Data Calculator

When you buy a "1 Terabyte" (1 TB) hard drive and plug it into your Windows computer, Windows says it only has about 931 Gigabytes of space. Did the manufacturer cheat you?

The confusion stems from two different ways of measuring data: Base-10 (Decimal) and Base-2 (Binary).

• Hard Drive Manufacturers use the metric SI standard (Base-10). To them, "Kilo" means exactly 1,000. So, 1 Kilobyte (KB) = 1,000 bytes. 1 Megabyte (MB) = 1,000,000 bytes.
• Operating Systems (like Windows) use binary computer logic (Base-2). Because computers work in powers of 2 (2, 4, 8, 16... 1024), to a computer, "Kilo" naturally means 1,024. So, 1 binary Kilobyte (KiB) = 1,024 bytes.

To fix this terminology conflict, the International Electrotechnical Commission (IEC) created new names for the binary units in 1998: Kilobyte (KB) = 1,000 bytes, while Kibibyte (KiB) = 1,024 bytes.

While macOS adopted the correct SI terminology for marketing numbers (showing exactly 1TB when you plug in a 1TB drive), Windows still uses the old, confusing terminology.

Windows labels files as "KB" or "GB" in the UI, but it actually calculates their sizes using the 1024 binary math (KiB/GiB). This is why storage drives always appear "smaller" when formatted in Windows.