Core answer: Storage is binary inside but sold in decimal: 1 KB = 1,024 B (binary) vs 1 kB = 1,000 B (decimal marketing). That gap explains why a "1 TB" drive shows 931 GB in your OS: 10¹² ÷ 2⁴⁰ = 0.909. Units ladder: B → KB → MB → GB → TB → PB, each ×1,024 in the binary world.
The two ladders (the source of all confusion)
| Unit | Decimal (drive makers) | Binary (your OS) |
|---|---|---|
| KB / KiB | 1,000 B | 1,024 B |
| MB / MiB | 1,000² = 10⁶ | 1,024² ≈ 1.049 M |
| GB / GiB | 10⁹ | 1.074×10⁹ |
| TB / TiB | 10¹² | 1.100×10¹² |
The shortfall grows with size: 2.4% at KB, 4.9% at GB, 9.1% at TB. A "2 TB" drive = 1.81 TiB.
Bits vs bytes (the ISP trick)
1 byte = 8 bits. Network speeds are quoted in bits: "500 Mbps broadband" downloads at most 500/8 = 62.5 MB/s. A 10 GB game at full speed: 80 Gb ÷ 500 Mbps ≈ 160 s ≈ 2.7 min.
Worked examples
Example 1 — Photo storage planning. 24 MP photos ≈ 8 MB each (JPEG): a 256 GB phone holds ~30,000 photos; shooting RAW (25 MB) cuts that to ~9,500.
Example 2 — Video budget. 1080p at 8 Mbps ≈ 3.6 GB/hour; 4K at 45 Mbps ≈ 20 GB/hour. A 128 GB card records ~6 h of 4K — plan cards for the shoot, not the camera manual's optimism.
Example 3 — NAS sizing. 5 years of family media: 40,000 photos (320 GB) + 200 h of video (1.5 TB) + documents (100 GB) ≈ 2 TB; with RAID-1 mirroring you need 2 × 4 TB drives for headroom.
Example 4 — Why the phone shows less. A "512 GB" phone = 512×10⁹ B = 476.8 GiB; minus ~20 GB system → ~455 GiB free. Nobody stole your storage.
Common mistakes and myths
- "The drive is defective" — 931 GB from 1 TB is math, not fraud; class-action lawsuits actually happened, and the industry answered with the KiB/MiB/GiB notation few people read.
- Confusing Mb and MB — an 8× factor; "100 Mbps fiber" ≠ 100 MB/s downloads.
- RAM vs storage — 16 GB RAM is working memory (volatile); 512 GB SSD is storage; "the computer is out of memory" is ambiguous for a reason.
- Assuming 4K = 4× the file of 1080p — bitrate, not resolution, sets file size; a well-encoded 4K stream can beat a bad 1080p one.
- Forgetting filesystem overhead — formatting, journaling, and reserved blocks eat 2–7% before you write a single file.