CalcAgent

RAID Usable Capacity Calculator

The calculator follows the standard RAID capacity formulas: total raw capacity equals (number of drives) × (capacity per drive). For RAID 5 and 6, usable data = raw – (parity disks / total drives × raw); for RAID 10, usable data is half the raw space. After subtracting reserved over‑provisioning space, the final usable capacity is shown.

Choose the RAID configuration you plan to use for your array. The calculator uses this choice to determine how many disks’ worth of space is used for parity or mirroring.
Enter how many identical drives will be in the RAID array.
Specify the usable capacity of a single drive (e.g., 1,500 GB or 3,000 GB). Use the same unit for all drives.
Amount of extra storage you intend to reserve (e.g., 5% or 10%). A higher value reduces the usable capacity reported here.

What it is

This calculator determines how much net storage a user can actually rely on when configuring a RAID array. In practice, each RAID level consumes part of the raw disk capacity for parity or mirroring to provide redundancy. For instance, RAID 5 reserves space equal to one drive, while RAID 10 splits the array into mirrored pairs, effectively halving usable data. The tool also accounts for an optional over‑provisioning reserve – a small percentage of capacity purposely withheld to improve performance and longevity on high‑density storage media. By presenting raw total, parity usage, and final usable figure in one view, architects can quickly assess whether the target volume meets business demands before committing hardware.

How to use it

Select your desired RAID level from the drop‑down menu (RAID 0, 5, 6 or 10). Enter how many identically sized drives will comprise the array. Specify each drive’s usable capacity in gigabytes. Optionally define an over‑provisioning percentage – a small value like 5 % tightens performance but reduces reported space. Press "Calculate" and read three results: total raw capacity, space reserved for parity or mirroring, and the final usable volume after reserving the overhead.

Worked example

In this example we configure a RAID 5 array with five 2,000 GB drives. The raw total capacity is 5 × 2,000 = 10,000 GB.

For RAID 5 the usable fraction is (drives – 1)/drives = (5 ‑ 1)/5 = 0.8. Multiplying yields a raw data space of 10,000 × 0.8 = 8,000 GB. The parity occupies the remaining 2,000 GB (the capacity of one drive).

We want to reserve 5 % for over‑provisioning; thus we keep only 95 % of the usable data: 8,000 × 0.95 = 7,600 GB. The calculator therefore reports a final usable capacity of 7,600 GB, raw total 10,000 GB and parity 2,000 GB.

Inputs

  • RAID Level: 5
  • Number of Drives: 5
  • Capacity per Drive: 2000
  • Over‑Provisioning (%): 5

Result

  • Usable Capacity: 7600
  • Raw Total Capacity: 10000
  • Parity / Mirrored Space Used: 2000

Frequently asked questions

Can I input drive capacities in terabytes instead of gigabytes?

This tool expects the capacity unit to be expressed consistently in gigabytes. If you have drives sized in terabytes, convert them by multiplying by 1,024 before entering the value.

For example, a 2 TB disk is 2 × 1,024 = 2,048 GB, so you would enter 2048 as the drive capacity.

Does the calculator consider file‑system block size or journaling overhead?

No. The formula focuses on parity and mirroring overhead that the RAID controller imposes. File‑system metadata uses a negligible fraction of the overall volume—usually well under one percent—and is omitted here for clarity. If you need precise figures, add an additional small percentage manually.

How should I decide what over‑provisioning value to use?

Typical performance‑oriented configurations reserve 5 %–10 % of raw data space. Higher reserves give the underlying flash SSDs more write headroom and can extend wear life, but they lower usable capacity. Match the over‑provisioning percentage to the workload’s write intensity: high‑write caches benefit from tighter provisioning.