RAID Calculator

RAID Calculator Guide

A RAID calculator estimates how much usable storage remains after drives are combined into a RAID array. The result depends on the RAID level, number of drives, and capacity of each drive.

This calculator supports RAID 0, RAID 1, RAID 5, RAID 6, and RAID 10 and shows raw capacity, usable capacity, redundancy overhead, storage efficiency, and drive-failure tolerance.

How Much Usable RAID Storage Will I Have?

Raw storage capacity is the combined capacity of all drives before RAID overhead is considered.

Usable capacity depends on how the selected RAID level uses drives for mirroring or parity. RAID levels that provide redundancy sacrifice some raw capacity in exchange for protection against drive failure.

RAID 0 Capacity

RAID 0 stripes data across two or more drives. It does not reserve capacity for parity or mirrored copies, so the full combined capacity of the drives is available for data.

RAID 0 provides no redundancy. If one drive fails, data across the array may become unusable.

RAID 1 Capacity

RAID 1 mirrors the same data across multiple drives. Under the full-mirror assumption used by this calculator, each drive contains a complete copy of the same data.

With a full mirror, the array can continue operating as long as at least one complete mirrored drive remains available. Some RAID implementations use different layouts, so verify the behavior of your specific storage system.

RAID 5 Capacity

RAID 5 combines striping with distributed single parity. The equivalent capacity of one drive is used for parity information across the array.

RAID 5 requires at least three drives and can tolerate the failure of one drive.

RAID 6 Capacity

RAID 6 is similar to RAID 5 but stores two independent sets of parity information. The equivalent capacity of two drives is therefore used for redundancy.

RAID 6 requires at least four drives and can tolerate the failure of two drives.

RAID 10 Capacity

RAID 10 combines mirroring and striping. Drives are arranged into mirrored pairs, and data is striped across those pairs.

RAID 10 requires at least four drives and an even number of drives. Half of the raw capacity is used for mirrored copies.

RAID 10 guarantees survival of at least one drive failure. It may survive multiple simultaneous drive failures when the failed drives are in different mirror pairs. If both drives in the same mirror pair fail, the array can fail.

RAID Capacity Comparison

The following example compares common RAID levels using four identical 4 TB drives.

*RAID 1 figures assume all four drives contain complete mirrors of the same data.

What Is RAID Storage Efficiency?

Storage efficiency is the percentage of raw drive capacity that remains available for user data.

RAID 0 has 100% storage efficiency because no space is reserved for redundancy. RAID 10 normally has 50% efficiency because half of the capacity stores mirrored copies.

RAID 5 and RAID 6 become more capacity-efficient as additional drives are added because their parity overhead remains equivalent to one or two drives respectively.

What Is RAID Redundancy Overhead?

Redundancy overhead is the difference between the total raw storage and the capacity available for normal data.

This capacity is not necessarily stored on one specific physical drive. In RAID 5 and RAID 6, parity information is normally distributed across the drives in the array.

TB vs TiB: Why Does My Storage Look Smaller?

Drive manufacturers generally advertise capacity using decimal units. One terabyte (TB) equals 1,000,000,000,000 bytes.

Some operating systems and storage tools display capacity using binary units. One tebibyte (TiB) equals 1,099,511,627,776 bytes.

Because a TiB contains more bytes than a TB, a drive advertised as a certain number of terabytes can appear as a smaller numerical value when displayed in tebibytes.

What If My RAID Drives Are Different Sizes?

This calculator assumes all drives have the same usable capacity. In many traditional RAID configurations, using drives of different sizes causes the larger drives to be treated as though they were the size of the smallest drive.

For example, an array containing three 4 TB drives and one 8 TB drive may not be able to use the full additional capacity of the 8 TB drive in a conventional RAID layout.

Some modern storage systems use more flexible allocation methods, so check the documentation for your RAID controller, NAS, operating system, or storage platform.

RAID 5 vs RAID 6

RAID 5 sacrifices the equivalent capacity of one drive for parity and can tolerate one drive failure. RAID 6 sacrifices the equivalent capacity of two drives and can tolerate two failures.

RAID 5 therefore provides more usable capacity for the same number and size of drives, while RAID 6 provides an additional level of redundancy.

RAID 5 vs RAID 10

RAID 5 uses distributed parity, while RAID 10 uses mirrored pairs. Their usable capacity can differ significantly depending on the number of drives.

With four equal drives, RAID 5 provides approximately 75% storage efficiency while RAID 10 provides 50%. Capacity alone, however, should not be the only factor when selecting a RAID level.

Performance characteristics, rebuild behavior, workload, controller implementation, drive type, and failure risk can also matter.

RAID 6 vs RAID 10

With four equal-size drives, RAID 6 and RAID 10 both provide 50% usable capacity. Their redundancy methods are different: RAID 6 uses dual parity, while RAID 10 mirrors pairs of drives and stripes data across the pairs.

As the number of drives increases, RAID 6 generally uses a larger percentage of the total raw capacity because its parity overhead remains equivalent to two drives.

Does RAID Replace a Backup?

No. RAID can improve availability when a drive fails, but it does not protect against every form of data loss.

RAID does not by itself protect against accidental deletion, file corruption, malware, theft, fire, controller failure, catastrophic array failure, or other events that can affect all copies of data in the array.

Important data should have a separate backup strategy in addition to RAID.

Which RAID Level Should I Use?

This calculator is designed to compare capacity and redundancy, not to select a RAID level for you. The appropriate configuration depends on the storage system, workload, performance requirements, number and type of drives, backup strategy, recovery requirements, and acceptable failure risk.

Check the requirements and recommendations for your NAS, RAID controller, server, or storage software before creating an array.

RAID Calculator Limitations

The results are theoretical capacity estimates based on equal-size drives. Real usable capacity may be somewhat lower because of filesystem formatting, filesystem metadata, RAID metadata, reserved space, manufacturer conventions, controller behavior, and storage-system implementation.

The calculator currently covers common RAID 0, RAID 1, RAID 5, RAID 6, and RAID 10 configurations. Nested or proprietary storage layouts and alternative systems such as RAID-Z, SHR, erasure coding, or vendor-specific hybrid RAID are not included.