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Digital Storage Explained: Decimal (GB) vs Binary (GiB)
Why does a 1TB hard drive only show 931 GB in Windows? Understand the crucial difference between SI decimal prefixes and binary IEC prefixes.
By UnitCompiler Computing Lab • 6 min read • Published 2026-08-30
The 1000 vs 1024 Confusion
When hard drive manufacturers market a “1 Terabyte” drive, they use SI decimal units:
- 1 TB = 10¹² bytes = 1,000,000,000,000 bytes.
However, operating systems like Microsoft Windows historically measure memory in binary powers of 2 (2¹⁰ = 1024):
- 1 TiB (Tebibyte) = 2⁴⁰ bytes = 1,099,511,627,776 bytes.
Why 1TB = 931.32 GiB
1,000,000,000,000 ÷ (1024 × 1024 × 1024 × 1024) ≈ 931.322 GiB. You did not lose 69 GB of space; it is purely a unit naming difference!
SI (Decimal) vs IEC (Binary) Standards
In 1998, the International Electrotechnical Commission (IEC) established official binary prefixes:
- Kibibyte (KiB) = 1,024 Bytes
- Mebibyte (MiB) = 1,024 KiB = 1,048,576 Bytes
- Gibibyte (GiB) = 1,024 MiB = 1,073,741,824 Bytes
- Tebibyte (TiB) = 1,024 GiB = 1,099,511,627,776 Bytes
Gigabytes (Decimal) to Gibibytes (Binary)
GiB = GB × (10⁹ / 2³⁰) ≈ GB × 0.93132257
Internet Speed (Mbps) to Download Speed (MB/s)
Megabytes (MB) = Megabits (Mb) ÷ 8
A 100 Mbps internet connection yields a theoretical maximum download rate of 12.5 MB/s
Comparison Table
Decimal (Base 10) vs Binary (Base 2) Comparison
| Unit Name | SI Prefix (Base 10) | IEC Binary Prefix (Base 2) | Discrepancy |
|---|---|---|---|
| Kilo / Kibi | 1 KB = 1,000 Bytes | 1 KiB = 1,024 Bytes | +2.4% |
| Mega / Mebi | 1 MB = 1,000,000 Bytes | 1 MiB = 1,048,576 Bytes | +4.86% |
| Giga / Gibi | 1 GB = 1,000,000,000 Bytes | 1 GiB = 1,073,741,824 Bytes | +7.37% |
| Tera / Tebi | 1 TB = 10¹² Bytes | 1 TiB = 2⁴⁰ Bytes | +9.95% |
| Peta / Pebi | 1 PB = 10¹⁵ Bytes | 1 PiB = 2⁵⁰ Bytes | +12.59% |