Is RAID 5 safe with 12TB or larger drives?

No. At 12TB per drive with a typical consumer HDD URE spec of 1 in 10^14 bits read, a RAID 5 rebuild has a ~62% probability of hitting an unrecoverable read error — meaning data loss during rebuild. Enterprise drives with 10^15 URE spec reduce this to ~8%, but RAID 6 (double parity) or erasure coding is the minimum safe configuration for large drives. See the RAID 5 Is Dead research briefing for the full math.

How long does a RAID rebuild take?

At 200 MB/s sustained rebuild speed (typical for 7200 RPM HDDs), a 20TB drive takes about 28 hours to rebuild. At 500 MB/s (enterprise SSD), the same rebuild takes ~11 hours. RAID 6 rebuild is only 13% slower than RAID 5 at 20TB scale — the real difference is the URE survival probability, not rebuild time. During rebuild, the array runs in degraded mode with no parity protection, making the window as short as possible critical.

RAID 10 vs RAID 6 — which should I choose?

RAID 10 (striped mirrors) gives better write IOPS (no parity calculation overhead), faster rebuilds (just copy the surviving mirror), and 50% usable capacity regardless of drive count. RAID 6 gives better usable capacity (N-2 drives), similar read IOPS, but slower writes and rebuilds. For databases and write-heavy workloads, RAID 10. For bulk storage and archives, RAID 6. At 8+ drives, RAID 6's capacity advantage grows significantly.

RAID Configuration Calculator — Capacity, IOPS, Rebuild Time, MTTDL

RAID Configuration Calculator

RAID 5 rebuild URE probability — the second disk failure during rebuild is math, not luck. A 4 TB consumer HDD with 1 in 10¹⁴ URE rate: during RAID 5 rebuild, you read 28 TB. Probability of hitting an unrecoverable read error: north of 90%. Compare RAID 0/1/5/6/10 across usable capacity, IOPS, rebuild time, and MTTDL. The rebuild window is your vulnerability surface.

📐 Drive Configuration

Configure drive count, capacity, type (HDD/SSD), and RPM. The engine computes usable capacity, IOPS, rebuild time, and MTTDL per RAID level. Flags URE risk when rebuilds exceed safe windows.

Number of Drives

Single Drive Capacity (TB)

Drive Type

Stripe Size (KB)

Workload Pattern

RAID Level

📊 Selected RAID Configuration

⚖️ All RAID Levels Compared

Side-by-side comparison of all five RAID levels with your current drive configuration.

RAID Calculator: Beyond Simple Capacity Math

Most RAID calculators answer one question: "How much usable space?" This one estimates IOPS performance under different workloads, rebuild time (make-or-break for large drives where rebuilds run for days), and MTTDL (Mean Time To Data Loss). The rebuild window is your vulnerability surface — quantify it.

Why Rebuild Time Matters in 2026

A 20 TB HDD in a RAID 5 array can take 36–72 hours to rebuild. During that window, a second drive failure (URE — Unrecoverable Read Error) causes complete data loss. This is why RAID 5 is increasingly discouraged for large-capacity HDD arrays, and why RAID 6 (double parity) or RAID 10 are the recommended configurations for arrays with drives larger than 2 TB. Our calculator flags this risk explicitly.

IOPS Estimation Methodology

We estimate IOPS using the standard formulas:

RAID Level	Read IOPS	Write IOPS	Write Penalty
RAID 0	N × Single Drive IOPS	N × Single Drive IOPS	1×
RAID 1	N × Single Drive IOPS	(N/2) × Single Drive IOPS	2×
RAID 5	N × Single Drive IOPS	(N/4) × Single Drive IOPS	4×
RAID 6	N × Single Drive IOPS	(N/6) × Single Drive IOPS	6×
RAID 10	N × Single Drive IOPS	(N/2) × Single Drive IOPS	2×

📎 Embed This Calculator

Embed the RAID calculator on your team wiki, internal docs, or storage planning page. Zero dependencies. Zero tracking. Pure client-side math.

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