RAID Capacity Calculator

TL;DR: To calculate RAID array capacity, divide the number of drives in the array by the RAID level's data-to-parity ratio (e.g., for RAID 5, subtract one drive's capacity for parity), then multiply by the size of the smallest drive in the array (e.g., 4x 1TB in RAID 5 = (4-1) × 1TB = 3TB usable, with 1TB reserved for redundancy).

What Is the RAID Capacity Calculator?

RAID (Redundant Array of Independent Disks) is a method of combining multiple physical hard drives into a single logical unit to improve performance, provide fault tolerance, or both. However, the total raw capacity of the drives is never fully available to you as usable storage, because the RAID level itself consumes some space for parity data (redundancy) or mirroring. A RAID Capacity Calculator is the tool that determines exactly how much usable space you will have after configuring a set of drives in a specific RAID level, and how much of that raw capacity is lost to redundancy.

This calculator is essential for system administrators, network engineers, homelab enthusiasts, and anyone building a Network Attached Storage (NAS) device, a server, or a video editing workstation. Without it, you might plan for 8TB of raw storage, purchase eight 1TB drives, and then discover after configuration that your usable space is only 6TB due to RAID 6's double parity. The calculator eliminates this guesswork, helping you budget for the correct number of drives, choose the right RAID level for your fault tolerance needs, and ensure you don't under-provision or over-purchase storage.

Furthermore, it helps you balance the trade-off between capacity and safety. A RAID 0 array gives 100% capacity but zero redundancy, while a RAID 1 array gives 50% capacity (with two drives) but full mirroring. The calculator makes these trade-offs explicit, allowing you to make an informed decision based on your data criticality and budget.

How to Use the Calculator

Using the RAID Capacity Calculator is a straightforward four-step process. Ensure you have the exact specifications of your hard drives handy (specifically their formatted capacity), as well as a firm decision on the RAID level you intend to deploy. Here is the step-by-step process:

1. Select the RAID Level: From the dropdown menu, choose the RAID configuration you plan to use. Common options include RAID 0, RAID 1, RAID 5, RAID 6, and RAID 10. The calculator uses this selection to determine which formula to apply for capacity loss.
2. Enter the Number of Drives: Input the total number of physical hard drives you will include in the array. Be aware that some RAID levels have minimum drive requirements (e.g., RAID 5 needs at least 3 drives, RAID 6 needs at least 4, and RAID 10 needs an even number of at least 4).
3. Input the Individual Drive Capacity: Enter the capacity of a single drive in Terabytes (TB) or Gigabytes (GB). If you have drives of different sizes, the calculator will typically use the smallest drive's capacity as the baseline, so it is crucial to check your drive specifications for the exact formatted capacity rather than the marketing label.
4. Click Calculate: The calculator will process the inputs and display the total usable capacity, the total raw capacity, and the capacity lost to redundancy. Review these outputs to verify that the array meets your storage needs.

For example, if you select RAID 5, enter 4 drives, and input a capacity of 1TB per drive, the calculator will show a usable capacity of 3TB. That is your actionable result.

Formula and Calculation Method

The calculation is based on a simple core principle: for most RAID levels with parity, one drive's worth of capacity is used for parity (RAID 5), and two drives' worth for double parity (RAID 6). For mirroring (RAID 1 and RAID 10), the usable capacity is half of the total raw capacity. The general formula is:

Usable Capacity = (Total Number of Drives - Parity Drives) × Capacity of Smallest Drive

The 'Parity Drives' value varies by RAID level:

• RAID 0: Parity Drives = 0 (No redundancy, full capacity)
• RAID 1: Parity Drives = Drives/2 (for a 2-drive mirror) or the equivalent for multiple mirrors
• RAID 5: Parity Drives = 1 (Single distributed parity)
• RAID 6: Parity Drives = 2 (Double distributed parity)
• RAID 10: Parity Drives = Total Drives / 2 (Mirrored sets)

Let's walk through a concrete worked example. Suppose you have four 1TB drives and you want to configure them as RAID 5. Using the formula:

Usable Capacity = (4 - 1) × 1TB = 3 × 1TB = 3TB.

The total raw capacity is 4TB (4 × 1TB). The redundancy capacity is 4TB - 3TB = 1TB. This single drive's worth of space is used to store parity information that allows the array to survive the failure of one physical drive.

Now consider the same four drives in RAID 10. The formula yields (4 - 2) × 1TB = 2 × 1TB = 2TB usable. The redundancy is 2TB, meaning half your raw capacity is used to create mirrors. This demonstrates why RAID 10 is more expensive in terms of capacity but offers better performance and fault tolerance than RAID 5 in some configurations.

Practical Examples

To illustrate the calculator's utility across different use cases, here are three realistic scenarios. The table below summarizes the inputs and outputs for each.

