Two definitions of "kilo"
Computers are binary, so early engineers used 1 KB = 1,024 bytes (2¹⁰) — close enough to a thousand to borrow the name. Storage manufacturers, meanwhile, used the SI-correct 1 KB = 1,000 bytes. Both called it "kilobyte," and decades of confusion followed.
The fix arrived in 1998: the IEC defined binary prefixes — kibibyte (KiB) = 1,024 bytes, mebibyte (MiB) = 1,024², gibibyte (GiB) = 1,024³ — while KB/MB/GB kept their decimal meanings (powers of 1,000). Correct usage today:
- KB, MB, GB, TB — decimal: ×1,000 each step. Used by drive makers, network speeds.
- KiB, MiB, GiB, TiB — binary: ×1,024 each step. Used by operating systems, RAM.
Where your gigabytes went
A "1 TB" drive holds 1,000,000,000,000 bytes (decimal). Your OS reports in binary gibibytes: 1,000,000,000,000 ÷ 1,073,741,824 ≈ 931 GiB — displayed as "931 GB." The ~7% gap per step compounds: at the tera scale you "lose" 69 GB to unit mismatch alone. Add filesystem overhead and recovery partitions, and the number shrinks further. Nobody stole your storage; the label and the OS are speaking different dialects.
The full ladder
| Decimal (SI) | Bytes | Binary (IEC) | Bytes |
|---|---|---|---|
| 1 KB (kilobyte) | 1,000 | 1 KiB (kibibyte) | 1,024 |
| 1 MB (megabyte) | 1,000,000 | 1 MiB (mebibyte) | 1,048,576 |
| 1 GB (gigabyte) | 1,000,000,000 | 1 GiB (gibibyte) | 1,073,741,824 |
| 1 TB (terabyte) | 10¹² | 1 TiB (tebibyte) | 2⁴⁰ ≈ 1.0995 × 10¹² |
Note the divergence grows: MB vs. MiB differ by ~4.9%, GB vs. GiB by ~7.4%, TB vs. TiB by ~10%.
Practical takeaways
- Buying storage: expect ~93% of the labeled terabytes in your OS. A "2 TB" drive ≈ 1.86 TiB usable before formatting.
- Internet speeds: "100 Mbps" means megabits — divide by 8 for megabytes per second (~12.5 MB/s theoretical).
- RAM: memory is always binary — 16 GB of RAM is really 16 GiB.
- Converting: our unit converter includes both unit families, so KB→KiB and friends are one tap.
Bits vs. bytes: the other confusion
Capitalization carries meaning here: b = bit, B = byte (8 bits). Network speeds are sold in bits (Mbps), file sizes measured in bytes (MB) — a factor-of-8 gap that confuses every buyer at least once. "100 Mbps" internet downloads at ~12.5 MB/s theoretical maximum; real-world overhead (protocol headers, congestion, Wi-Fi) typically delivers 70–90% of that.
Download-time math: seconds ≈ (file MB × 8) ÷ speed Mbps. A 5 GB (5,000 MB) game on 100 Mbps: (5,000 × 8) ÷ 100 = 400 seconds ≈ 7 minutes theoretical, ~9 minutes realistic. Streaming needs are modest by comparison: 4K video ≈ 15–25 Mbps, video calls ≈ 2–5 Mbps — which is why a "slow" 50 Mbps connection handles most households fine.
When comparing plans or diagnosing speeds, convert carefully: our unit converter handles the byte-family units (B through GiB), and dividing or multiplying by 8 bridges the bit/byte gap. The single most useful habit: always check whether the "b" is big or little before doing any math.
Storage buyers face a related numbers game: advertised vs. usable capacity. Beyond the decimal/binary gap, file systems reserve space for their own bookkeeping (typically 5–10% on large drives), and SSDs hold back additional over-provisioning for wear leveling and performance. A "1 TB" SSD therefore yields roughly 850–900 GB of truly usable space — completely normal, and worth knowing before you plan a photo archive or game library around the label number.
For the broader measurement landscape, see metric vs. imperial and Celsius to Fahrenheit.