RAID Calculator

About RAID Calculator

RAID (Redundant Array of Independent Disks) combines multiple drives into a single logical volume for speed, redundancy, or both. This calculator shows the usable capacity, space efficiency, fault tolerance, and minimum drive count for RAID 0, 1, 5, 6, and 10 across any drive configuration you enter.

RAID Levels at a Glance

RAID Level	How It Works	Min Drives	Usable Capacity	Fault Tolerance	Read Speed	Write Speed
RAID 0 (Stripe)	Data split across all drives in blocks	2	N x drive size (100%)	None - one failure loses everything	Excellent (N x single drive)	Excellent (N x single drive)
RAID 1 (Mirror)	Identical copy on each drive	2	1 x drive size (50%)	N-1 drives can fail	Good (reads from any mirror)	Normal (writes to all mirrors)
RAID 5 (Stripe + Parity)	Data striped with distributed parity	3	(N-1) x drive size	1 drive	Good	Slower (parity calculation)
RAID 6 (Stripe + Double Parity)	Data striped with two parity blocks	4	(N-2) x drive size	2 drives	Good	Slowest (double parity)
RAID 10 (Mirror + Stripe)	Striped across mirrored pairs	4 (even)	N/2 x drive size (50%)	1 per mirror pair	Excellent	Good

Usable Capacity Formulas

The calculator applies these formulas to determine how much space you can actually use:

RAID Level	Formula	Example: 4 x 4 TB Drives	Usable	Efficiency
RAID 0	N x S	4 x 4 TB	16 TB	100%
RAID 1	S	4 TB (mirrored 4 ways)	4 TB	25%
RAID 5	(N - 1) x S	3 x 4 TB	12 TB	75%
RAID 6	(N - 2) x S	2 x 4 TB	8 TB	50%
RAID 10	(N / 2) x S	2 x 4 TB	8 TB	50%

N is the number of drives, S is the size of each drive. If drives have different sizes, most RAID controllers treat every drive as the size of the smallest one, wasting the extra capacity on larger drives. Use the storage converter if you need to translate between TB, GB, and other units.

How Space Efficiency Changes with Drive Count

RAID 5 and RAID 6 become more space-efficient as you add drives, because the fixed parity overhead is spread across more drives:

Drive Count	RAID 0	RAID 1	RAID 5	RAID 6	RAID 10
2 drives	100%	50%	N/A	N/A	N/A
3 drives	100%	33%	67%	N/A	N/A
4 drives	100%	25%	75%	50%	50%
6 drives	100%	17%	83%	67%	50%
8 drives	100%	13%	88%	75%	50%
12 drives	100%	8%	92%	83%	50%

RAID 10 stays at 50% regardless of drive count because every drive has exactly one mirror. RAID 5 with 8 drives reaches 88% efficiency, which is why it is popular for NAS devices with many bays.

Choosing the Right RAID Level

Use Case	Recommended RAID	Why
Video editing scratch disk	RAID 0	Maximum speed and capacity, data is temporary and backed up elsewhere
Operating system / boot drive	RAID 1	Simple redundancy, fast reads, easy to recover from a failure
Home NAS / media server	RAID 5	Good balance of capacity and protection for 3-5 drives
Business file server	RAID 6	Survives two simultaneous failures, important for large arrays where rebuild times are long
Database server	RAID 10	Best random I/O performance with redundancy
Enterprise storage (large arrays)	RAID 6 or RAID 60	Double parity is essential when rebuild of a single large drive takes 12-24 hours

RAID Rebuild Times and Risk

When a drive fails in a RAID 5 or 6 array, the controller reconstructs the missing data from parity during a rebuild. This process stresses the remaining drives and takes time proportional to drive capacity:

Drive Size	Approximate Rebuild Time (RAID 5)	Risk During Rebuild
1 TB HDD	2-4 hours	Low - short window of vulnerability
4 TB HDD	8-16 hours	Moderate - sustained stress on remaining drives
8 TB HDD	16-36 hours	High - another drive failure during rebuild loses everything
16 TB HDD	24-72 hours	Very high - this is why RAID 6 exists for large drives
4 TB SSD	1-3 hours	Low - SSDs rebuild much faster

With modern drive sizes (8 TB+), RAID 5 is increasingly risky because a second drive failure during the long rebuild window means total data loss. This is why RAID 6 has become the standard recommendation for arrays with large HDDs.

RAID Is Not a Backup

This is the single most important thing to understand about RAID. RAID protects against drive failure only. It does not protect against:

Threat	RAID Protects?	What Protects You
Single drive failure	Yes (RAID 1/5/6/10)	RAID redundancy
Accidental file deletion	No	Backups, snapshots, versioning
Ransomware / malware	No - encrypts all drives simultaneously	Offline or immutable backups
Controller failure	No - may corrupt the array metadata	Backups, matching spare controller
Fire, flood, theft	No	Off-site backup (cloud or physical)
Silent data corruption (bit rot)	Not in standard RAID	ZFS, Btrfs, or other checksumming file systems
Multiple simultaneous drive failures	Depends on RAID level	RAID 6 or 10 for two failures, backups for more

The 3-2-1 backup rule applies regardless of RAID: three copies of data, on two different media types, with one copy off-site.

Hardware RAID vs Software RAID

Type	How It Works	Pros	Cons
Hardware RAID	Dedicated RAID controller card with its own processor and cache	Fastest performance, offloads CPU, battery-backed cache prevents data loss during power failure	Expensive, controller failure can make data inaccessible, locked to that controller model
Software RAID (mdadm, Windows Storage Spaces)	OS manages RAID using the CPU	Free, portable between systems, no special hardware needed	Uses CPU resources, no battery-backed write cache
ZFS / Btrfs	File system with built-in RAID-like functionality	Data checksumming (catches bit rot), flexible configuration, snapshots	Higher RAM requirements (ZFS wants 1 GB per TB), learning curve
Fake RAID (motherboard RAID)	BIOS-level RAID using the chipset	Included with motherboard, no extra hardware	Slower than true hardware RAID, limited features, tied to that chipset

To estimate bandwidth when transferring files to and from your array, check the file transfer calculator. All calculations run in your browser with no data sent anywhere.