Data Storage Calculator
Show the math
What Your Result Means
- Decimal (SI) Values: These use powers of 1,000 (KB, MB, GB, TB, PB). This is how storage manufacturers label drives — a "1 TB" drive contains exactly 1,000,000,000,000 bytes.
- Binary (IEC) Values: These use powers of 1,024 (KiB, MiB, GiB, TiB, PiB). This is how Windows and most operating systems report file sizes. A "1 TB" drive shows as about 931 GiB in your OS.
- The Gap: The difference between decimal and binary grows with size. At the terabyte level, the gap is roughly 7–10%. This is not missing storage — it is purely a labeling difference.
How This Calculator Works
You enter a value and select its unit. The tool first converts your input to bytes using the decimal (SI) multiplier, then divides by every unit's factor to show equivalents in both decimal and binary systems side by side. No rounding is applied until display — the underlying arithmetic uses full floating-point precision.
Quick Questions
Why does my hard drive show less space than advertised?
Drive manufacturers use decimal units (1 GB = 1,000,000,000 bytes), while your OS typically reports in binary units (1 GiB = 1,073,741,824 bytes). The drive has exactly as many bytes as advertised — the number just looks smaller in binary units.
What is the difference between KB and KiB?
KB (kilobyte) is 1,000 bytes in the SI/decimal system. KiB (kibibyte) is 1,024 bytes in the IEC/binary system. The IEC prefixes (KiB, MiB, GiB) were introduced in 1998 to eliminate this ambiguity.
Does macOS use decimal or binary?
Since macOS 10.6 (Snow Leopard), Apple uses decimal units — 1 GB = 1,000,000,000 bytes. This means the reported capacity matches the drive label. Windows still uses binary units with decimal labels, which causes the apparent discrepancy.
How much usable space does a formatted drive have?
Formatting typically uses 1–5% of the raw capacity for the filesystem structure (allocation tables, journals, etc.). So a 1 TB drive may show around 930 GiB after both the binary conversion and formatting overhead.
Sources
- Wikipedia — Binary Prefix (IEC binary prefix standard history and definitions)
- NIST — Prefixes for Binary Multiples (official IEC prefix definitions)
Method & review
Estimate only. Results reflect your inputs and standard formulas. Double-check important decisions independently.