You buy a 1 terabyte drive, plug it in, and the computer reports 931 gigabytes. Seven percent of your purchase has apparently evaporated before you stored a single file. Every generation of computer buyers rediscovers this moment of betrayal, and the explanation is always the same: nothing is missing. The manufacturer and your operating system are counting the same bytes with different-sized units and, confusingly, the same unit names.
Drive makers use decimal units, where 1 TB means exactly 1,000,000,000,000 bytes — consistent with how “tera” works everywhere else in science. Windows divides that same byte count by powers of 1024 and reports 931 “GB”, which are really gibibytes.
Following the arithmetic
Take the trillion bytes on that drive and divide by 1024 three times — for kibi, mebi, gibi — and you get about 931.3. The drive contains precisely what the box claimed; Windows is just measuring it with a bigger gigabyte. The gap widens with each prefix level: about 2.4 percent at kilo, 4.9 at mega, 7.4 at giga, and nearly 10 percent at tera. A “4 TB” drive shows as roughly 3.64 TB in binary units, so bigger purchases produce bigger apparent losses, which is why the complaint gets louder as drives grow.
Everyone is inconsistent differently
The mess is not one villain but a patchwork of conventions. Storage and network industries use decimal. RAM is genuinely binary — a 16 GB memory kit really holds 16 times 1024 cubed bytes. macOS switched to decimal reporting in 2009, so a drive shows its full advertised terabytes on a Mac while shrinking on Windows next to it. Linux tools commonly offer both, sometimes labeled properly as GiB. Lawsuits over the discrepancy were filed against drive manufacturers years ago and mostly resolved with clearer fine print rather than changed units.
The space that really is used
A small portion of the discrepancy people notice is genuine overhead: file-system structures, recovery partitions, and preinstalled software consume a few gigabytes of a new machine’s drive. That part is real consumption, unlike the units gap. On a new laptop the two effects stack, which is why a “512 GB” machine may show something like 460-plus binary gigabytes free out of the box.
When you shop for storage, do the mental haircut in advance — multiply advertised terabytes by about 0.91 to predict the Windows number — and the unboxing will hold no surprises.
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