You bought a 500 GB drive, plugged it in, and Windows says it has 465 GB. Were you scammed? No. It's the difference between the decimal system (KB, MB, GB) and the binary one (KiB, MiB, GiB) — and understanding it saves you surprises every time you buy storage.
Decimal vs binary: the two ways of counting bytes
- Decimal (SI): 1 KB = 1000 bytes. Used by drive, USB stick and SD card manufacturers, and by internet speeds.
- Binary (IEC): 1 KiB = 1024 bytes. Used by operating systems when showing files and by RAM sizes.
Neither is "wrong": they're two conventions. The problem is that manufacturers advertise in decimal while your operating system displays in binary (even though it labels it GB).
Quick table: KB vs KiB, MB vs MiB
| Unit | Bytes (decimal) | Unit | Bytes (binary) |
|---|---|---|---|
| 1 KB | 1000 | 1 KiB | 1024 |
| 1 MB | 10⁶ = 1,000,000 | 1 MiB | 2²⁰ ≈ 1,048,576 |
| 1 GB | 10⁹ = 1,000,000,000 | 1 GiB | 2³⁰ ≈ 1,073,741,824 |
| 1 TB | 10¹² | 1 TiB | 2⁴⁰ ≈ 1,099,511,627,776 |
The 500 GB drive case, explained
A 500 GB drive (decimal) holds 500 × 10⁹ = 500,000,000,000 bytes. Windows divides that by 2³⁰ (1,073,741,824) and gets 465.66 → it shows 465 GB (actually GiB). Nobody tricked you: it's just two ways of counting the same bytes.
Why it matters in practice
The gap grows with every scale:
- A 1 TB drive (decimal) shows 931 GiB.
- A 2 TB drive shows 1.81 TiB.
- SD cards, USB sticks and cloud storage: exactly the same.
Trick: to know the real space your system will show, multiply decimal TB by 0.931 (or GB by 0.931). A "1 TB" drive is really 931 GiB usable.
Internet speeds: another disguised decimal
Providers advertise 300 Mb/s, but those are megabits, not megabytes: 300 Mb/s ÷ 8 = 37.5 MB/s actual download speed. And those "megs" are decimal (1 Mb = 1,000,000 bits), just like drives. That's why you never download at the "advertised" speed: first you divide by 8 (bits → bytes), and on top of that your OS counts in binary.
Common KB and KiB mistakes
- Assuming 1 MB = 1024 KB: in decimal it's 1000, in binary 1024. Depends on who labels it.
- Mixing notations: 1 MB ≠ 1 MiB. They differ by 2.4 % at the MB scale, 7.4 % at the GB scale and 10 % at the TB scale.
- Using "megs" for internet speed: there it's always decimal and in bits, not bytes.
FAQ about KB, KiB and storage
How many bytes are in a KiB?
1024 bytes.
Why do manufacturers use decimal?
Because the SI defines the kilo, mega and giga prefixes in base 10 — and the numbers look bigger.
Do I use KB or KiB?
Almost every operating system shows GiB but labels it GB. Now you know how to read it.
How much real space does a 1 TB drive have?
1 TB decimal = 10¹² bytes ≈ 931 GiB visible.
How much do I actually download with 300 Mb/s?
300 ÷ 8 = 37.5 MB/s theoretical; in practice, a bit less.
Convert between KB, KiB, MB, MiB, GB, GiB and more with the unit calculator — no decimal/binary confusion.
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