RAID Calculator: Usable Capacity and Drives Needed
Work out the usable capacity of a RAID 0, 1, 5, 6, 10, 50 or 60 array, or enter the capacity you need and get the number of drives. Results assume identical drives and are shown in decimal TB (as drives are sold) and binary TiB (as most operating systems report space), before filesystem overhead.
Example: 200TB usable in RAID 6
- 11 × 24TB in RAID 6 = 216TB usable (196.45 TiB), 82% of raw capacity; survives 2 drive failures.
- 200TB usable from 24TB drives in RAID 6 takes 11 drives (216TB usable).
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.
RAID levels compared
| Level | Usable capacity | Minimum drives | Drive failures survived | Disk I/Os per random write |
|---|---|---|---|---|
| RAID 0 | n × S | 2 | None | 1 |
| RAID 1 | S | 2 | n − 1 | 2 |
| RAID 5 | (n − 1) × S | 3 | 1 | 4 |
| RAID 6 | (n − 2) × S | 4 | 2 | 6 |
| RAID 10 | (n ÷ 2) × S | 4 | 1 (up to 1 per mirrored pair) | 2 |
| RAID 50 | (n − g) × S | 6 | 1 (up to 1 per group) | 4 |
| RAID 60 | (n − 2g) × S | 8 | 2 (up to 2 per group) | 6 |
n = drives in the array, S = drive size, g = number of RAID 5 or RAID 6 groups.
Drives needed with 24TB drives
| Usable target | RAID 5 | RAID 6 | RAID 10 | RAID 60 |
|---|---|---|---|---|
| 50TB | 4 drives (72TB) | 5 drives (72TB) | 6 drives (72TB) | 8 drives (96TB) |
| 100TB | 6 drives (120TB) | 7 drives (120TB) | 10 drives (120TB) | 10 drives (144TB) |
| 200TB | 10 drives (216TB) | 11 drives (216TB) | 18 drives (216TB) | 14 drives (240TB) |
| 500TB | 22 drives (504TB) | 23 drives (504TB) | 42 drives (504TB) | 26 drives (528TB) |
RAID 60 shown with 2 groups. Hot spares not included.
How usable capacity is calculated
Every level gives up some raw capacity for redundancy. With n identical drives of size S:
RAID 0: n × S, no redundancy. RAID 1: S, every drive holds a full copy. RAID 5: (n − 1) × S, one drive of parity. RAID 6: (n − 2) × S, two drives of parity. RAID 10: (n ÷ 2) × S, mirrored pairs. RAID 50: (n − g) × S and RAID 60: (n − 2g) × S, where g is the number of RAID 5 or RAID 6 groups striped together.
Why the operating system shows less than the label
Drive makers count in decimal: a 24TB drive holds 24 × 10¹² bytes. Most operating systems count in binary units, so the same drive appears as 21.83 TiB. Nothing is missing; it is the same number of bytes in a different unit. The filesystem then reserves some space for its own metadata, which is why the calculator reports capacity before filesystem overhead.
Hot spares
A hot spare is an installed drive that sits idle until another drive fails, then the controller rebuilds onto it automatically. It adds no capacity, so the calculator counts spares on top of the drives in the array.
Mixed drive sizes
Traditional RAID treats every drive as the size of the smallest drive in the array; the extra space on larger drives goes unused. Plan arrays with drives of the same capacity.
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
How many 24TB drives do I need for 200TB usable in RAID 6?
11 drives. RAID 6 uses two drives' worth of capacity for parity, so 11 × 24TB gives 9 × 24TB = 216TB usable (196.45 TiB), before filesystem overhead. Add any hot spares on top.
What is the usable capacity of RAID 5 with 4 × 8TB drives?
24TB (21.83 TiB). RAID 5 keeps (n − 1) drives of data: 3 × 8TB. The array survives one drive failure.
Does a hot spare add capacity?
No. A hot spare is idle until a drive fails, then the array rebuilds onto it. Count it in the drives you buy, not in usable capacity.
Why does my array show less space than the calculator?
Two reasons. The operating system usually shows binary TiB rather than the decimal TB drives are sold in (24TB appears as 21.83 TiB), and the filesystem and RAID controller reserve a small amount of space for their own metadata.
Does this calculator work for Synology, QNAP or ZFS?
For standard RAID 0, 1, 5, 6 and 10 on any NAS or server controller, yes: the capacity math is the same. Synology Hybrid RAID (SHR) and ZFS RAID-Z are calculated differently (SHR can use mixed drive sizes; RAID-Z adds allocation overhead that depends on the pool layout), so use the vendor’s own tool for those.
How many drives can fail in RAID 5, 6 and 10?
RAID 5 survives 1 drive failure, RAID 6 survives any 2, and RAID 10 survives at least 1 and up to one per mirrored pair. RAID 0 survives none.
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
- TB vs TiB: Why a 24TB Drive Shows 21.8 TiB
- 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
Related
Updated 2026-10-22.