RAID Capacity, Performance & Cost Calculator
Compare common RAID levels using equal-size disks, with live usable capacity, theoretical speed gain, fault tolerance, storage efficiency, and disk-cost metrics.
RAID type and disks
Live results
Capacity breakdown
Where the raw capacity goes
Calculation detail
| Metric | Value | Interpretation |
|---|---|---|
| Raw capacity | 5.00 TB | Five 1 TB disks before RAID overhead. |
| Usable capacity | 3.00 TB | Three disk-equivalents remain for data. |
| Unavailable capacity | 2.00 TB | Two disk-equivalents support dual parity. |
| Capacity utilization | 60.00% | Usable capacity divided by raw capacity. |
| Read speed gain (max) | 3× | Theoretical maximum relative to one disk. |
| Write speed gain (max) | 1× | Reference-style theoretical comparison; real workloads vary. |
| Fault tolerance | 2 disks | Up to two disk failures before data loss for RAID 6. |
| Total disk cost | $250.00 | Five disks at $50.00 each. |
| Cost per usable TB | $83.33/TB | Total disk cost divided by 3.00 usable TB. |
How to use this RAID calculator
What this calculator does
This calculator estimates how a common RAID layout changes the usable capacity of an equal-size disk set, how much raw capacity is consumed by mirrors, parity, or integrated spare space, and how the configuration compares with a single disk on theoretical maximum read and write gain. It also estimates disk-only cost when a per-drive price is supplied. It is a planning and comparison tool, not a reliability forecast, benchmark, backup plan, or controller-compatibility checker. RAID implementation details vary, and a RAID array does not replace an independent backup. The Linux kernel documentation provides a useful technical overview of software RAID array behavior.
When to use it
- Compare RAID 5, RAID 6, or RAID 10 before sizing a NAS or server.
- Estimate the storage overhead created by parity, mirroring, or integrated spare capacity.
- Compare disk-only acquisition cost per usable terabyte across candidate layouts.
- Sanity-check how changing disk count or disk size affects capacity utilization and theoretical throughput scaling.
How to calculate
- The calculator opens with a ready-to-use demonstration: RAID 6, 1 TB per disk, 5 disks, and $50 per disk. The results and example XLSX are available immediately.
- Choose RAID level. Supported layouts are RAID 0, 1, 1E, 10, 5, 50, 5E, 5EE, 6, and 60. The calculator validates the minimum disk count and the even-count requirement used here for RAID 10, 50, and 60.
- Replace Disk size (single), choose the Disk size unit, and enter the Number of disks. Changing the unit converts the current disk-size value rather than merely relabeling it.
- Optionally enter Disk cost in USD. Leave it blank if you only need capacity and performance metrics.
- Read the live result cards, capacity allocation donut, and detail table. Select Download Excel to export the current validated model as a real XLSX workbook. Select Reset to clear the demonstration values; export is then disabled until a complete valid configuration is entered again.
Input guide
RAID level is required and selects the formula family. For example, RAID 6 uses two parity disk-equivalents, while RAID 0 uses none. Choosing a level with too few disks produces a visible validation message. A common mistake is assuming every controller supports every listed level; confirm support in the hardware or operating-system documentation.
Disk size (single) is a required positive decimal. Enter the capacity of one disk, such as 1. The model assumes all disks contribute the capacity of the smallest member, so mixed-size arrays should use the smallest drive size. Higher disk size increases raw, usable, and unavailable capacity proportionally but does not change the utilization percentage for a fixed level and disk count.
Disk size unit is required and can be GB, TB, or PB using decimal SI storage units. For example, 1 TB equals 1,000 GB. This calculator intentionally reports vendor-style decimal capacities rather than binary GiB/TiB. The NIST SI units guidance is a useful reference for decimal prefixes. Changing the unit converts the current numeric disk-size value.
Number of disks is required and must be a whole number. Typical examples are 3 disks for RAID 5, 4 for RAID 6, 4 for RAID 10, 6 for RAID 50, and 8 for RAID 60. RAID 10, RAID 50, and RAID 60 are modeled as even-drive layouts; RAID 50 and RAID 60 use two equal-sized sub-arrays because there is no separate group-count input. Adding disks usually increases usable capacity; utilization also improves for parity layouts because fixed parity overhead is spread across more drives.
