Hard Drive Capacity Calculator — Why 1 TB Shows as 931 GB
A hard drive capacity calculator converts the decimal size on the box into the binary GiB your operating system displays — and shows that nothing is actually missing.
Quick answer: 1 TB = 1,000,000,000,000 bytes on the box, but the OS divides by 1,073,741,824 bytes per GiB → 931.32 GiB. The same drive is 931 GiB in Windows, 1.0 TB in macOS (decimal) and 1,000 GB in Linux tools configured for SI.
Recovery partitions, over-provisioning and vendor tools can hide another 1–5%
| On the box (decimal) | Windows (GiB) | Bytes |
|---|---|---|
| 128 GB | 119.209 GiB | 128,000,000,000 B |
| 256 GB | 238.419 GiB | 256,000,000,000 B |
| 500 GB | 465.661 GiB | 500,000,000,000 B |
| 1 TB | 931.323 GiB | 1,000,000,000,000 B |
| 2 TB | 1,862.65 GiB | 2,000,000,000,000 B |
| 4 TB | 3,725.29 GiB | 4,000,000,000,000 B |
| 8 TB | 7,450.58 GiB | 8,000,000,000,000 B |
| 16 TB | 14,901.2 GiB | 16,000,000,000,000 B |
| 500 GB → | 465.66 GiB in Windows |
|---|---|
| 1 TB → | 931.32 GiB |
| 2 TB → | 1,862.65 GiB |
| 4 TB → | 3,725.29 GiB |
| 8 TB → | 7,450.58 GiB |
| Difference | Exactly 1024³ ÷ 10⁹ = 1.0737 — a constant 7.4% reduction |
The arithmetic, once and for all
Drive vendors follow SI: 1 TB = 10¹² bytes. Operating systems traditionally use binary: 1 GiB = 2³⁰ = 1,073,741,824 bytes. Divide one by the other and every advertised capacity shrinks by 7.38% when displayed in GiB: 500 GB → 465.7 GiB, 1 TB → 931.3 GiB, 4 TB → 3,725 GiB. Nothing is hidden, formatted away or reserved by the manufacturer.
Over-provisioning and hidden partitions
There is a second, much smaller reduction worth knowing: SSDs reserve 1–7% for wear management, and prebuilt systems sometimes hide recovery partitions. Those are real deductions, but they happen after the GiB conversion — so a 1 TB SSD shows 931 GiB and then reports about 20 GB unallocated for over-provisioning. If you need the full figure, check Disk Management (Windows) or diskutil list (macOS) rather than the file manager.
Comparing drives fairly
When comparing drives from different vendors, compare bytes or price per terabyte of decimal capacity — that is how retail pricing works. Comparing “931 GiB” against “1,000 GB” makes an identical product look different. Cloud storage is quoted the same way, which is why a 1 TB plan holds a 931 GiB local copy exactly, and why backup tools that count in binary report quotas as reached slightly early.
When to use this calculator
- Explaining a “missing” 69 GB on a new drive.
- Verifying that a purchased drive is genuine rather than counterfeit.
- Comparing drive capacity claims across vendors and operating systems.
- Planning partitions in GiB while the drive is sold in GB.
- Understanding why a 1 TB cloud plan fills up at 931 GiB locally.