You chose your NAS storage RAID level, installed the drives—and the usable storage is still smaller than you expected. Where did it go? Beyond parity and mixed-drive limits, your NAS also reserves space for the system itself—UGOS Pro, for example, uses about 16 GB (≈15.3 GiB) for system partitioning. That’s why a RAID calculator is so useful: it estimates space utilization across different drive configurations and RAID types so you can set expectations early (and then confirm the final number in the built-in storage manager).

What a RAID Calculator Does

A RAID calculator translates raw drive specifications into realistic storage expectations. Enter the size and number of drives along with your RAID level, and the tool estimates usable capacity, redundancy, and overhead.

To begin, input details such as drive size (in terabytes), drive type (HDD or SSD), and RAID level (0, 1, 5, 6, or 10). Some tools also let you include hot spares for extra fault tolerance.

Keep in mind that RAID calculators focus on capacity, not overall system performance or rebuild risk. For a complete view, use them alongside other evaluation tools. It’s also worth understanding what happens when things go wrong—if a drive fails during or after setup, knowing the steps to recover a failed RAID array can save you from unnecessary data loss.

Key Factors in RAID Calculations

Drive Count and Size

The number and size of drives are the main factors affecting RAID calculator results. With identical drives, the math is simple: capacity follows the RAID level’s rules. In mixed setups, the smallest drive defines the effective size for all drives in the array. For example, pairing a 4TB drive with 6TB drives means each will count as 4TB. This is one reason why choosing the right drives for your NAS matters—matching drive sizes and selecting NAS-rated models from the start helps you get the most accurate and usable results from any RAID calculation.

Take four 6TB drives as an example. RAID 5 yields around 18TB of usable capacity after reserving one drive’s worth for parity. RAID 6 drops that to about 12TB since it uses two parity drives.

RAID Level Selection

Each RAID level balances storage, protection, and performance differently. RAID 0 offers full capacity but no redundancy. RAID 1 mirrors data, cutting usable space in half. RAID 5 uses single parity for (N-1) drives’ worth of storage, and RAID 6 uses dual parity for (N-2). RAID 10 provides high speed and strong protection but uses half of total capacity.

Choose based on your priorities: RAID 5 or 6 for better space efficiency, or RAID 10 for reliability and speed.

Scalability Considerations

RAID calculators also help when planning future growth. Adding drives of the same size is standard, but scalability depends on the RAID level. RAID 6 becomes more efficient with larger arrays, while RAID 10 always maintains a 50 percent overhead. Combine RAID calculators with NAS planning tools to forecast how expansions affect usable capacity over time.

Frequently Asked Questions (FAQ)

What’s the difference between raw and usable capacity?
Raw capacity is the total size of all drives combined, while usable capacity reflects what remains after parity and redundancy. RAID calculators make this clear so you can set accurate expectations before building your array.

Why does rebuild time matter for large drives?
Large-capacity drives can take hours or days to rebuild after failure. While RAID calculators show usable space, they don’t highlight the operational risks of lengthy rebuilds. Accounting for this helps protect your data during recovery.

Should I plan for hot spares and future expansion?
Yes. Hot spares reduce available capacity but add resilience. Likewise, data growth can quickly consume storage, so plan ahead to avoid costly or complex expansions later.

Conclusion

RAID calculators are valuable tools for NAS capacity planning. They help you visualize usable space, understand parity impact, and make smarter configuration choices. Combine these insights with broader RAID knowledge to build a setup that balances protection, performance, and storage efficiency.