# RAID Capacity Calculator

> Work out usable storage for RAID 0, 1, 5, 6 and 10 from the number and size of drives, in decimal TB and binary TiB, with fault tolerance and efficiency.

Category: IT, networking & security · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/raid-capacity-calculator

RAID gives up some capacity for redundancy: one drive’s worth for RAID 5, two for RAID 6, half for RAID 10. Drive makers quote decimal TB, while many operating systems show binary TiB.

Also called: RAID storage calculator, NAS capacity calculator.

## Inputs
- RAID level (options: RAID 0 (striping), RAID 1 (mirror), RAID 5 (single parity), RAID 6 (double parity), RAID 10 (striped mirrors); default 6)
- Number of drives (default 6)
- Hot spares (default 0; optional)
- Drive size (default 4)
- Size unit (options: TB, GB; default TB)

## Results
- Usable capacity
- Usable capacity (binary)
- Raw capacity
- Redundancy and spares
- Storage efficiency
- Drive failures survived

## Formula

```
Usable = Data drives × Drive size, where data drives = n (RAID 0), 1 (RAID 1), n − 1 (RAID 5), n − 2 (RAID 6), n ÷ 2 (RAID 10), n = drives − hot spares;  TiB = bytes ÷ 2⁴⁰
```

Where:
- n: active drives in the array
- Drive size: decimal: 1 TB = 10¹² bytes

## Worked example
1. RAID 6 uses two drives’ worth of parity: 6 − 2 = 4 data drives.
2. 4 × 4 TB = 16 TB = 16 × 10¹² bytes.
3. 16 × 10¹² ÷ 2⁴⁰ = 14.552 TiB; 16 ÷ 24 TB raw = 66.67%.

**Result:** Six 4 TB drives in RAID 6 give 16 TB (14.55 TiB) usable and survive any two drive failures.

## What the result means

Leave free space for file-system metadata and snapshots, and remember RAID is not a backup: deletion, corruption and ransomware are copied to every drive.

## Assumptions and limits
- All drives are the same size; with mixed sizes most RAID levels use only the smallest size on every drive.
- File-system overhead, ZFS/Btrfs specifics and vendor RAID variants (RAID-Z, SHR) are not modelled.

## Questions

### Why does my 4 TB drive show 3.64 TB?

The drive has 4 × 10¹² bytes. Windows divides by 2⁴⁰ and shows 3.64, but labels it “TB” rather than TiB.

### RAID 6 or RAID 10?

RAID 6 survives any two failures and gives more space with many drives; RAID 10 gives better write performance and faster rebuilds but can be lost if both drives in one mirror fail.

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Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/raid-capacity-calculator