RAID Calculator: Usable Capacity and Fault Tolerance Explained

Buying a stack of hard drives is the easy part of building a storage server. The hard part is deciding how to arrange them. Combine six 4 TB disks one way and you get 24 TB of blazing-fast but fragile storage; arrange them another way and you get just 4 TB that can survive multiple drive failures. The technology that governs this trade-off is RAID—a Redundant Array of Independent Disks—and a RAID calculator is the fastest way to see exactly what each configuration will give you.

This guide walks through what RAID does, how each common level converts raw disks into usable space, and how to read the three numbers that matter most: usable capacity, fault tolerance, and efficiency. By the end you will be able to size any array with confidence and pick the level that balances space, speed, and safety for your needs.

What Is RAID?

RAID combines several physical disks into a single logical volume to achieve one or more goals: more capacity, faster performance, redundancy against disk failure, or some blend of all three. The operating system sees one drive, while behind the scenes the RAID controller spreads data across the members according to a chosen scheme called a RAID level.

The key insight is that redundancy always costs space. Any protection against a failed disk requires storing extra copies or parity information somewhere, and that overhead reduces how much of your raw capacity you can actually use. Different RAID levels strike different balances between the space you keep and the safety you gain.

How Each RAID Level Works

Assume you have n identical disks, each of size capacity. The raw capacity—the total you paid for—is always:

raw capacity = n × size

Usable capacity depends on the level:

RAID 0 — Striping

Data is split across all disks with no redundancy at all. Usable capacity is the full n × size, and read/write speed is excellent. The catch: fault tolerance is zero. If any single disk dies, the entire array is lost. RAID 0 suits scratch space and caches where speed matters and the data is disposable.

RAID 1 — Mirroring

Every disk holds an identical copy of the data. Usable capacity equals a single disk, size, no matter how many mirrors you add (minimum two disks). Because every member is a full copy, the array survives as long as at least one disk remains—so with n disks it tolerates n − 1 failures. Excellent safety, poor space efficiency.

RAID 5 — Single Parity

Data and distributed parity are spread across all disks, reserving the equivalent of one disk for parity. Usable capacity is (n − 1) × size, and it needs a minimum of three disks. It tolerates exactly one disk failure. RAID 5 is the classic balance of capacity and protection.

RAID 6 — Double Parity

Like RAID 5 but with two independent parity blocks, reserving two disks’ worth of space. Usable capacity is (n − 2) × size with a minimum of four disks, and it tolerates two simultaneous failures. This matters for large arrays where a second disk can fail during the lengthy rebuild after the first.

RAID 10 — Mirror Plus Stripe

Disks are paired into mirrors, and those mirrors are striped together. It requires an even number of disks, at least four. Usable capacity is (n ÷ 2) × size. It always survives at least one failure, and in the best case one disk per mirror—up to n ÷ 2 total—can fail. RAID 10 offers strong performance and fast rebuilds, which is why databases favor it.

A Worked Example

Suppose you have four 4 TB disks and want to compare RAID 5 and RAID 10.

Raw capacity is the same for both: 4 × 4 TB = 16 TB.

RAID 5:

usable = (n − 1) × size = 3 × 4 TB = 12 TB
efficiency = 12 ÷ 16 = 75%
fault tolerance = 1 disk

RAID 10:

usable = (n ÷ 2) × size = 2 × 4 TB = 8 TB
efficiency = 8 ÷ 16 = 50%
fault tolerance = 1 disk (up to 2 if one per mirror fails)

RAID 5 gives you 50% more usable space (12 TB versus 8 TB), while RAID 10 offers faster writes and quicker recovery. Neither is universally “better”—the right choice depends on whether you prioritize capacity or performance and rebuild safety.

The Efficiency Formula

Efficiency expresses how much of your raw purchase becomes usable storage:

efficiency = (usable capacity ÷ raw capacity) × 100%

RAID 0 is 100% efficient but unsafe; RAID 1 with two disks is only 50% efficient but very safe; RAID 5 climbs toward 100% as you add disks because the single parity disk becomes a smaller fraction of the total.

How to Use the RAID Calculator

The tool needs three inputs:

1. Number of disks – how many identical drives are in the array.
2. Single disk size and unit – the capacity of one drive, in GB or TB.
3. RAID level – choose from 0, 1, 5, 6, or 10.

It validates the minimum disk count for the chosen level (for example, refusing RAID 6 with fewer than four disks or RAID 10 with an odd number) and then reports usable capacity, raw capacity, fault tolerance, and efficiency instantly.

Tips for Choosing a RAID Level

• Match the level to the disk count. RAID 5 shines with 4–6 disks; beyond that, RAID 6 is safer because rebuild times grow and the odds of a second failure rise.
• Never treat RAID as a backup. RAID protects against hardware failure only—not deletion, corruption, theft, or ransomware. Keep separate, ideally off-site, backups.
• Consider rebuild stress. Rebuilding a large RAID 5 array hammers the surviving disks for hours, and a second failure during that window is catastrophic. RAID 6 or RAID 10 hedge against this.
• Use identical disks. Arrays size themselves to the smallest member, so mixing capacities wastes space.

Common Use Cases

A RAID calculator is indispensable when planning a NAS or home server, sizing an array to hit a target usable capacity before ordering drives, or comparing the space cost of redundancy across levels. System administrators use it to document trade-offs for stakeholders, and enthusiasts use it to decide whether that fourth drive should add capacity or safety.

Conclusion

RAID turns a pile of disks into storage that can be faster, larger, safer, or a deliberate mix of the three—but every gain in redundancy costs usable space. Knowing the formulas behind RAID 0, 1, 5, 6, and 10 lets you predict exactly how much capacity you will keep, how many disks can fail, and how efficiently your investment is used. Plan your next array the smart way and try our free RAID Calculator for instant results.