TB vs TiB: Why a 24TB Drive Shows 21.8 TiB
Nothing is missing. Drive makers count in decimal units and many operating systems count in binary units, so the same number of bytes shows up as two different numbers.
The conversion
- 1 TB = 1,000,000,000,000 bytes (10¹²).
- 1 TiB = 1,099,511,627,776 bytes (2⁴⁰).
- 1 TB = 0.9095 TiB, so multiply TB by 0.9095 to get TiB.
Identical drives; decimal TB as drives are sold, binary TiB as most operating systems show it; before filesystem overhead. Hot spares are extra drives and add no capacity.
TB to TiB for common drive and array sizes
| Decimal (as sold) | Binary TiB | Binary GiB |
|---|---|---|
| 1 TB | 0.91 TiB | 931 GiB |
| 2 TB | 1.82 TiB | 1,863 GiB |
| 4 TB | 3.64 TiB | 3,725 GiB |
| 8 TB | 7.28 TiB | 7,451 GiB |
| 12 TB | 10.91 TiB | 11,176 GiB |
| 16 TB | 14.55 TiB | 14,901 GiB |
| 20 TB | 18.19 TiB | 18,626 GiB |
| 24 TB | 21.83 TiB | 22,352 GiB |
| 30.72 TB | 27.94 TiB | 28,610 GiB |
| 100 TB | 90.95 TiB | 93,132 GiB |
| 216 TB | 196.45 TiB | 201,166 GiB |
| 500 TB | 454.75 TiB | 465,661 GiB |
Which systems show which
Windows reports binary sizes but labels them "TB" and "GB", which is where most confusion starts. macOS has reported decimal sizes since Mac OS X 10.6. On Linux it depends on the tool: df -h shows binary units and df -H shows decimal.
Filesystem overhead is separate
The TB-to-TiB gap is only a change of unit. A filesystem then reserves some space for its own metadata, which lowers the free space you see a little further.
24TB drives for this array
In-stock 24TB drives from our catalogue, with the price for the 11 drives in the example above. Hot spares are extra.
- Western 0F62798 Western Digital 0F62798 Ultrastar DC HC580 24TB 7200RPM 6Gb/s SATA 512e $960.00 each · 11 drives $10,560.00
- Seagate ST24000NM007H Exos X24 24TB 7200RPM 12Gb/s SAS 512MB Cache 512e 3.5-Inch Hard $1,450.00 each · 11 drives $15,950.00
- Western WUH722424ALE604 Western Digital WUH722424ALE604 Ultrastar DC HC580 24TB 7200RPM $960.00 each · 11 drives $10,560.00
- Dell GPP63 Dell 0GPP63 / GPP63 24TB 7200RPM 12Gb/s SAS 512MB Cache 512e Hot-Pluggable $1,589.47 each · 11 drives $17,484.17
- Western WD240EDGZ Western Digital WD240EDGZ 24TB 7200RPM SATA 6Gb/s 3.5-inch Internal $960.00 each · 11 drives $10,560.00
- 400-BRRZ - Dell 24TB SAS 12Gb/s 7.2K 512e 3.5-inch LFF Brand New Hard Drive for PowerEdge $1,950.00 each · 11 drives $21,450.00
- Seagate ST24000NM000H Exos X24 24TB SATA 6Gb/s 3.5-Inch Hard Drive $987.37 each · 11 drives $10,861.07
- Western 0F62835 Western Digital 0F62835 Ultrastar DC HC580 24TB 7200RPM 12Gb/s SAS 512MB $2,068.42 each · 11 drives $22,752.62
Frequently asked questions
Why does my 24TB drive show about 21.8TB in Windows?
Windows counts in binary units (TiB) but labels them TB. 24 × 10¹² bytes is 21.83 TiB.
Is a TiB bigger than a TB?
Yes. 1 TiB is 1,099,511,627,776 bytes, about 1.0995 TB.
How do I convert TB to TiB?
Multiply by 0.9095. For example, a 216TB RAID 6 array is 196.45 TiB.
RAID calculators by level
- RAID calculator (all levels)
- RAID 0 calculator
- RAID 1 calculator
- RAID 5 calculator
- RAID 6 calculator
- RAID 10 calculator
- RAID 50 calculator
- RAID 60 calculator
RAID planning guides
- How Many Drives Can Fail in Each RAID Level?
- RAID Hot Spare: What It Does and How It Affects Capacity
- RAID Write Penalty by Level
- Best RAID Level for 2, 3, 4, 5, 6 or 8 Drives
- RAID 50 vs RAID 60
- RAID 6 vs RAID 60
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Updated 2026-10-22.