Disk cost is optional and accepts a nonnegative USD amount such as 50 or $1,250.00 using U.S. comma grouping and a period decimal separator. Unsupported currency symbols, decimal-comma input, and scientific notation are rejected rather than silently reinterpreted. Disk cost affects Total disk cost and Cost per usable TB only.
Output guide
Usable capacity is the primary result: the estimated data capacity left after RAID overhead, displayed in the selected size unit. Raw capacity is disk size multiplied by disk count. Unavailable capacity is the difference between raw and usable capacity. Capacity utilization is usable capacity divided by raw capacity; 100% means no RAID storage overhead, while lower percentages mean more capacity is devoted to redundancy or spare space.
Read speed gain (max) and Write speed gain (max) are theoretical multipliers relative to one disk, not guaranteed throughput. The Linux device-mapper RAID documentation describes the major RAID families and their resilience-oriented layouts. Fault tolerance shows the reference-style number of disk failures the modeled layout can tolerate under its stated arrangement; for nested mirror layouts, survival depends on which members fail, not only the count.
Total disk cost is disk count multiplied by the optional per-disk price. Cost per usable TB divides total disk cost by usable capacity converted to TB. The three summary pills repeat the active RAID level, disk count, and utilization for quick scanning. In the detail table, Metric names each calculated quantity, Value shows its formatted result, and Interpretation explains what the value means. The donut chart uses the same canonical values as the table: blue is usable data capacity and teal is parity, mirror, or spare overhead. If a valid layout has no overhead, such as RAID 0, the two-part donut is replaced by a compact 100%-usable message rather than a misleading one-segment chart.
Worked example
The startup example uses RAID 6 with five 1 TB disks. Raw capacity is 5 × 1 TB = 5 TB. RAID 6 devotes the equivalent of two disks to dual parity, so usable capacity is (5 – 2) × 1 TB = 3 TB and unavailable capacity is 2 TB. Capacity utilization is 3 ÷ 5 × 100 = 60.00%. The reference-style maximum read gain is 5 – 2 = 3×, write gain is 1×, and fault tolerance is 2 disks. At $50 per disk, total disk cost is $250.00, and cost per usable TB is $250 ÷ 3 TB = $83.33/TB. These are the exact values shown on first open and exported in the startup workbook.
How the RAID formulas are interpreted
Capacity arithmetic treats every member disk as the same size. RAID 0 uses all disks for data. RAID 1 mirrors the full data set to every member, so usable capacity is one disk. RAID 5 reserves one disk-equivalent for distributed parity, RAID 6 reserves two, and RAID 5E/5EE reserve one parity-equivalent plus one integrated spare-equivalent. RAID 1E and RAID 10 use 50% of raw capacity for mirrored copies. RAID 50 and RAID 60 are modeled as two equal sub-arrays, so they reserve two and four disk-equivalents respectively.
The original RAID literature emphasizes that disk arrays trade capacity, cost, and write overhead for performance and reliability. Patterson, Gibson, and Katz introduced the RAID levels and compared their relative cost and performance in the original UC Berkeley RAID technical report. For RAID 6 specifically, the Linux RAID-6 technical note explains the mathematics behind tolerating any two drive failures through two independent parity syndromes in The mathematics of RAID-6.
Important planning limits
The performance multipliers here are intentionally theoretical. Real throughput depends on controller bandwidth, cache policy, filesystem, queue depth, I/O size, sequential versus random access, read/write mix, SSD or HDD behavior, and degraded or rebuild state. Likewise, “fault tolerance” is not a probability of data loss. In RAID 10, for example, multiple failures can be survivable when they occur in different mirror pairs but catastrophic when both copies of the same data are lost.
RAID is also not a backup. It can keep a system available through some device failures, but it does not independently protect against accidental deletion, malware, file corruption, theft, controller failure, or site loss. Use separate, tested backups for data you cannot replace, and check the exact RAID levels, maximum disk counts, rebuild behavior, and drive-size rules supported by your actual controller or software stack.