RAID Storage Capacity

Trading Raw Drive Capacity for Redundancy

RAID trades some raw drive capacity for redundancy — how much you keep depends on the RAID level. Enter your drive count, each drive’s capacity, and a RAID level to find your usable capacity.

The Formula

$$\vD{\text{RAID 0}} = \vA{n} \times \vB{\text{Capacity}} \qquad \vD{\text{RAID 1}} = \vB{\text{Capacity}}$$ $$\vD{\text{RAID 5}} = (\vA{n}-1) \times \vB{\text{Capacity}} \qquad \vD{\text{RAID 6}} = (\vA{n}-2) \times \vB{\text{Capacity}}$$ $$\vD{\text{RAID 10}} = \frac{\vA{n}}{2} \times \vB{\text{Capacity}}$$

Worked Example

6 drives of 2 TB each in RAID 6:

1. $\vD{\text{Usable}} = (\vA{6}-2) \times \vB{2 \text{ TB}} = \vD{8 \text{ TB}}$.

Key Factors to Consider

• RAID is not a backup, even the redundant levels. RAID protects against a physical drive failure, but it doesn’t protect against accidental deletion, ransomware, file corruption, or a disaster affecting the whole array (like fire or theft) — a real backup strategy (ideally following the “3-2-1” principle: 3 copies, 2 different media types, 1 offsite) is still needed alongside RAID, not instead of it.
• Different RAID levels trade off capacity, redundancy, and performance in different ways. RAID 0 maximizes capacity and speed but offers zero redundancy (a single drive failure loses everything); RAID 1 mirrors data for strong redundancy at the cost of half the capacity; RAID 5 and 6 balance capacity against tolerating one or two drive failures respectively; RAID 10 combines mirroring and striping for both redundancy and speed at a bigger capacity cost — choosing the right level depends on which of capacity, speed, or redundancy matters most for a specific use case.
• RAID 5 carries a real, well-documented risk during a rebuild after one drive fails, especially with large modern drives. Rebuilding a RAID 5 array puts significant read stress on every remaining drive at once, and if a second drive fails (or develops an unreadable sector) during that rebuild window, the whole array can be lost — this risk grows with larger drive sizes, which is part of why RAID 6 (tolerating two simultaneous failures) is often preferred for bigger, more critical arrays.
• All drives in most RAID arrays are typically assumed to be the same capacity for this calculation to apply cleanly. Mixing drive sizes in most standard RAID configurations means the array is effectively limited by the SMALLEST drive’s capacity for each drive slot used — this calculator assumes uniform drive capacity, matching the standard, most common real-world RAID setup.

Common Mistakes

• Treating RAID itself as a backup. As covered above, RAID protects against a drive failure, not against deletion, corruption, or a disaster that takes out the whole array — always keep a real backup regardless of RAID level.
• Picking RAID 5 for a large, critical array without considering the rebuild risk. For big modern drives, RAID 6 or RAID 10’s extra redundancy is often worth the additional capacity cost, especially where downtime or data loss would be expensive.
• Assuming usable capacity equals (number of drives) × (capacity per drive). That’s only true for RAID 0 — every redundant RAID level reserves some capacity for mirroring or parity, which this calculator accounts for automatically per level.

Useful to Know

• Comparing storage sizes across different units (GB, TiB, etc.)? Unit Converter converts between measurement units, including digital storage.
• Want to estimate how long a file transfer will take on your network? Bandwidth (Download Time) Calculator calculates download/transfer time from file size and connection speed.