Synology RAID Capacity Calculator
Estimate usable capacity for SHR, SHR-2, RAID 0/1/5/6/10, JBOD, and Basic using your own drive sizes.
What this Synology RAID calculator helps you answer
When building a NAS, the biggest question is simple: How much usable storage do I really get? Raw disk capacity is never the same as usable capacity once parity, mirroring, and overhead are included. This calculator gives a fast planning estimate so you can compare layouts before buying drives.
- Compare SHR vs RAID 5/6/10 for total usable space.
- See how mixed drive sizes affect traditional RAID arrays.
- Estimate practical capacity after a reserve percentage.
- Review fault tolerance so you understand risk during rebuilds.
How Synology RAID types differ
SHR (Synology Hybrid RAID)
SHR is usually the most flexible option for home and small office users. It is designed to use uneven disk sizes more efficiently than classic RAID in many scenarios. With SHR, you typically get one-disk fault tolerance while still taking advantage of larger replacement drives over time.
SHR-2
SHR-2 adds two-disk fault tolerance. You lose more capacity than SHR, but gain stronger resilience for larger arrays where rebuild windows are longer and unrecoverable read error risk is higher.
Classic RAID levels
- RAID 0: Maximum capacity and speed, zero redundancy.
- RAID 1: Mirroring, strong redundancy, low efficiency.
- RAID 5: One-disk parity protection, good efficiency.
- RAID 6: Two-disk parity protection, safer for large arrays.
- RAID 10: Mirrored pairs striped together, strong performance and redundancy.
Important planning tips before you buy drives
1) Mixed sizes are usually constrained by the smallest disk in classic RAID
RAID 5/6/10 and RAID 1 commonly treat larger disks as if they were the size of the smallest member. That can leave significant space unused. If you upgrade gradually, SHR can be easier to grow efficiently.
2) Fault tolerance is not backup
RAID protects against drive failure, not accidental deletion, malware, sync mistakes, or fire/theft. Keep a separate backup strategy (3-2-1 is a strong baseline).
3) Leave working headroom
Performance and stability are usually better when the volume is not near 100% full. Many users keep 10-20% free for snapshots, index jobs, and healthy filesystem behavior.
Capacity formulas used by this calculator
This tool is intended for planning estimates. It applies standard quick formulas:
- RAID 0 / JBOD / Basic: usable ≈ sum of all drives.
- RAID 1: usable ≈ smallest drive (full mirror set).
- RAID 5: usable ≈ (N - 1) × smallest drive.
- RAID 6: usable ≈ (N - 2) × smallest drive.
- RAID 10: usable ≈ (N / 2) × smallest drive.
- SHR: usable ≈ total - largest drive equivalent (for protection).
- SHR-2: usable ≈ total - two largest drive equivalents.
Actual DSM results can vary slightly due to partitioning, binary vs decimal display (TiB vs TB), and implementation details. Use this as a practical estimate, not an exact provisioning contract.
Quick example
If you install 4 TB, 4 TB, 8 TB, and 8 TB drives:
- RAID 5: estimated usable = 12 TB (limited by 4 TB smallest disk).
- SHR: estimated usable = 16 TB (often better with mixed sizes).
- SHR-2: estimated usable = 8 TB with two-disk protection.
Bottom line
If your Synology NAS will use mixed drive capacities or staged upgrades, SHR is often the easiest and most space-efficient starting point. If you need stronger protection on larger arrays, consider SHR-2 or RAID 6. Use the calculator above to test your own disk combinations before committing to hardware.