Quick answer
Four 10 TB drives in RAID 5 provide 30 TB of usable capacity before any additional allowance. The same bytes are about 27.28 TiB. With a 20% free-space target, plan to store up to 24 TB; a 15 TB data set leaves 9 TB of planning headroom.
Change the hot-spare, allowance, current-data, and free-space inputs to separate what RAID consumes from what you choose to keep available.
How to use the RAID 5 calculator
- Enter the total installed drive count, including any dedicated hot spares.
- Enter the smallest drive's label capacity in decimal TB. Conventional RAID usually cannot use the extra size of larger members.
- Add the number of dedicated hot spares. At least three active drives must remain.
- Enter an additional filesystem or vendor allowance only when you have a basis for it.
- Enter the data you need to store now and the percentage of usable capacity you prefer to keep free.
- Read usable capacity for array size, then use plan-within capacity and headroom for the migration or purchase decision.
What each RAID 5 result means
- Usable after allowance: active capacity after one drive equivalent for parity and the extra percentage you entered, shown in TB and TiB.
- Plan-within capacity: usable capacity after also preserving the selected free-space target.
- Headroom for entered data: plan-within capacity minus the current data set; a negative result is a shortfall.
- Parity capacity: one smallest-drive equivalent distributed across the active RAID 5 members.
- Hot-spare capacity: installed drive capacity held outside the active array.
- Filesystem/vendor allowance: the entered percentage applied after parity.
- Installed-capacity efficiency: usable capacity after the additional allowance divided by all installed raw capacity, including hot spares.
Decisions this calculator helps with
Size a new RAID 5 array
Compare installed raw capacity with usable TB and TiB before purchasing drives.
Assess a data migration
Enter the current data set and a free-space target to see headroom or shortfall.
Model a hot spare
Compare the same drive set with and without a dedicated spare while keeping parity separate.
Compare protection layouts
Use the same drive assumptions in RAID 6 and RAID 10 to compare capacity with different failure boundaries.
RAID 5 usable-capacity formula
Active drives = installed drives − dedicated hot spares.
Usable before additional allowance = (active drives − 1 parity drive equivalent) × smallest drive size.
Usable after allowance = usable before allowance × (1 − allowance percentage).
Binary capacity = decimal TB × 1012 ÷ 240 TiB.
Plan-within capacity = usable after allowance × (1 − free-space target).
Headroom = plan-within capacity − data to store now.
The free-space target does not change array geometry. It creates a separate decision threshold so parity, device allowances, and operational headroom are not combined into one unexplained number.
Assumptions and limitations
- Drive size is entered as the decimal TB printed on a drive label. TiB is the binary-unit view of the same byte count.
- All active members are modeled at the smallest drive size. Vendor-specific systems may use mixed drives differently.
- Parity is modeled as one active drive equivalent. A hot spare is removed before that parity calculation.
- The filesystem/vendor allowance is a user-entered percentage. The calculator does not infer metadata, snapshot, thin-provisioning, block-size, reserved-pool, or controller-specific losses.
- The free-space target is a planning preference. It is not parity, fault tolerance, or a universal requirement.
- The calculator estimates capacity, not read/write speed, rebuild duration, unrecoverable-read-error probability, controller compatibility, or workload suitability.
- RAID 5 tolerates one drive failure. A second failure before recovery can lose the array.
Worked RAID 5 capacity examples
| Installed layout | Capacity calculation | Planning result |
|---|---|---|
| 4 × 10 TB, no spare, 0% allowance | (4 − 1) × 10 = 30 TB = 27.28 TiB | 20% free target gives 24 TB; 15 TB data leaves 9 TB headroom |
| 6 × 12 TB, 1 spare, 5% allowance | (6 − 1 spare − 1 parity) × 12 × 0.95 = 45.6 TB = 41.47 TiB | 20% free target gives 36.48 TB; 30 TB data leaves 6.48 TB headroom |
| 5 × 8 TB, no spare, 2% allowance | (5 − 1) × 8 × 0.98 = 31.36 TB = 28.52 TiB | 25% free target gives 23.52 TB; 26 TB data is 2.48 TB over the plan-within amount |
Common mistakes in RAID 5 capacity planning
- Multiplying every installed drive by its label size and forgetting parity.
- Counting a dedicated hot spare as active usable capacity.
- Subtracting parity before removing hot spares; RAID 5 parity uses one of the active-drive equivalents.
- Treating the lower TiB number as missing storage instead of a unit conversion.
- Using the largest mixed drive instead of the smallest drive as the generic capacity limit.
- Planning to fill every usable byte without a separate free-space target.
- Assuming a zero additional allowance guarantees that a device will expose the entire arithmetic result.
- Treating one-drive fault tolerance as a substitute for separate backups.
RAID 5 capacity calculator FAQ
How much usable capacity does RAID 5 provide?
With equal-size drives and no hot spare, usable capacity before extra allowances is (drive count minus one) multiplied by the smallest drive size. One active drive worth of capacity is distributed as parity across the array.
How many drives does RAID 5 require?
RAID 5 requires at least three active drives. A hot spare is not active array capacity, so four installed drives with one hot spare leave the three active drives needed for RAID 5.
Why does the calculator show both TB and TiB?
Drive labels commonly use decimal terabytes, while many operating systems report binary tebibytes. One decimal TB is about 0.9095 TiB, so the two numbers describe the same bytes with different units.
Does a hot spare reduce RAID 5 capacity?
Yes. A dedicated hot spare is installed but does not contribute active data or parity capacity until it replaces a failed drive. The calculator removes hot spares before applying the one-drive parity rule.
What should I enter for filesystem or vendor allowance?
Enter a percentage only when you want to model capacity not available for ordinary data after parity, such as filesystem metadata, vendor reserves, or your own additional allowance. Use vendor documentation for a device-specific number.
What does the free-space reserve change?
It changes the planning comparison, not RAID geometry. The calculator multiplies usable capacity by one minus the free-space percentage, then compares that plan-within amount with the data you entered.
Can I use different drive sizes in RAID 5?
Many conventional RAID 5 implementations treat each drive as if it were the size of the smallest member. Use the smallest drive size here for a conservative generic estimate, or use the mixed-drive RAID calculator and your vendor documentation.
Is RAID 5 a backup?
No. RAID 5 can keep an array available after one drive failure, but it does not protect against deletion, corruption, malware, theft, controller failure, or a second failure during rebuild. Keep separate copies of important data.
Sources and model boundaries
References for units and storage terminology:
- Capacity basis: installed drives, smallest drive size, one active-drive equivalent for RAID 5 parity, and any dedicated hot spares.
- Unit basis: decimal TB uses 1012 bytes; binary TiB uses 240 bytes.
- User assumptions: additional filesystem/vendor allowance, current data size, and desired free-space percentage.
- Excluded factors: controller and filesystem implementation details, performance, rebuild duration, compression, deduplication, snapshots, and workload behavior.
- Calculation method: RAID5-1.1.
The inputs that drive the estimate most: active drive count, smallest drive size, hot-spare count, and the free-space target used for the fit comparison.
Updated: July 24, 2026.
What to do next
Run the same drive set through the RAID 6 calculator and RAID 10 calculator, then compare usable capacity with each layout's failure boundary. For mixed capacities, use the mixed-drive RAID calculator. For a complete appliance plan, continue with the NAS capacity calculator and your vendor's supported-drive, expansion, and rebuild guidance. If the arithmetic differs from the displayed formula, report an issue with all inputs and the expected result.