Raid Size Calculator
See exactly how much usable space N drives give you at each RAID level before you buy or build. Enter the drive count, the size of one drive, and the RAID level: the tool reports usable capacity in both TB and TiB, raw capacity, parity/mirror overhead, storage efficiency, and fault tolerance, plus a comparison table across all five levels.
Assumes identical drives; mixed arrays are limited by the smallest drive. Usable figures are pre-filesystem: formatting and the OS reserve a few percent more.
Worked example, checked against this exact tool
Eight 4 TB drives in RAID 5: raw = 8 × 4 = 32 TB; usable = (8 − 1) × 4 = 28 TB (25.47 TiB); overhead = 4 TB of parity; efficiency = 87.5%; survives 1 drive failure. The same drives give 24 TB in RAID 6 (75%), 16 TB in RAID 10 (50%), 32 TB in RAID 0 (100%, no safety net), and 16 TB in RAID 1. Enter 8 drives of 4 TB and the tool’s comparison table reproduces every row.
The formulas
With N identical drives of size S:
RAID 0: N × S. Every byte is usable; any single failure destroys the array.
RAID 1: N/2 × S with the classic two-drive mirror (usable = one drive). More mirrors add redundancy, not space.
RAID 5: (N − 1) × S. One drive’s worth of distributed parity; survives any single failure.
RAID 6: (N − 2) × S. Two drives’ worth of parity; survives any two failures.
RAID 10: N/2 × S. Striped mirror pairs; survives at least one failure per pair. Requires an even drive count: this tool rejects odd counts, which the old version silently miscomputed.
TB vs TiB: where your “missing” space goes
Drive makers sell decimal terabytes (1 TB = 10¹² bytes); operating systems report binary tebibytes (1 TiB = 2⁴⁰ bytes ≈ 1.0995 TB). A “4 TB” drive therefore shows as about 3.64 TiB before you store a single file. The tool shows both, so the number on the box and the number in your OS finally agree. Nothing is wrong with your drives; the units just differ.
Mixed drive sizes and hot spares
Traditional RAID is limited by the smallest drive: three 4 TB drives plus one 2 TB drive in RAID 5 yields (4 − 1) × 2 = 6 TB usable, stranding 2 TB on each larger drive. If you upgrade gradually, a hybrid system (e.g. Synology SHR) wastes less. Hot spares sit outside the array entirely: a spare does not add usable space, it shortens the window you spend degraded after a failure. Count spares separately from the N drives you enter here.
RAID 5 and large drives: the rebuild caveat
RAID 5’s single parity assumes rebuilds succeed. With multi-terabyte drives, rebuilds take many hours, and an unrecoverable read error (URE) during rebuild can fail the array. That is why dual parity (RAID 6) or RAID 10 is the usual recommendation once drives exceed ~8 TB or the array holds irreplaceable data. RAID is availability, not backup: it keeps you running through a drive failure, it does not protect against deletion, ransomware, or fire.
What competing calculators miss
Several rivals compute the same five formulas but accept impossible configurations (odd drive counts for RAID 10) or report one unit and leave the TB/TiB confusion to the reader. The softonic tool is the most complete rival with mixed-drive and spare handling. This page’s edge: invalid RAID 10 configurations are rejected rather than miscomputed, TB and TiB are shown together, and the comparison table puts the capacity-vs-safety trade-off for your exact drives on one screen.
Frequently asked questions
How much usable space does RAID 5 give?
(N − 1) × drive size. Eight 4 TB drives give 28 TB usable at 87.5% efficiency, surviving one drive failure.
How much usable space does RAID 6 give?
(N − 2) × drive size. Eight 4 TB drives give 24 TB usable at 75% efficiency, surviving two drive failures.
How much usable space does RAID 10 give?
Half the raw capacity: N/2 × drive size. It needs at least 4 drives and an even count.
Why does RAID 10 need an even number of drives?
It is built from mirrored pairs striped together. An odd drive cannot form a pair, so controllers require even counts.
What is the difference between TB and TiB?
1 TB = 10¹² bytes (decimal, as drives are sold); 1 TiB = 2⁴⁰ bytes (binary, as operating systems report). 28 TB ≈ 25.47 TiB.
Can I mix drive sizes in RAID?
Traditional RAID uses the smallest drive as the baseline and strands the rest. Use identical drives, or a hybrid system designed for mixed sizes.
What is a hot spare?
A drive installed but idle until a failure, when it automatically takes over the failed drive’s role. It adds no usable space; it shortens rebuild time.
Is RAID a backup?
No. RAID protects against drive failure (availability). It does not protect against deletion, corruption, ransomware, theft, or disaster: keep real backups.
RAID 5 or RAID 6 for large drives?
RAID 6 for drives above ~8 TB or irreplaceable data: long rebuilds make a second failure during rebuild a real risk with single parity.
How many drives can fail in RAID 10?
At least one per mirror pair is guaranteed survivable; more can survive if failures land in different pairs. Losing both drives of any single pair fails the array.