Drive Capacity Calculator

The Drive Capacity Calculator is an essential Data Storage and IT Management tool. Its primary function is to estimate the real-world, usable storage space on a digital storage device or a combination of devices in a RAID configuration. It accounts for factors that reduce the available space compared to the manufacturer's advertised capacity.

This calculator assists users, IT professionals, and system builders to:

• Determine the approximate usable storage space after formatting a drive with a specific filesystem.
• Understand the difference between advertised capacity (often in decimal gigabytes/terabytes) and the capacity reported by operating systems (often in binary gibibytes/tebibytes).
• Calculate the total usable capacity of various RAID (Redundant Array of Independent Disks) configurations, considering redundancy overhead.
• Plan storage needs more accurately for personal computers, servers, or Network Attached Storage (NAS) devices.
• Make informed decisions when purchasing storage drives or configuring storage systems.

By providing realistic estimates, the calculator helps manage expectations and prevent potential shortfalls in storage capacity planning.

Formula of Drive Capacity Calculator

Calculating usable drive capacity involves considering formatting overhead and, for RAID arrays, the specific configuration's redundancy scheme.

1. Single Drive Usable Capacity:

Operating systems and filesystems require a portion of the drive's space for management structures (like file allocation tables, journaling, metadata). The basic formula is:

Usable Capacity = Raw Capacity × (1 - Overhead Factor)

Where:

• Usable Capacity is the actual storage space available for user data.
• Raw Capacity is the advertised capacity of the drive (e.g., 1 TB, 500 GB). Note: There's also a difference between manufacturer GB (10^9 bytes) and OS GiB (2^30 bytes) which contributes to perceived loss.
• Overhead Factor is a decimal representing the percentage of space used by the filesystem and formatting process.

Typical Overhead Factors (approximate):

• FAT32: 0.03 - 0.04 (3-4%)
• NTFS: 0.03 - 0.08 (3-8%)
• exFAT: 0.01 - 0.03 (1-3%)
• EXT4: 0.01 - 0.05 (1-5%)
• APFS: Varies, typically low single-digit percentage.

2. RAID Capacity:

RAID configurations combine multiple drives for performance or redundancy, affecting total usable capacity. Assuming all drives in the array have the same capacity (best practice):

• RAID 0 (Striping): Focuses on performance, no redundancy.
  Total Capacity = Number of Drives × Capacity per Drive

• RAID 1 (Mirroring): Focuses on redundancy, duplicates data.
  Total Capacity = Capacity of one Drive (effectively, capacity of the smallest drive if mixed)

• RAID 5 (Striping with Distributed Parity): Balance of performance and redundancy. Requires at least 3 drives.
  Total Capacity = (Number of Drives - 1) × Capacity per Drive

• RAID 6 (Striping with Dual Distributed Parity): Enhanced redundancy. Requires at least 4 drives.
  Total Capacity = (Number of Drives - 2) × Capacity per Drive

• RAID 10 (or 1+0) (Mirroring + Striping): Combines mirroring and striping. Requires at least 4 drives (even numbers).
  Total Capacity = (Number of Drives / 2) × Capacity per Drive

Note: These RAID formulas give the raw usable capacity before filesystem overhead is applied.

Reference Table: Estimated Usable Capacity Drive Capacity Calculator

Manufacturers often advertise capacity in decimal units (GB/TB, powers of 10), while operating systems typically report capacity in binary units (GiB/TiB, powers of 2). This difference, combined with filesystem overhead (assumed ~7% here for NTFS/HFS+), leads to the perceived "loss" of space.

Advertised Capacity (TB)	Advertised Capacity (GB)	Approx. Capacity (TiB)	Approx. Capacity (GiB)	Estimated Usable Space (TiB/GiB after ~7% overhead)
0.5 TB	500 GB	0.45 TiB	465 GiB	~0.42 TiB / ~432 GiB
1 TB	1000 GB	0.91 TiB	931 GiB	~0.85 TiB / ~866 GiB
2 TB	2000 GB	1.82 TiB	1862 GiB	~1.69 TiB / ~1731 GiB
4 TB	4000 GB	3.64 TiB	3725 GiB	~3.38 TiB / ~3464 GiB
8 TB	8000 GB	7.28 TiB	7450 GiB	~6.77 TiB / ~6928 GiB
12 TB	12000 GB	10.91 TiB	11175 GiB	~10.15 TiB / ~10393 GiB
16 TB	16000 GB	14.55 TiB	14901 GiB	~13.53 TiB / ~13858 GiB
20 TB	20000 GB	18.19 TiB	18626 GiB	~16.92 TiB / ~17320 GiB

Note: 1 TB = 10^12 bytes, 1 TiB = 2^40 bytes. 1 GB = 10^9 bytes, 1 GiB = 2^30 bytes. Actual overhead varies.

Example of Drive Capacity Calculator

Let's calculate the usable capacity for a RAID 1 array using two advertised 4 TB drives, formatted with NTFS (assuming ~7% overhead including binary conversion difference).

Given:

• Number of Drives: 2
• Advertised Capacity per Drive: 4 TB
• RAID Level: RAID 1 (Mirroring)
• Estimated Overhead (including binary conversion + filesystem): ~7% (or 0.07)

Calculation Steps:

1. Calculate Raw RAID Capacity:
   For RAID 1, the total capacity is equal to the capacity of one drive.
   Raw RAID Capacity = Capacity of one Drive = 4 TB (advertised)

2. Convert Advertised TB to Binary GiB (Approximate):
   From the table, 4 TB (advertised) is approximately 3725 GiB as reported by the OS before filesystem overhead.

3. Apply Overhead Factor to Estimate Usable Capacity:
   Usable Capacity (GiB) = Raw Capacity (GiB) × (1 - Overhead Factor) ≈ 3725 GiB × (1 - 0.07)
   Usable Capacity (GiB) ≈ 3725 GiB × 0.93 ≈ 3464 GiB

Alternatively, using the table's final estimate directly:
A 4 TB drive yields roughly 3464 GiB usable space after overhead. Since RAID 1 mirrors, the total usable space for the array is the usable space of one drive.

Therefore, a RAID 1 array built with two 4 TB advertised drives will offer approximately 3464 GiB (or ~3.38 TiB) of usable storage space after formatting.

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