Raid Calculator - Usable Capacity, Read/Write Gain, Fault Tolerance, Cost

A raid calculator turns a single disk size, total disk count, and chosen RAID level into usable capacity, utilization, read/write speed gain, fault tolerance, and optional cost per usable terabyte. Use it before buying drives for a NAS or small business array.

• • Size a homelab NAS before buying drives: Enter disk size and count, then switch between RAID 5, 6, and 10 to compare usable capacity, utilization, and fault tolerance.
• • Compare RAID 6 vs RAID 10 on cost per usable TB: Pick a per-disk price and read cost per usable TB.
• • Plan a video editing scratch array: Pick RAID 0 with several large disks for maximum read/write gain, accepting zero fault tolerance.
• • Check parity overhead: Enter a five-disk array and switch between RAID 5 and 6 to see the parity penalty.

RAID stands for Redundant Array of Inexpensive Disks, from the original Patterson, Gibson, and Katz paper. Every level is a different answer to the same trade-off: capacity, performance, and fault tolerance for a given set of disks. Values are theoretical maxima; real gains depend on controller, workload, and stripe size.

Once a RAID tool returns a usable capacity in terabytes, the Data Storage Converter in our Tools cluster turns that number into MB, GB, or TB the moment a disk vendor spec sheet uses a different unit.

The tool pulls parity, mirroring, and minimum-disk rules from a per-level table, multiplies data-bearing disks by the single-disk size, then derives read speed, write speed, fault tolerance, and cost outputs.

• RAID level: Selects the rule set: RAID 0, 1, 1E, 10, 5, 50, 5E, 5EE, 6, or 60.
• Single disk size: Capacity of one drive in TB; assumes identical disks.
• Number of disks: Total drives; per-level rules reject combinations that violate minimums or parity-of-2 constraints.
• Data disks: Disks whose capacity contributes to usable storage; RAID 1 always counts as one.
• Cost per disk: Optional price input for total cost and cost per usable TB.

More reserved space usually means more failures survived: RAID 0 reserves none, RAID 5 reserves one drive for parity, RAID 6 reserves two, and RAID 10 reserves half the drives as mirrors. Cost per usable TB is the cleanest way to compare arrays delivering the same usable storage.

After the raid calculator reports the read speed gain, the Data Transfer Calculator in our Tools cluster shows how long it takes to move a multi-terabyte project onto the array at realistic network and disk speeds. The original term RAID and the striping, mirroring, and parity levels come from the Patterson, Gibson, and Katz RAID paper and the per-level parity and mirror rules used here also match the Microsoft Storage Spaces overview for parity and mirror configurations in Windows Server.

Four concepts come up every time RAID levels are compared; once clear, the numbers stop being magic.

Nested levels (RAID 10, 50, 60) build a higher-level stripe out of lower-level sub-arrays, so each sub-array's parity or mirror overhead applies on top of the stripe. A six-disk RAID 50 from two three-disk RAID 5 sub-arrays keeps 4 TB usable (n - 2 data disks out of n); an eight-disk RAID 60 from two four-disk RAID 6 sub-arrays keeps 4 TB (n - 4 data disks out of n). According to IBM FlashSystem RAID documentation, parity-based RAID tolerates one failure for RAID 5 and two for RAID 6, RAID 10 tolerates one failure per mirror pair, and RAID 60 adds double-parity tolerance from its RAID 6 sub-arrays.

When the RAID array is reached over a network rather than a local bus, the Upload Time Calculator gives the wall-clock hours needed to populate the array with a multi-terabyte dataset, the practical second question after this calculator sizes usable capacity.

The form takes three required inputs and one optional input and returns eight outputs. The flow walks through a 4-disk NAS.

1. 1 Pick a RAID level: Open the RAID Level dropdown and select the configuration, from RAID 0 for raw performance to RAID 60 for two-failure tolerance with a stripe.
2. 2 Enter the single disk size: Type the per-disk capacity in TB. If your disks differ, use the smallest so the calculator matches what the controller will use.
3. 3 Enter the number of disks: Type the total number of drives. The calculator rejects combinations that violate the level's minimum or parity rules.
4. 4 Add a disk price for cost outputs: Leave Cost Per Disk at zero to skip the cost rows, or enter a price to read total cost and cost per usable TB.
5. 5 Read usable capacity and utilization: Usable Capacity is the storage you can fill with data. Capacity Utilization shows that as a percentage of raw capacity.
6. 6 Compare fault tolerance and cost: Fault Tolerance is the drive failures the array survives. Cost Per Usable TB lets you compare arrays delivering the same usable storage.

Plan a 4-disk NAS of 2 TB drives: select RAID 5, type 2 for disk size, type 4 for disk count, and $60 for cost per disk. The calculator returns 6 TB usable, 75 percent utilization, 4x read, 1x write, one tolerated failure, $240 total, $40 per usable TB. Switch to RAID 6 and usable drops to 4 TB but two failures are tolerated. For a serial-attached RAID shelf the read speed gain is gated by the link rate, so pair the raid calculator with the Baud Rate Calculator in our Tools cluster to confirm the SAS or SATA connection can carry the throughput the array promises.

A raid calculator compresses the math from ten RAID levels into one form, useful for picking a first NAS or rebuilding a production array.

• • Compare all ten RAID levels: Hold disk size and count fixed and flip between levels.
• • Catch disk-count rule violations: RAID 10, 50, and 60 need an even disk count; RAID 5, 6, 5E, and 5EE each have minimums.
• • Quantify parity vs mirroring: Parity RAID trades write speed for usable capacity; mirrored RAID trades usable capacity for write speed.
• • Translate fault tolerance into rebuild risk: Tolerated failures are not the same as data safety.
• • Compare cost per usable TB: Two arrays can deliver the same usable capacity at different prices.
• • Document the configuration: The output doubles as a record of RAID level, disk size, and disk count.

The biggest practical benefit is catching a configuration that looks fine on paper but fails a controller rule. RAID 10 with five disks does not exist, RAID 60 needs eight, and RAID 50 needs six disks split into two even sub-arrays. The calculator surfaces those constraints before the drives are ordered, and the Is It Worth It Calculator tests whether the chosen layout clears the cost-versus-benefit threshold.

The headline numbers depend on the inputs and on rules baked into the level.

• Read/write speed gains are theoretical maxima; real performance is lower because of controller overhead, stripe size, queue depth, and disk model.
• Fault tolerance counts tolerated failures under the best case; RAID 10 fails if both disks in a mirror pair die, and parity RAID is vulnerable during rebuild.
• Cost outputs cover disk price only and exclude controller, chassis, power, and backup storage.

RAID protects against drive failure, not file deletion, controller failure, ransomware, or site loss. The calculator is a sizing tool, not a backup.

If the RAID array is provisioned over iSCSI, the IP Address Converter in our Tools cluster converts between decimal and CIDR so the storage subnet the calculator output assumes actually fits the IP plan you are deploying.