RAID Capacity & Rebuild Time Calculator

Capacity is only one part of a NAS choice

A drive bay used by a spare is unavailable for normal storage. Parity or mirror copies also consume space, and a free-space allowance reduces the amount you plan to fill. The outputs keep those deductions separate from TB-to-TiB conversion. This tool supports conventional equal-drive arrays, not mixed-size optimized pools, distributed spares, compression, deduplication, or nested layouts other than two-copy RAID 10.

Method: active members, parity, and reserve

Active drives A = installed drives - hot spares. With drive size S TB, RAID 5 capacity is (A - 1)S and needs at least 3 active drives. RAID 6 capacity is (A - 2)S and needs at least 4. RAID 10 capacity is AS / 2 and needs an even number of at least 4 active drives. An invalid layout is rejected rather than reported as zero usable space.

Data budget = array capacity x (1 - reserve / 100). Unused budget = max(0, budget - data); excess data = max(0, data - budget). Decimal TB means 10^12 bytes, while TiB means 2^40 bytes. Thus TiB = TB x 10^12 / 2^40. Binary unit conversion does not remove bytes; formatting and metadata can remove usable space and are not included here.

Rebuild-time assumptions

This deliberately models reconstruction of one full member, regardless of your entered stored data: hours = S x 1,000,000 / (effective MB/s x 3,600). Required effective rate for a time budget H is S x 1,000,000 / (H x 3,600). Rate means bytes successfully reconstructed on that member per second, not all survivor reads combined.

Some systems rebuild only allocated regions; others scan the whole member. Controller limits, disk zones, user workloads, throttling, errors, and consistency checks alter the effective rate. A faster interface does not guarantee a faster rebuild. Try a slower observed rate to see how the degraded window changes. The result is a conditional time estimate, not a recovery deadline.

Worked example: six 8 TB drives, one spare, RAID 6

There are 5 active members and 48 TB installed. Dual parity leaves 24 TB array capacity, about 21.8279 TiB. A 20% reserve creates a 19.2 TB data budget; 18 TB of planned data leaves 1.2 TB budget unused. Rebuilding a full 8 TB member at an effective 100 MB/s takes about 22.2222 hours. A 24-hour budget needs about 92.5926 MB/s effective progress.

Protection is not backup

RAID 5 tolerates one member failure; RAID 6 tolerates any two. RAID 10 can lose one drive per mirror pair, but losing both drives in a pair loses the array. A spare can shorten the wait to begin recovery but does not change those failure rules. RAID does not protect against deletion, ransomware, theft, or every controller and filesystem failure. Keep separate backups and test restores.

Sources and supported bounds

Seagate's Business Storage NAS OS RAID Modes manual documents the conventional layouts and backup limitations (undated product manual; its product-specific mode restrictions are not general guarantees). Synology's RAID calculator illustrates why mixed-drive and vendor-specific layouts need a separate model. This page does not implement SHR.

Bounds: 2-32 installed drives, 0-30 spares subject to layout validity, 0.1-100 TB per drive, 0-90% reserve, 0-3,200 TB planned data, 1-2,000 MB/s effective rate, and 0-1,000 hours for the inverse plan. All capacity and timing assumptions should be checked against your actual system.

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Last updated: 2026-10-09. Planning estimate, not a storage reliability guarantee.