Calculate RAID usable capacity, advertised decimal-to-binary drive size, or bytes from sector count, with parity loss, reserve allowance, GB/GiB, and TB/TiB results.
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Drive Capacity Formula
The drive capacity calculator uses different formulas depending on the selected mode: decimal/binary capacity, RAID usable capacity, or logical-sector capacity.
Advertised Decimal and Binary Capacity
- A = entered drive capacity; choose its actual decimal or binary unit
- U_d = bytes per selected unit: GB = 10⁹, TB = 10¹², PB = 10¹⁵, GiB = 2³⁰, TiB = 2⁴⁰ bytes
- Bytes = total raw byte count
- GiB = gibibytes, using 1024-based units
- TiB = tebibytes, using 1024-based units
RAID Array Capacity
- n = number of drives in the array
- S = size of each drive
- RAID 0 = striped capacity with no redundancy
- RAID 1 = one mirror set: every active member holds the same data, giving one drive’s capacity; controller support for more than two members varies
- RAID 5 = one drive’s worth of capacity used for parity
- RAID 6 = two drives’ worth of capacity used for parity
- RAID 10 = half the raw capacity is usable because drives are mirrored in pairs
Capacity From Sectors
- N_s = number of sectors
- S_b = sector size in bytes
- Bytes = total raw capacity
- GB = decimal gigabytes
- TB = decimal terabytes
The decimal/binary mode converts the same byte count between units; it does not measure lost capacity. RAID estimates use equal-sized active members and exclude hot spares and controller/filesystem overhead. The editable planning reserve is subtracted after RAID allocation. A single-drive RAID 0 virtual disk is a capacity-only case supported by some controllers, with no striping across members. The sectors mode multiplies logical sector count by logical bytes per sector: 512e uses 512 logical bytes, while 4Kn uses 4096. Enter a count, not the final LBA address.
Common Drive Capacity Unit Differences
| Label or Unit | Bytes | Used For |
|---|---|---|
| 1 GB | 1,000,000,000 bytes | Drive marketing and decimal capacity |
| 1 GiB | 1,073,741,824 bytes | Binary capacity shown by many operating systems |
| 1 TB | 1,000,000,000,000 bytes | Drive marketing and decimal capacity |
| 1 TiB | 1,099,511,627,776 bytes | Binary capacity shown by many storage tools |
RAID Capacity Reference
| RAID Level | Minimum Drives | Usable Capacity | Redundancy Note |
|---|---|---|---|
| RAID 0 | 1 | All drives | No redundancy |
| RAID 1 | 2 | One drive | Mirrored copy |
| RAID 5 | 3 | n – 1 drives | One drive can fail |
| RAID 6 | 4 | n – 2 drives | Two drives can fail |
| RAID 10 | 4, even number required | Half of raw capacity | Mirrored stripe sets |
Example Drive Capacity Calculations
Example 1: Advertised 1 TB Drive
You have a drive advertised as 1 TB.
The actual binary capacity is about 0.91 TiB, often shown by an operating system as roughly 0.91 TB even though the drive contains 1,000,000,000,000 bytes.
Example 2: RAID 5 With Four 4 TB Drives
You have 4 drives, each advertised as 4 TB, configured as RAID 5.
The usable RAID capacity is 12 TB before formatting and filesystem overhead. The raw capacity is 16 TB, with 4 TB used for parity.
Drive Capacity Calculator FAQ
Why does a 1 TB drive show as about 0.91 TB?
Drive makers use decimal units, where 1 TB equals 1,000,000,000,000 bytes. Many operating systems calculate capacity using binary units, where 1 TiB equals 1,099,511,627,776 bytes. The bytes are still present, but the displayed number is smaller because the unit is larger.
Does RAID usable capacity include formatting overhead?
No. RAID capacity is the usable array size before filesystem formatting, metadata, snapshots, reserved space, and operating system overhead. The final available space shown after creating a filesystem may be slightly lower.
Can drives of different sizes be used in the RAID formula?
This calculator assumes all drives are the same size. In many RAID setups, mixed-size drives are limited by the smallest drive in the array. For a mixed array, use the smallest drive size as the size of each drive to get a conservative estimate.
