Storage Capacity — Advertised vs Actual Capacity

When purchasing external storage devices, users may notice a discrepancy between the advertised storage capacity and the actual usable storage. This is due to a difference in how manufacturers calculate storage (in base 10) and how operating systems interpret it (in base 2). To explain this discrepancy, it’s important to understand how storage devices are marketed and the fundamental differences in units used to measure storage.

In the past, manufacturers decided to rate storage devices using the metric system (base 10), which is simpler for most consumers to understand. For instance, 1 Megabyte is defined as 1,000,000 bytes and 1 Gigabyte as 1,000,000,000 bytes. However, computers, including hard drives, use binary mathematics (base 2) to calculate storage. In the binary system, 1 Kilobyte equals 1,024 bytes, 1 Megabyte equals 1,024 Kilobytes (or 1,048,576 bytes), and so on. This means that the actual capacity is often lower than the advertised capacity when you factor in the difference between the base 10 and base 2 systems.

For example, when you purchase a 1TB hard drive, the manufacturer’s claim is based on a decimal system (1,000,000,000,000 bytes), while your computer, interpreting the size based on binary, views the drive as having only 0.91TB (1,099,511,627,776 bytes). The difference between these two calculations results in the discrepancy you observe between advertised and actual storage capacity.

While this difference may seem minor on a smaller scale, it becomes significant as storage sizes increase, leading to a noticeable gap between the advertised and actual capacity. This article helps clarify these differences and shows how various advertised storage capacities differ from what you actually get on your device.