8% |
| 18 TB | 16.37 TiB | 16,763 GB | −6.8% |
| 20 TB | 18.19 TiB | 18,626 GB | −6.8% |
| 22 TB | 20.01 TiB | 20,488 GB | −6.8% |
| 24 TB | 21.83 TiB | 22,351 GB | −6.8% |
How to Use This Table for NAS Planning
Find the drive size you are buying and multiply by your number of data drives (after RAID parity). For RAID 5 with four 4TB drives, multiply 3.64 TiB by 3 (data drives) = 10.91 TiB before system and filesystem overhead.
The RAID calculator does this automatically for all 8 NAS ecosystems, including system overhead, filesystem metadata, and snapshot reserves specific to your brand.
TB vs TiB: Which Should I Use?
Your NAS's Storage Manager will show capacity in TiB (or GB using 1 GiB = 1,024 MiB). This is the binary standard. Drive packaging and product listings use TB (decimal). Both are correct; they measure the same physical space with different definitions.
When comparing NAS configurations, always work in TiB to avoid confusion. The RAID calculator outputs results in both TiB and approximate GB for comparison.
Frequently Asked Questions
Is the 6.8% loss the same for every drive?
Yes. The loss is constant at exactly 6.8% regardless of drive size. This is because the ratio of 1,000,000,000,000 to 1,099,511,627,776 is fixed. Every drive from 1TB to 24TB loses exactly 6.8% to binary conversion.
Does SSD storage have the same issue?
Yes. SSDs use the same decimal TB convention as HDDs. A 1TB NVMe SSD delivers 0.91 TiB (931 GB). The same conversion applies. Some SSDs reserve additional space for wear levelling and over-provisioning, which further reduces usable capacity.
Why do some NAS units show capacity in GB rather than TiB?
Some NAS interfaces display storage in GiB (gibibytes) but label them as "GB" for simplicity. If your NAS shows a 4TB drive as approximately 3,726 GB, it is using binary GiB. Consistent with the TiB figures in this table. If it shows exactly 4,000 GB, it may be using decimal measurement.