RAID 5 Capacity Calculator
Calculate usable RAID 5 storage in decimal TB and binary TiB, separate parity and hot-spare capacity, and see whether your data fits while preserving free space.
Interactive calculator · Method RAID5-1.3 · Last reviewed: 2026-11-04
Calculate RAID 5 usable space and data headroom
Enter installed drives, the drive size used for the model, optional hot spares, and the allowances you want to keep outside ordinary data use.
Your inputs
Default worked example. Edit the inputs to calculate your scenario.
Default worked example
After parity, the array provides 30 TB (27.28 TiB) after the 0% additional allowance. Keeping 20% free leaves 24 TB for planned data. The same allowances and entered data are used in each comparison row.
How to use the calculator
Count all installed drives, including dedicated hot spares. Enter the decimal-TB size used for every position, an additional filesystem/vendor allowance, the data to store and your chosen free-space percentage. At least three active drives must remain.
What the result means
Zero headroom is an exact fit at the chosen policy. Negative headroom is a shortfall. The comparison applies the same drives, spares, allowance, free-space target and data to every supported layout; reserved free space is applied once.
Formula and units
Active drives = installed drives − hot spares. Capacity before allowance = (active drives − 1) × drive TB. Usable = that capacity × (1 − allowance/100). Plan-within capacity = usable × (1 − free-space target/100). Headroom = plan-within capacity − entered data. Binary TiB converts the same bytes using 2^40 bytes per TiB.
Worked examples
4 × 10 TB in RAID 5: Installed drive count: 4 drives; Smallest drive size (decimal TB): 10 decimal TB; Dedicated hot spares: 0 drives; Filesystem/vendor allowance (%): 0 %; Data to store now (TB): 15 TB; Free-space target (%): 20 %.
Example result: Usable after allowance: 30 TB / 27.28 TiB; Plan-within capacity: 24 TB / 21.83 TiB; Headroom for entered data: 9 TB; Fit assessment: Fits with 9 TB headroom; Parity capacity: 10 TB; Hot-spare capacity: 0 TB; Filesystem/vendor allowance: 0 TB; Raw capacity at entered drive size: 40 TB; Efficiency at entered drive size: 75%; Failure tolerance: 1 drive.
Eight 8 TB drives with one spare: Installed drive count: 8 drives; Smallest drive size (decimal TB): 8 decimal TB; Dedicated hot spares: 1 drives; Filesystem/vendor allowance (%): 5 %; Data to store now (TB): 15 TB; Free-space target (%): 20 %.
Example result: Usable after allowance: 45.6 TB / 41.47 TiB; Plan-within capacity: 36.48 TB / 33.18 TiB; Headroom for entered data: 21.48 TB; Fit assessment: Fits with 21.48 TB headroom; Parity capacity: 8 TB; Hot-spare capacity: 8 TB; Filesystem/vendor allowance: 2.4 TB; Raw capacity at entered drive size: 64 TB; Efficiency at entered drive size: 71.3%; Failure tolerance: 1 drive.
RAID 5 capacity questions
Why do TB and TiB differ?
They describe the same bytes: decimal TB uses 1,000,000,000,000 bytes and binary TiB uses 1,099,511,627,776 bytes. Changing the label does not consume capacity.
How do spares and the free-space target differ?
Dedicated hot spares are outside the active array before parity. The entered filesystem allowance reduces post-parity capacity, then the chosen free-space percentage is kept within that usable capacity. The data comparison applies each once.
How should mixed drive sizes be modeled?
Use the smallest active member for a conventional usable-capacity estimate. Raw capacity and efficiency on this page use that size for every position; use Mixed Drive RAID to account for larger members.
Does this estimate rebuild safety?
No rebuild time or failure likelihood is calculated. The RAID 5 layout has one parity-drive equivalent; vendor behavior and recovery conditions need separate assessment. Keep a separate backup.
Limitations and common mistakes
Raw capacity and efficiency use the entered size for every installed position. For mixed physical capacities, use the smallest active member for a conventional usable-capacity estimate and the Mixed Drive RAID tool for larger-member loss. The entered allowances are planning choices; check controller support, vendor reserves, rebuild time and failure probability against separate evidence. Two equal groups are assumed for RAID 50/60 comparisons. RAID 10 leaves an unpaired active member unused. Keep a separate backup.
Compare a data set with plan-within capacity rather than installed raw capacity. Do not apply the free-space percentage a second time or interpret capacity as a reliability assessment.
Sources and assumptions
Capacity arithmetic is a mathematical identity; SNIA terminology and NIST unit terminology do not validate rebuild reliability.
Method RAID5-1.3; last formula review 2026-11-04. Sources and their supported claims. Calculation methodology.
NIST byte and binary-prefix definitions support the unit conversions. Western Digital generic RAID capacity definitions describe parity, mirrors, smallest-drive use and JBOD aggregate capacity.