RAID 10 Capacity Calculator
Enter your drive configuration to estimate usable capacity, overhead, and fault tolerance.
RAID 10 requires an even number of active drives and at least 4 active drives.
What this RAID 10 calculator does
RAID 10 (also called RAID 1+0) combines mirroring and striping. In plain terms, every block of data is duplicated on a partner drive (the mirror), and then data is striped across those mirrored pairs for speed. This tool helps you quickly estimate how much storage you can actually use once that redundancy is accounted for.
The calculator gives you four practical outputs:
- Installed raw capacity: all physical drives combined, including spares.
- Active RAID raw capacity: drives participating in the RAID set.
- Usable RAID 10 capacity: data space after mirroring overhead.
- Fault tolerance range: minimum guaranteed and theoretical maximum failures.
RAID 10 formula explained
If all drives are the same size, the usable capacity formula is straightforward:
Usable Capacity = (Active Drives ÷ 2) × Smallest Drive Capacity
In most home lab and SMB setups with equal-size drives, that means RAID 10 gives you about 50% of active raw capacity as usable storage. If you reserve hot spare drives, they improve recovery behavior but do not add usable space until they are activated.
Quick example
With 8 drives at 4 TB each and no spares:
- Installed raw = 8 × 4 TB = 32 TB
- Usable RAID 10 = (8 ÷ 2) × 4 TB = 16 TB
- Efficiency = 50%
How to choose a drive count
Start with your required usable storage, then double it for RAID 10 overhead, and finally add spares if uptime is critical. For example, if you need around 20 TB usable and are using 4 TB drives:
- Required active raw ≈ 40 TB
- Active drives needed = 40 ÷ 4 = 10 drives (must be even)
- Add 1–2 hot spares for faster recovery, depending on risk tolerance
Performance and reliability notes
Why RAID 10 is fast
RAID 10 can deliver strong read performance because reads can be serviced from either mirror member, and stripe-level parallelism helps with throughput. Writes are generally faster than parity RAID in many workloads because no parity calculations are required.
Failure behavior
RAID 10 can survive multiple drive failures only if failed drives are in different mirror pairs. If both drives in the same mirror pair fail, the array fails. That's why the calculator reports both:
- Minimum guaranteed survival: 1 drive failure
- Theoretical maximum survival: up to one failed drive per mirror pair
Common mistakes to avoid
- Using an odd number of active drives in RAID 10.
- Mixing drive sizes and expecting full larger-drive capacity.
- Confusing raw capacity with usable capacity.
- Assuming RAID replaces backups. It does not.
RAID 10 calculator FAQ
Does RAID 10 always provide exactly 50% usable capacity?
For equal-size active drives, yes: usable is effectively half of active raw. With mixed capacities, practical usable space is constrained by the smallest drives or controller behavior.
Should I add hot spares?
If availability matters, hot spares are often worth it. They do reduce immediate usable capacity, but they can reduce risk during rebuild windows and speed recovery after a failure.
Is RAID 10 better than RAID 5/6?
"Better" depends on workload and priorities. RAID 10 usually favors performance and rebuild simplicity, while parity RAID levels often offer better capacity efficiency. Choose based on IOPS needs, rebuild risk, and budget.
Final planning tip
Use this calculator for sizing, then verify your final design against your controller or software RAID implementation. Vendor-specific behavior, filesystem overhead, and binary vs decimal reporting can slightly change the numbers you see in production.