You just unboxed a shiny new 1TB external hard drive, plugged it into your Windows PC, and opened File Explorer — only to see "931 GB free." You check the box again. It clearly says 1TB. Have you been cheated? Not exactly. You have stumbled into one of the most persistent and costly confusions in consumer technology: the difference between megabytes (MB) and mebibytes (MiB).

Two Different Counting Systems

The root of the problem is that the word "byte" can be counted in two fundamentally different ways. Hard drive manufacturers use the decimal (SI) system, where prefixes mean powers of 10 — just like in everyday life. Under this system, 1 kilobyte (KB) = 1,000 bytes, 1 megabyte (MB) = 1,000,000 bytes, 1 gigabyte (GB) = 1,000,000,000 bytes, and 1 terabyte (TB) = 1,000,000,000,000 bytes (10^12).

Operating systems like Windows, however, were built in an era when engineers counted in powers of 2, because binary arithmetic is natural to computers. Under this system, 1 "kilobyte" = 1,024 bytes, 1 "megabyte" = 1,048,576 bytes, and 1 "gigabyte" = 1,073,741,824 bytes. Windows still labels these binary quantities with the familiar GB and MB symbols — which is where the confusion begins.

The IEC Steps In: MiB, GiB, TiB

In 1998, the International Electrotechnical Commission (IEC) introduced unambiguous names for binary units. The new prefixes — kibi-, mebi-, gibi-, tebi- — combine the first two letters of the SI prefix with "bi" for binary:

  • 1 KiB (kibibyte) = 2^10 = 1,024 bytes
  • 1 MiB (mebibyte) = 2^20 = 1,048,576 bytes (about 4.9% more than 1 MB)
  • 1 GiB (gibibyte) = 2^30 = 1,073,741,824 bytes (about 7.4% more than 1 GB)
  • 1 TiB (tebibyte) = 2^40 = 1,099,511,627,776 bytes (about 9.95% more than 1 TB)

So that 1TB drive contains exactly 1,000,000,000,000 bytes. Divide by 2^30 and you get approximately 931.32 GiB — which Windows faithfully reports as "931 GB" (using the wrong label, but the right binary number). Nothing is missing; the units simply don't match.

Real-World Examples That Hit Your Wallet

The gap grows with scale. Consider a "256GB iPhone." Apple follows the decimal convention: 256GB means 256 × 10^9 = 256,000,000,000 bytes. Convert to GiB and you get roughly 238 GiB. Subtract iOS itself (around 7–10 GB of system software) and you might see only 225–230 GB available in Settings. That is not a bug — it is the math.

Enterprise storage makes this even more visible. A "10TB" NAS drive holds about 9.09 TiB. Across a rack of 20 such drives, the discrepancy amounts to roughly 1.8 TiB of "missing" space — nearly two full drives worth of capacity that was never missing, just mislabeled.

The Megabits Trap: Internet Speeds

There is a second common confusion layered on top: bits vs bytes. Internet service providers advertise speeds in megabits per second (Mbps), but file sizes are measured in megabytes (MB). Since 1 byte = 8 bits, a 100 Mbps connection can transfer at most 12.5 MB/s — not 100 MB/s. So if you are downloading a 1 GB file on a "100 Mbps" plan, expect it to take at least 80 seconds under ideal conditions, not 10.

Why Haven't Labels Been Fixed?

Linux and macOS have largely moved to reporting storage in decimal GB (matching manufacturers), which eliminates the apparent discrepancy. Windows still uses binary GiB values but calls them GB. The IEC MiB/GiB labels appear in scientific and engineering software, many Linux distributions, and network equipment documentation — but consumer devices resist change because "238 GiB iPhone" is a harder sell than "256GB iPhone." The confusion, in other words, is partly by design.

The practical takeaway: when comparing storage across devices or planning backups, always check whether you are working in decimal GB or binary GiB. The difference is not rounding error — on a 4TB drive, it is nearly 400 GB.