Example Data Table
| Drives |
Disk Size |
Columns |
Parity |
Reserve |
Estimated Use |
| 6 |
8 TB |
3 |
Single |
10% |
About 28.22 TB |
| 8 |
12 TB |
4 |
Single |
10% |
About 63.50 TB |
| 10 |
16 TB |
5 |
Dual |
15% |
About 79.97 TB |
Formula Used
Raw capacity = Drive count × Drive size.
Aligned drives = floor(Drive count ÷ Columns) × Columns.
Aligned raw = Aligned drives × Drive size.
Parity factor = (Columns − Parity blocks) ÷ Columns.
Usable before reserve = Aligned raw × Parity factor.
Estimated usable = Usable before reserve − Rebuild reserve − Metadata allowance.
Efficiency = Estimated usable ÷ Raw capacity × 100.
How to Use This Calculator
- Enter the total number of planned drives.
- Enter the size of one drive in terabytes.
- Choose the column count used for striping.
- Select single or dual parity.
- Add rebuild reserve and metadata allowance.
- Enter a target usable capacity if planning expansion.
- Press calculate to see the result above the form.
- Use CSV or PDF export for saved planning records.
Why Parity Planning Matters
Storage Spaces parity can turn several disks into one resilient pool. The design looks simple, yet capacity math can surprise new builders. Raw drive size is not the same as usable space. Parity data, reserves, metadata, and column alignment all reduce capacity. This calculator exposes each layer before money is spent.
How the Estimate Works
The tool starts with the selected drive count and drive size. It then groups capacity by the chosen column count. Full columns are treated as aligned storage. Any remainder is reported as unaligned raw space. The parity factor is based on single or dual parity. Single parity uses one parity block per stripe. Dual parity uses two. The tool then subtracts rebuild reserve and metadata allowance.
Why Columns Matter
Columns control how data is striped across disks. More columns can improve throughput. They can also raise the minimum disk need. A column count that does not match the disk plan can leave capacity stranded. Keep the setting realistic for the pool layout. Use the warning message when the drive count is below the practical minimum.
Planning With Reserve
Parity pools need spare headroom. A rebuild can fail when the pool is too full. Reserve space also helps future expansion. Many home labs choose ten to twenty percent. Business pools often use stricter rules. The right value depends on backup policy, disk size, rebuild time, and risk tolerance.
Reading the Output
The usable estimate is a planning value. It is not a promise from the operating system. Real results can differ because of sector size, slab allocation, file system overhead, and mixed disk behavior. The CSV and PDF exports help save the assumptions. Share them with team members before buying drives or changing a live pool.
Example Use Case
A small office can compare six drives with eight drives. It can see whether dual parity is worth the capacity loss. This makes budget and risk conversations easier before purchase.
Best Practice
Avoid filling a parity pool to its limit. Keep backups outside the pool. Test recovery before trusting important data. Add drives in planned batches. Review resiliency settings after every hardware change. A clear estimate reduces surprises and supports safer storage decisions.
FAQs
What does this calculator estimate?
It estimates raw capacity, parity overhead, reserve space, metadata allowance, usable capacity, efficiency, and suggested drive count for a Storage Spaces parity plan.
Is the result exact?
No. It is a planning estimate. Actual usable capacity can differ because of allocation slabs, sector format, file system overhead, and real pool settings.
What is single parity?
Single parity stores one parity block per stripe. It usually offers better capacity efficiency than dual parity, but it gives less failure protection.
What is dual parity?
Dual parity stores two parity blocks per stripe. It reduces usable capacity, but it can improve protection against multiple disk failures.
Why does column count matter?
Column count controls striping width. It affects usable capacity, performance planning, alignment, and minimum practical drive count for the pool.
What is rebuild reserve?
Rebuild reserve is free capacity kept for repair operations. It helps the pool recover after a failed disk is replaced.
Why is unaligned raw space shown?
The calculator groups drives by full column sets. Extra drives outside those sets are shown as unaligned raw space for conservative planning.
Can I export the result?
Yes. Use the CSV button for spreadsheet records. Use the PDF button for a simple planning report.