RAID capacity calculator

Add your HDDs or SSDs, then compare two RAID types using the same drives. See usable, protection and unused capacity as you build your pool.

Choose your drives

Nominal drive capacity · TB

HDD SSD

Use one media type per pool. Switching HDD / SSD changes the quick choices only; existing drive capacities stay the same.

Quantity to add

Click a capacity to add HDDs

Or enter capacity per drive (TB)

Add up to 24 drives, each from 0.001 to 1,000 TB.

Selected drives

Start with a capacity above, or load an example drive set.

Replace drives with an example:

Compare RAID capacity

Updates automatically as you edit

Comparison A

Comparison B

Defaults show theoretical capacity before system and filesystem deductions. SHR / SHR-2 and RAID TP require supported systems. RAID 10 guarantees at least one drive failure; additional tolerance depends on mirror pairs.

Additional assumptions and system deductions

RAID 0 with mixed drive sizesMatch your controller or software. Full-sum capacity is not universal for RAID 0.

System reserve per drive (GB)Deducted from each drive before RAID. Must be less than the smallest drive capacity.

Filesystem deduction from usable space (%)0–50% of capacity after RAID. This is your planning assumption, not a verified device allowance.

Shared links contain only drive media, capacities and calculation settings. No customer details are included.

sync.co.th

RAID capacity estimate

New-pool planning · settings and results

Understand the capacity before choosing RAID

Let N be the drive count and S the smallest drive capacity. These formulas give usable capacity before system and filesystem deductions. Mixed drive sizes can leave unused excess capacity.

RAID 0: Sum of all drives, or N × S under the selected policy · 2+ drives · no failure protection
RAID 1: S · 2–4 drives in this tool’s scope · full mirroring
RAID 5 / 6 / TP: (N − 1) × S / (N − 2) × S / (N − 3) × S · at least 3 / 4 / 5 drives respectively
RAID 10: (N ÷ 2) × S · at least 4 drives, with an even drive count
SHR / SHR-2: Allocate capacity in tiers. SHR uses tiers on at least 2 drives with 1 share for protection; SHR-2 uses tiers on at least 4 drives with 2 shares for protection. A single-drive SHR pool has no failure protection.

Example: 4 + 4 + 8 + 8 TB gives RAID 5 with 12 TB usable, 4 TB for protection and 8 TB unused. SHR gives 16 TB usable, 8 TB for protection and no unused excess, before system deductions.

Units: 1 TB = 10¹² bytes · 1 TiB = 2⁴⁰ bytes. Protection capacity is mirror / parity space, not a hot spare. Calculations use unrounded values; only displayed figures are rounded.

RAID capacity questions

Can I combine different drive capacities?

Yes, for planning. RAID 1, 5, 6, TP and 10 size their members to the smallest drive, leaving excess capacity unused. SHR and SHR-2 allocate mixed sizes differently according to the number of drives in each capacity tier. Check supported drive types and device models before implementation.

Why is the TiB figure smaller than the drive’s TB rating?

TB is decimal: 1 TB equals 1,000,000,000,000 bytes. TiB is binary: 1 TiB equals 1,099,511,627,776 bytes. For example, 8 TB is about 7.28 TiB before RAID and system deductions. The unit conversion does not remove any bytes.

How many drive failures can RAID 10 tolerate?

RAID 10 guarantees tolerance of at least one drive failure. Further failures may be tolerated only when they affect different mirror pairs. Losing both drives in one pair can lose data, so half of all drives failing is not a guaranteed tolerance.

Does this equal the final usable capacity on a NAS?

This estimates a new pool. Defaults exclude system and filesystem deductions; you can enter both assumptions in the additional options. Actual capacity also depends on hardware, software, metadata and configuration. This tool does not confirm expansion eligibility, compatibility or performance. RAID is not a backup.

Primary references

Reviewed 5 November 2026. This independent model is not the official Synology calculator and can differ from device-specific system allowances.

Reference links open in a new tab.