Quick answer
RAID 10 stripes data across mirrored pairs. With equal-size drives, each complete pair contributes one drive's capacity: four 10 TB drives form two pairs and provide about 20 TB before filesystem or vendor reserves.
Failure tolerance depends on placement. Several drives can fail when no mirror pair loses both members; two failures in the same pair can stop the array. Confirm the exact layout and supported drive count in the controller or NAS documentation before purchase.
How to use the calculator
- Enter every installed drive, including any disk reserved as a dedicated hot spare.
- Use the smallest drive's advertised decimal-TB capacity when sizes differ.
- Choose hot spares so the remaining active-drive count is even; otherwise one active drive cannot complete a mirror pair.
- Add a filesystem allowance only if you want a planning deduction beyond the mirror copies and spares.
- Compare usable capacity, mirror pairs, spare capacity, unpaired capacity, and the failure warning together.
What each result means
- Estimated usable capacity: capacity contributed by complete mirror pairs after the filesystem allowance.
- Installed raw capacity: every entered drive multiplied by the smallest drive size, including hot spares.
- Mirror-copy capacity: the second copy inside each two-drive mirror.
- Unpaired active capacity: a drive left outside a complete pair when the active count is odd.
- Installed-capacity efficiency: usable capacity divided by all installed capacity, so spares and unpaired drives reduce the percentage.
Decisions this calculator helps with
Choose an even active-drive count
Subtract dedicated spares from installed drives. The remainder should be even so every active drive belongs to a two-drive mirror.
Price the capacity you can use
Compare usable TB with the full cost of active drives and spares, not only the raw capacity printed on each disk.
Plan for pair-specific failures
RAID 10 does not mean that any two drives may fail safely. Map the actual mirror pairs in your controller or NAS interface.
Keep the scenario
Share, copy, or print the inputs and results, then compare them with the RAID 6 vs RAID 10 guide.
What the capacity calculation separates
A single “half the raw capacity” shortcut hides hot spares, incomplete pairs, and filesystem deductions. This calculator reports each one separately so an eight-bay purchase can be compared with the actual active layout.
What changes the result most: complete mirror-pair count and the smallest drive size drive the estimate. Dedicated spares and an odd active-drive count reduce efficiency against all installed capacity.
- Active drives: installed drives minus dedicated hot spares.
- Complete pairs: active drives rounded down to a whole number of two-drive mirrors.
- Usable before allowance: one drive-size contribution from each complete pair.
- Final estimate: pair capacity minus the optional filesystem allowance.
RAID 10 capacity formula
Active drives = installed drives − dedicated hot spares.
Complete mirror pairs = floor(active drives ÷ 2).
Usable before filesystem allowance = complete mirror pairs × smallest drive size.
Estimated usable capacity = usable before allowance × (1 − allowance percentage ÷ 100).
The model leaves an odd active drive outside a pair and reports its capacity separately instead of quietly counting it as usable.
How RAID 10 drive failures work
Each mirror pair stores two copies of its data, and the stripe depends on every pair. The array can continue after one member of a pair fails. It may also continue after several simultaneous failures when each failed drive belongs to a different pair.
The array loses access when both members of any one mirror pair fail. That is why “two-drive fault tolerance” is not a safe universal description of RAID 10. Rebuild time, controller behavior, drive error rates, and pair placement are outside this capacity model; maintain a separate backup.
For a capacity and risk comparison, use the RAID 6 vs RAID 10 guide and the RAID 6 calculator.
Assumptions and limitations
- The model uses standard two-drive mirrors striped together; vendor hybrid layouts may allocate capacity differently.
- Every active member is treated as the size of the smallest entered drive. Use the mixed-drive RAID calculator when disks differ.
- Drive size is entered and displayed in manufacturer-style decimal TB. Operating systems may report a smaller-looking TiB value; see TB vs TiB explained.
- Dedicated hot spares reserve entire drives and do not contribute normal usable capacity.
- The filesystem allowance is a user-selected planning deduction, not a prediction of a specific NAS, controller, filesystem, snapshot policy, or reserved system partition.
- This calculator does not estimate IOPS, throughput, rebuild duration, unrecoverable read errors, compression, deduplication, or backup needs.
Worked RAID 10 examples
| Scenario | Pair accounting | Estimated result |
|---|---|---|
| 4 × 10 TB, no spare, 0% allowance | 4 active drives = 2 complete pairs | 20 TB usable from 40 TB installed; 50% efficiency |
| 8 × 12 TB, 1 spare, 0% allowance | 7 active drives = 3 pairs + 1 unpaired drive | 36 TB usable, 12 TB spare, 12 TB unpaired; change the layout to an even active count |
| 6 × 12 TB, no spare, 5% allowance | 6 active drives = 3 complete pairs | 36 TB before allowance; 34.2 TB after a 1.8 TB deduction |
Common mistakes when planning RAID 10
- Entering only active drives and forgetting that a purchased hot spare still occupies a bay and adds cost.
- Leaving an odd number of active drives after subtracting spares, then assuming every drive contributes to a mirror pair.
- Using the largest disk size in a mixed array instead of the smallest member's effective size.
- Describing RAID 10 as safe from any two-drive failure without checking which disks share a mirror.
- Treating RAID redundancy as a substitute for backups, snapshots, or off-site copies.
RAID 10 calculator FAQ
How much usable space do four 10 TB drives provide in RAID 10?
Four equal 10 TB drives form two mirror pairs. Each pair contributes 10 TB, so the estimate is 20 TB before filesystem and vendor reserves.
Can RAID 10 use an odd number of active drives?
A standard two-drive-mirror layout needs an even active-drive count. This calculator forms all complete pairs, reports the remaining drive as unpaired, and warns that the layout should be changed.
How many drive failures can RAID 10 survive?
It can survive one failed member in each mirror pair. Multiple drives may fail safely when they are in different pairs, but losing both members of any one pair can stop the array.
How do hot spares change RAID 10 capacity?
A dedicated hot spare occupies an installed bay but does not add normal usable capacity. Subtract spares before forming mirror pairs, and keep the remaining active-drive count even.
Why can the operating system show fewer units than this result?
Drive makers normally label capacity in decimal TB, while some systems display binary TiB. The bytes have not disappeared; the unit scale changed. The TB vs TiB guide shows the conversion.
What does the filesystem allowance represent?
It is an optional planning deduction from mirror-pair capacity. It can represent filesystem or system reserves, but it is not a vendor-specific prediction; use the product documentation for the exact implementation.
Sources and assumptions for the capacity model
References used for the terminology and unit model:
- Capacity basis: complete equal-size two-drive mirrors contribute one drive's capacity per pair.
- Failure statement: one member of each pair may fail; loss of both members in a pair loses that stripe member.
- Excluded factors: controller metadata, vendor-reserved partitions, rebuild behavior, performance, snapshots, compression, and backup policy.
- Method date: July 22, 2026. Calculation method: RAID10-1.1.
Updated: July 22, 2026.
What to do next
Use the NAS capacity calculator to compare the result with a target free-space requirement, then compare the layout with RAID 6 vs RAID 10. If the output differs from your controller, report an issue with the exact inputs and product model.