The mystery of the 931 GB drive
You buy a 1 TB drive, plug it into a Windows computer, and File Explorer reports a capacity of 931 GB. Nearly 70 GB have gone missing — more than an entire operating system takes up. Did the manufacturer lie? Did formatting eat the space? The short answer is that nothing is missing: the manufacturer and Windows use the same abbreviation, GB, for two different amounts. This guide explains where the confusion comes from, how to do the math, what each operating system shows, and where the space you do lose actually goes. To convert any amount, the storage unit converter shows every equivalent at once.
Where the confusion comes from
Computers work in binary, and memory is naturally organized in powers of 2. In the early days of computing, 2¹⁰ = 1,024 was close enough to 1,000 that people got into the habit of calling it "kilo": a kilobyte of memory was 1,024 bytes. The 2.4% difference seemed negligible.
Drive manufacturers, on the other hand, never had a technical reason to use powers of 2: a drive's capacity depends on how many sectors fit on the platters, not on an address bus. From early on they advertised capacities using International System prefixes, where kilo means 1,000, mega 1,000,000, and giga 1,000,000,000 — exactly as in kilometer or megawatt.
For decades both meanings coexisted, sometimes within the same product. The most famous example is the "1.44 MB" floppy disk: its real capacity is 1,474,560 bytes, which is 1,440 × 1,024. That "MB" is neither 1,000² nor 1,024² but a blend of the two: 1,000 × 1,024.
The IEC's fix
In 1998, the International Electrotechnical Commission (IEC) published dedicated prefixes for powers of 2, now part of the IEC 80000-13 standard:
| Decimal prefix | Value | Binary prefix | Value |
|---|---|---|---|
| kilo (k) | 10³ = 1,000 | kibi (Ki) | 2¹⁰ = 1,024 |
| mega (M) | 10⁶ | mebi (Mi) | 2²⁰ = 1,048,576 |
| giga (G) | 10⁹ | gibi (Gi) | 2³⁰ = 1,073,741,824 |
| tera (T) | 10¹² | tebi (Ti) | 2⁴⁰ = 1,099,511,627,776 |
| peta (P) | 10¹⁵ | pebi (Pi) | 2⁵⁰ |
Under this convention, GB always means 10⁹ bytes and GiB means 2³⁰ bytes. Adoption has been uneven: Linux, macOS, and much of the open-source world use IEC prefixes or decimal ones correctly; Windows still calculates in powers of 2 and labels the result GB and TB.
The 1 TB math
The manufacturer sells 1 TB = 1,000,000,000,000 bytes. Windows divides that by 1,024³ to express it in "GB":
1,000,000,000,000 ÷ 1,073,741,824 = 931.32
Those 931.32 are GiB, even though Windows displays them as "931 GB." The gap widens with each prefix, because each one compounds the 1,000-versus-1,024 difference:
| Advertised | Equals | Apparent shortfall |
|---|---|---|
| 1 kB | 0.98 KiB | 2.3% |
| 1 MB | 0.95 MiB | 4.6% |
| 1 GB | 0.93 GiB | 6.9% |
| 1 TB | 0.91 TiB | 9.1% |
| 1 PB | 0.89 PiB | 11.2% |
That's why the bigger the drive, the more space seems to be "missing." An 8 TB drive shows up in Windows as 7.27 TB, and a server with a petabyte of storage appears to have 11% less.
What each system shows
| System or tool | Calculation | A 1 TB drive shows as |
|---|---|---|
| Manufacturer's box and specs | Decimal | 1 TB |
| Windows File Explorer | Binary, labeled as decimal | 931 GB |
| macOS Finder (since 10.6, 2009) | Decimal | 1 TB |
df -h, du -h, ls -lh (Linux) | Binary, with G and T suffixes | 932G |
df -H (Linux) | Decimal | 1.0T |
lsblk (Linux) | Binary | 931.5G |
Apple switched macOS to decimal in 2009 precisely so the Finder's number would match the box. On Linux it pays to know which option each command uses: df -h and df -H show different figures for the same drive. The gap between lsblk (931.5G) and the theoretical math (931.32) is because real drives usually ship with a few hundred million bytes more than the round number on the label.
The space you really do lose
Once the unit difference is out of the way, some space genuinely isn't available for files:
- The file system. Internal structures (the MFT on NTFS, inode tables on ext4, the journal) take anywhere from a few to several GB depending on drive size.
- Reserved blocks on ext4. By default, ext4 reserves 5% of the space for the root user, so the system keeps working when the disk fills up. On an 8 TB data drive that's nearly 400 GB; you can lower it with
tune2fs -m 1. - Recovery and boot partitions. Off-the-shelf computers often ship with several hidden partitions totaling anywhere from a few hundred MB to several GB.
- SSD over-provisioning. SSDs set aside part of their flash memory to replace worn-out cells and keep performance up. That's why 480 GB drives exist that are built from 512 GiB of flash: the reserve doesn't show up anywhere.
- System files. The hibernation file, the page file, and restore points can take tens of GB on Windows.
It ended up in court
The gap between advertised capacity and what the operating system shows led to class-action lawsuits in the United States against drive makers. Western Digital in 2006 and Seagate in 2007 reached settlements that included compensation for buyers, though neither changed how it measures. Ever since, the fine print on nearly every drive or flash drive spells out that "1 GB = 1,000,000,000 bytes."
Bits and bytes: the other misunderstanding
Network speeds come with their own confusion. Internet providers advertise speeds in megabits per second (Mbit/s or Mbps), while browsers and download managers usually show megabytes per second (MB/s). A byte has 8 bits, so divide by 8:
| Connection | Maximum in MB/s | Downloading 50 GB takes at least |
|---|---|---|
| 100 Mbit/s | 12.5 MB/s | 1 h 6 min 40 s |
| 300 Mbit/s | 37.5 MB/s | 22 min 13 s |
| 1 Gbit/s | 125 MB/s | 6 min 40 s |
In practice, add about 5% for TCP, IP, and Ethernet headers, plus whatever limits the source server, the Wi-Fi network, or the destination disk impose. A 50 GB download on a 300 Mbit/s connection takes, in the best real-world case, about 23 to 24 minutes. Network speeds always use decimal prefixes: 1 Mbit/s is 1,000,000 bits per second, never 1,048,576.
RAM, the exception
RAM is the one case where nearly everyone, manufacturers included, uses GB in the binary sense. Memory chips are built in powers of 2, so a "16 GB" module holds exactly 16 GiB = 17,179,869,184 bytes. JEDEC, the body that standardizes memory, officially keeps that traditional definition for RAM.
How to read any capacity figure
- Drive, SSD, or flash drive on the box: decimal. 1 TB = 10¹² bytes.
- Windows: binary labeled as decimal. Read "GB" as GiB.
- macOS and iOS: decimal, matching the box.
- Linux terminal: depends on the command and option (
-hbinary,-Hdecimal). - RAM: binary.
- Network speed: decimal and in bits; divide by 8 to get bytes.
Summary
A 1 TB drive holds 10¹² bytes, and Windows reports it as 931 "GB" because it divides by 1,024³ instead of 1,000³: that's 931 GiB. No space is missing; two different rulers share one abbreviation. The IEC prefixes (KiB, MiB, GiB, TiB) have existed since 1998 to remove the ambiguity. What really does eat capacity is the file system, reserved space, and hidden partitions. And for internet connections, the figure is in bits: divide by 8.