RAID Calculator & Storage Planner
Calculate RAID capacity and fault tolerance, or plan the number, size and type of drives your storage needs — for RAID 0, 1, 5, 6 and 10.
This tool runs entirely in your browser. Anything you type here is never sent to a server.
- Raw capacity
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- Usable capacity
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- Capacity used for redundancy
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- Storage efficiency
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- Minimum number of drives
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- Fault tolerance
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Drive manufacturers advertise capacity in decimal TB (1 TB = 1,000,000,000,000 bytes), but operating systems and NAS interfaces usually report it in binary TiB (1 TiB = 2⁴⁰ bytes) — so the same drive shows a smaller number there. Nothing is missing; it is the same capacity in a different unit.
Possible configurations
No valid configuration fits these requirements.
Try adjusting one or more of the following:
- Increase the available drive bays.
- Reduce the capacity reserve.
- Allow a lower minimum fault tolerance.
- Consider larger drive sizes.
Check hardware compatibility before purchasing drives.
This tool calculates RAID capacity and storage configurations, but it cannot verify compatibility with your specific NAS, server or RAID controller.
What should I check?
- Maximum drive capacity supported by the NAS/controller
- Number of available bays
- SATA / SAS / NVMe support
- 2.5" / 3.5" form factor
- HDD / SSD support
- RAID levels supported by the NAS/controller
- Manufacturer compatibility list (HCL)
- Controller/NAS firmware requirements
- 512e / 4Kn sector format, when relevant
- Whether a bay is needed for the hot spare
For HDDs: check whether the drive uses CMR or SMR and follow the manufacturer/controller/NAS guidance for RAID use — not every SMR drive is unusable, but some RAID/NAS setups handle it poorly.
For SSDs, especially in write-intensive workloads: check endurance specifications such as TBW or DWPD.
RAID is not a backup
RAID can improve availability and tolerate certain drive failures, but it does not protect against:
- Accidental deletion
- Ransomware
- Filesystem or data corruption
- Theft
- Fire
- Catastrophic server/NAS failure
- Loss of the complete array
How RAID capacity is calculated
Each RAID level combines several physical drives differently to balance performance, capacity and fault tolerance. Raw capacity is the sum of every drive; usable capacity is what remains available for data once the space set aside for redundancy is subtracted. Switch to Plan Storage above to work the other way around: tell it how much usable capacity you need and it will suggest valid drive/RAID combinations.
Example
For 4 × 2 TB drives in RAID 5, raw capacity is 8 TB, usable capacity is 6 TB, and 2 TB go to redundancy.
Supported levels
- RAID 0: Pure striping, no redundancy. 100% storage efficiency and the best performance, but zero fault tolerance — a single drive failure loses everything on the array. Minimum 2 drives. Typical use: scratch or temporary data where speed matters more than safety.
- RAID 1: Mirrors data across every drive. Usable capacity always equals a single drive's capacity, however many drives you add — efficiency drops as the mirror grows, but guaranteed fault tolerance rises with it (N-1 of N drives). Minimum 2 drives. Typical use: small, critical volumes such as a boot drive or database log.
- RAID 5: Striping with distributed parity: roughly (N-1)/N storage efficiency, and it guarantees the failure of exactly 1 drive. Minimum 3 drives. Typical use: general-purpose file storage that needs some redundancy without giving up too much capacity.
- RAID 6: Like RAID 5 but with double parity: roughly (N-2)/N storage efficiency, guaranteeing the failure of 2 drives. Minimum 4 drives. Typical use: larger arrays or long rebuild times, where surviving 2 simultaneous failures matters.
- RAID 10: Combines mirroring and striping (mirrored pairs, then striped): 50% storage efficiency. It guarantees the failure of 1 drive, and can survive more if every failure lands in a different mirror pair — that extra tolerance is never a guarantee. Requires an even number of drives, minimum 4. Typical use: workloads needing both strong performance and fault tolerance, such as databases or virtualization.
Frequently asked questions
Why does RAID 10 require an even number of drives?
Because drives are organized into mirrored pairs that are then combined with striping; an odd number would leave one drive without a partner.
Is a RAID 10 array with 6 drives guaranteed to survive 3 drive failures?
No. RAID 10 only guarantees surviving 1 drive failure. With 6 drives (3 mirror pairs), it can survive up to 3 failures in the best case — but only if each failure hits a different pair. If 2 failures hit the same pair, the array is lost, so 3 is a best case, never a promise.
Why does my NAS or operating system show less capacity than the drive label promises?
Drive manufacturers advertise capacity in decimal TB (1 TB = 1,000,000,000,000 bytes), while operating systems typically report it in binary TiB (1 TiB = 2⁴⁰ bytes ≈ 1.1 TB). A 24 TB drive is genuinely about 21.83 TiB — no capacity is missing, it is the same number in a different unit.
Does RAID replace backups?
No. RAID protects against the physical failure of one or more drives, but not against human error, ransomware or accidental deletion. An independent backup strategy is still necessary.