Digital Storage Units Explained
Digital storage units can be confusing because two different systems are in common use: the decimal (SI) system used by hard drive manufacturers, and the binary (IEC) system used by operating systems. This difference is why a "1 TB" hard drive shows only about 931 GB when you plug it into a Windows or macOS computer — and it's not a scam. It's just a units mismatch.
The Two Systems: Decimal vs Binary
Decimal (SI) Prefixes — Used by Hard Drive Manufacturers
In the decimal system, every prefix multiplies by 1,000:
- 1 Kilobyte (KB) = 1,000 bytes
- 1 Megabyte (MB) = 1,000,000 bytes
- 1 Gigabyte (GB) = 1,000,000,000 bytes
- 1 Terabyte (TB) = 1,000,000,000,000 bytes
- 1 Petabyte (PB) = 1,000,000,000,000,000 bytes
Hard drive manufacturers (Seagate, WD, Samsung) advertise capacity in decimal GB and TB. This is technically correct per the SI standard and is larger-looking on the box.
Binary (IEC) Prefixes — Used by Operating Systems
In the binary system, every prefix multiplies by 1,024 (2¹⁰):
- 1 Kibibyte (KiB) = 1,024 bytes
- 1 Mebibyte (MiB) = 1,048,576 bytes
- 1 Gibibyte (GiB) = 1,073,741,824 bytes
- 1 Tebibyte (TiB) = 1,099,511,627,776 bytes
Windows reports file sizes in what it labels "GB" but actually means GiB. macOS Monterey and later correctly reports decimal GB. This historical inconsistency is the source of endless confusion.
Why Does a 1 TB Drive Show ~931 GiB?
A 1 TB drive contains exactly 1,000,000,000,000 bytes. When Windows reads it and reports in GiB (labeling it "GB"), it divides by 1,073,741,824 (= 1024³): 1,000,000,000,000 ÷ 1,073,741,824 = 931.3 GiB. That's the "missing" 69 GB — it hasn't gone anywhere, it's just a unit label mismatch.
Storage Unit Reference Table
| Unit | Symbol | Bytes | System |
|---|---|---|---|
| Bit | b | 0.125 | Both |
| Byte | B | 1 | Both |
| Kilobyte | KB | 1,000 | Decimal (SI) |
| Kibibyte | KiB | 1,024 | Binary (IEC) |
| Megabyte | MB | 1,000,000 | Decimal (SI) |
| Mebibyte | MiB | 1,048,576 | Binary (IEC) |
| Gigabyte | GB | 1,000,000,000 | Decimal (SI) |
| Gibibyte | GiB | 1,073,741,824 | Binary (IEC) |
| Terabyte | TB | 1,000,000,000,000 | Decimal (SI) |
| Petabyte | PB | 1,000,000,000,000,000 | Decimal (SI) |
Real-World Storage Benchmarks
To put storage sizes in context: a typical high-resolution JPEG photo is 3–8 MB; an MP3 song runs 3–10 MB; a 1080p HD movie is 4–8 GB; a 4K movie is 40–100 GB. A 1 TB hard drive holds approximately 200,000 photos, 125,000 songs, or 125–250 HD movies. Internet connection speeds (Mbps, Gbps) use bits per second — so a 100 Mbps connection can theoretically download at 12.5 MB/s.
Frequently Asked Questions
What is the difference between GB and GiB?
A gigabyte (GB) = 1,000,000,000 bytes (10⁹). A gibibyte (GiB) = 1,073,741,824 bytes (2³⁰). That's about a 7.4% difference. Hard drive manufacturers use GB (decimal); operating systems like Windows historically report in GiB while labeling it "GB," which causes the apparent "missing storage."
How many MB are in a GB?
In the decimal system: 1 GB = 1,000 MB. In the binary system: 1 GiB = 1,024 MiB. The confusion arises because both are commonly labeled "GB" or "MB" in consumer software, even when the binary definition is being used.
How many bits are in a byte?
Exactly 8 bits = 1 byte. This is why internet speeds (in Mbps or Gbps) and file sizes (in MB or GB) require a conversion: a 100 Mbps connection can download at 100 ÷ 8 = 12.5 megabytes per second (MB/s). Always check whether a spec uses bits (lowercase b) or bytes (uppercase B).
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Storage unit definitions sourced from IEC 80000-13 and IEEE 1541 standards. Accuracy note.