Scenario 1: Home NAS (RAID 5, 4x 4TB). A user building a media server has four 4TB drives. Using RAID 5, they get 12TB usable, sacrificing 4TB for parity. This allows them to store a large movie library while tolerating a single drive failure.

Scenario 2: Performance Server (RAID 10, 6x 2TB). A database administrator needs fast read/write speeds and better fault tolerance than parity RAID provides. With six 2TB drives in RAID 10, they get 6TB usable. The 6TB of redundancy is the cost of having every piece of data mirrored, but the array can survive multiple drive failures as long as no mirrored pair loses both drives.

Scenario 3: Archival Server (RAID 6, 8x 8TB). An IT team building a large archival system with 8TB drives chooses RAID 6 for dual parity. They get 48TB usable out of 64TB raw. This allows the array to survive two simultaneous drive failures, which is critical when dealing with high-capacity drives where rebuild times are long.

Tips for Accurate Results

To ensure the calculator provides accurate results, you must pay attention to several technical details. The most common mistakes can derail an entire storage planning project.

• Always use the formatted capacity, not the marketing capacity. A drive advertised as "1TB" actually has a formatted capacity of approximately 931GB (in binary gibibytes). The difference lies in decimal vs. binary measurement. If you enter 1TB but the drive is actually 931GB, your calculated usable capacity will be slightly off. Check your drive's OS-reported capacity for the most accurate number.
• Never mix drive sizes in a parity RAID array. The calculator (and the RAID controller) will assume the capacity of the smallest drive in the array. If you have three 1TB drives and one 500GB drive, the array will treat all drives as 500GB, resulting in a total raw capacity of 2TB, not 3.5TB. The extra 500GB on the larger drives becomes unusable in that array.
• Verify you have the minimum number of drives for the selected RAID level. RAID 5 requires a minimum of 3 drives; RAID 6 requires a minimum of 4. Using RAID 5 with only 2 drives will either fail to configure or the calculator will return an error, as the array cannot create valid parity with only one data drive.
• Consider a dedicated hot spare when planning total capacity. If you plan to have a hot spare (a standby drive that automatically replaces a failed drive), you must exclude that drive from the capacity calculation. A hot spare is not part of the active array; it sits idle, waiting for a failure. If you have 5 drives and one is a hot spare, your active array only has 4 drives, and your usable capacity should be calculated based on those 4 drives.
• Remember that RAID 0 has no redundancy. The calculator will show 100% usable capacity, but this is a mathematical output, not a recommendation. If a single drive fails in RAID 0, all data on the entire array is lost permanently. The calculator's "redundancy" output of zero is a critical warning.

Frequently Asked Questions

1. Why is my usable capacity less than the sum of my hard drives?

This is entirely normal and expected. The difference between the sum of your raw drive capacities and the usable capacity is the space allocated for redundancy data. In RAID 5, the controller writes parity information across all drives, which consumes the equivalent of exactly one drive's capacity regardless of how many drives are in the array. In RAID 6, it consumes the equivalent of two drives. In RAID 1 and RAID 10, it consumes half the total capacity for mirroring. This is the 'price' you pay for the ability to survive a drive failure without data loss. For example, with 4x 1TB drives in RAID 5, you lose 1TB to parity, leaving 3TB usable. With RAID 0, there is no parity, so you get the full 4TB, but you have zero fault tolerance, meaning a single drive failure destroys all data.

2. Can I mix different size drives in a RAID 5 array?

Technically yes, but it is highly discouraged. The RAID controller will standardize on the capacity of the smallest drive in the array. Any extra capacity on the larger drives is simply ignored and becomes unusable. For instance, if you have two 2TB drives and one 1TB drive in a RAID 5 array, the controller will treat all three drives as 1TB drives. Your total raw capacity will be 3TB (not 5TB), and your usable capacity will be 2TB (3 - 1 for parity). You would lose the entire extra 1TB from the two larger drives. To avoid this waste, it is best to use identical drives with the same model and capacity for the entire array.

3. What RAID level gives me the most usable capacity?

RAID 0 gives you the absolute highest usable capacity—100% of your raw storage. Because it uses striping without any parity or mirroring, no space is dedicated to redundancy. However, this comes at the extreme cost of zero fault tolerance. RAID 5 is the next most space-efficient level that still offers redundancy, sacrificing only one drive's worth of capacity regardless of the total number of drives in the array. For example, with 4x 1TB drives, RAID 5 gives you 3TB usable and 1TB redundancy. RAID 6 sacrifices two drives' worth (2TB), and RAID 10 sacrifices half your drives (2TB with 4 drives). If maximum capacity is your primary goal and data loss is acceptable, RAID 0 wins. If you need safety but want to maximize space, RAID 5 is the most efficient redundant option.