Calculate usable drive capacity from advertised size, RAID level, or sector count and see the actual GB, GiB, TB, or TiB output in one place.
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Drive Capacity Formula
The drive capacity calculator uses different formulas depending on the selected mode: advertised vs actual capacity, RAID usable capacity, or sector-based capacity.
Advertised vs Actual Capacity
Bytes = A * U_d
GiB = Bytes / 1024^3
TiB = Bytes / 1024^4
- A = advertised drive size
- U_d = decimal bytes per advertised unit, where 1 GB = 1,000,000,000 bytes and 1 TB = 1,000,000,000,000 bytes
- Bytes = total raw byte count
- GiB = gibibytes, using 1024-based units
- TiB = tebibytes, using 1024-based units
RAID Array Capacity
RAID 0 = n * S
RAID 1 = S
RAID 5 = (n - 1) * S
RAID 6 = (n - 2) * S
RAID 10 = (n / 2) * S
- n = number of drives in the array
- S = size of each drive
- RAID 0 = striped capacity with no redundancy
- RAID 1 = mirrored capacity
- RAID 5 = one drive 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
Bytes = N_s * S_b
GB = Bytes / 10^9
TB = Bytes / 10^12
- N_s = number of sectors
- S_b = sector size in bytes
- Bytes = total raw capacity
- GB = decimal gigabytes
- TB = decimal terabytes
The advertised mode converts drive labels such as 1 TB or 500 GB into the binary units often shown by operating systems. The RAID mode estimates usable array capacity before filesystem overhead. The sectors mode multiplies sector count by sector size to get the raw byte capacity.
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 | 2, 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.
Bytes = 1 * 1,000,000,000,000
TiB = 1,000,000,000,000 / 1024^4
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.
RAID 5 = (4 - 1) * 4 TB
RAID 5 = 12 TB
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.
