Two capacity numbers can differ because they use different unit scales. That is separate from space used by formatting, system structures or files. Start with the exact byte quantity and the unit definition before deciding what a displayed difference means.
Apple's capacity guidance describes decimal units such as 1 GB = 1,000,000,000 bytes and notes that formatted usable capacity can be smaller than labeled capacity. Do not use that observation to claim that every operating system displays capacity in the same way.
Put decimal and binary units side by side
| Unit | Bytes | Meaning in this guide |
|---|---|---|
| MB | 1,000,000 | Decimal megabyte |
| GB | 1,000,000,000 | Decimal gigabyte |
| MiB | 1,048,576 | Binary mebibyte |
| GiB | 1,073,741,824 | Binary gibibyte |
An illustrative 1,000,000,000-byte quantity is exactly 1 GB and about 0.931323 GiB. The bytes have not changed. The divisor used to express the quantity has changed. Likewise, 1,000,000,000,000 bytes is 1,000 GB or about 931.323 GiB before any additional use of space.
Separate unit conversion from usable capacity
If a product label says 1 TB and a system reports a smaller number, first record how the system labels and defines that display. Then distinguish the total device or volume capacity from available free space. A file-system volume can occupy only part of a device, and existing data can reduce free space without changing total physical capacity.
Do not declare that an unexplained difference is entirely formatting overhead. The unit conversion is one calculable component; partitions, files and the actual device need their own evidence.
Keep a reconciliation sheet
Write product model, labeled capacity, labeled unit definition, system and version, exact displayed label, total-volume figure and free-space figure. If the interface exposes a byte count, record it. Compare like with like: total versus total or free versus free, not the product total against the remaining free space.
For transfer calculations, choose the matching units explicitly. Ten GB and ten GiB are different byte quantities, so identical rate numbers can produce different times. The calculator preserves that distinction.
Stop before destructive changes
A numerical difference alone is not a reason to erase or repartition a disk. Investigate the unit and volume facts first, preserving existing data. This guide explains arithmetic and documentation; it does not diagnose a physical drive's capacity or certify its condition.
Sources and scope
Official documentation checked October 17, 2026. The exact models, versions and conditions below matter.
- Apple: storage capacity shown by devices — Decimal storage units use 1 GB = 1 billion bytes; formatted usable capacity can be smaller than labeled capacity.