Binary vs Decimal Storage (GB vs GiB) Explained

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Binary vs Decimal Storage

A “1TB” drive often shows as ~931 in Windows because drive labels use decimal (TB) while many OS tools report using binary (GiB/TiB).

Converter

Decimal ↔ Binary

Amount

From unit

To unit

Tip: Try 1 TB → GiB to see why a “1TB” drive shows as ~931 in many OS displays.

Exact

Conversion	Formula	Notes
GB → GiB	GiB = GB × 10⁹ ÷ 2³⁰	Convert to bytes, then divide by binary unit size
TB → TiB	TiB = TB × 10¹² ÷ 2⁴⁰	Rule of thumb: TB × 0.909 ≈ TiB
GiB → GB	GB = GiB × 2³⁰ ÷ 10⁹	Rule of thumb: GiB × 1.074 ≈ GB
TiB → TB	TB = TiB × 2⁴⁰ ÷ 10¹²	Rule of thumb: TiB × 1.100 ≈ TB

Same bytes, different grouping

Storage is counted in bytes. The confusion comes from two systems:

Decimal: 1 KB = 1,000 bytes Binary: 1 KiB = 1,024 bytes Decimal: 1 GB = 10⁹ bytes Binary: 1 GiB = 2³⁰ bytes

Most drive labels use GB/TB (decimal). Many OS tools show sizes in GiB/TiB (binary) but still label them as “GB/TB”, which adds confusion.

Why it looks smaller

A 1TB drive (decimal) contains 1,000,000,000,000 bytes. If your OS reports in GiB (binary), it divides by 2³⁰ = 1,073,741,824:

1,000,000,000,000 ÷ 1,073,741,824 ≈ 931.32 GiB

Formatting and partitions can reduce usable space slightly, but the big drop is usually unit math.

Practical uses

Buying drives: labels are usually decimal (GB/TB).
Backup planning: compare in bytes or convert TB ↔ TiB to avoid surprises.
NAS / RAID: usable space is further affected by parity/overhead (in addition to unit differences).

What you often see

Label (decimal)	Bytes	Binary shown	Math
256 GB	256 × 10⁹	~238.42 GiB	256e9 ÷ 2³⁰
512 GB	512 × 10⁹	~476.84 GiB	512e9 ÷ 2³⁰
1 TB	1 × 10¹²	~931.32 GiB	1e12 ÷ 2³⁰
2 TB	2 × 10¹²	~1.82 TiB	2e12 ÷ 2⁴⁰
4 TB	4 × 10¹²	~3.64 TiB	4e12 ÷ 2⁴⁰

Formatting, reserved blocks, and partitions can reduce usable space further.

Handy ratios

Decimal	≈ Binary	Binary	≈ Decimal
1 GB	0.93 GiB	1 GiB	1.07 GB
1 TB	0.91 TiB	1 TiB	1.10 TB
10 TB	9.09 TiB	10 TiB	11.00 TB

Short answers

Is my drive missing space? Usually no — it’s the same bytes reported using different units.

Why do drive makers use decimal? Decimal aligns with SI prefixes (kilo = 1000), and it’s standard for marketing and manufacturing.

Why does the OS use binary? Computers naturally work in powers of two, so GiB/TiB align with that math.

Does formatting reduce size? Yes. File system metadata, reserved space, and partitions reduce usable space slightly.

What should I use for planning? Convert everything to bytes, or use TB↔TiB conversions when comparing drive labels to OS displays.

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