Guide
Why your drive never shows the full advertised capacity.
It isn't an error, it isn't a manufacturer scam, and it doesn't mean your drive is defective. Four documented factors combine to explain why a '1 TB' drive shows between 850 and 931 GB available.
Reason 1: decimal vs. binary (the biggest, ~7-10%)
Drive manufacturers measure in decimal TB (1 TB = 1,000,000,000,000 bytes), but Windows calculates and displays the result in GiB, even though it labels it "GB" (1 GiB = 1,073,741,824 bytes). A 1 TB drive, measured in that binary unit, becomes about 931 GB visible — without a single physical byte having disappeared. This is by far the largest contributor to the gap between what the box says and what File Explorer shows. You can see the full math in the TB vs TiB guide.
Reason 2: reserved system partitions
When you buy a laptop or a drive with a preinstalled operating system, the manufacturer usually reserves space for a recovery partition (to reinstall Windows without needing an external USB drive) and, in some cases, for the manufacturer's own diagnostic tools. These hidden partitions typically subtract anywhere from 500 MB to several GB, and they don't show up as "available" space even though they technically still occupy part of the physical drive.
Reason 3: filesystem formatting
Formatting a drive with a filesystem (NTFS on Windows, APFS on Mac, exFAT for cross-platform compatibility) isn't free in terms of space: the filesystem needs to reserve internal structures (allocation tables, metadata, journaling) to organize and protect the files you're going to store. This loss is usually small compared to the other three reasons, but it adds to the total.
Reason 4: SSD over-provisioning
SSDs have a quirk mechanical drives don't: flash memory cells wear out with every write cycle. To compensate, manufacturers reserve a percentage of the chip at the factory (often close to 7-10% of total physical capacity) as spare cells invisible to the user, which the SSD's controller uses to replace worn cells and maintain performance over the years. This space never shows up as "available," nor should it — it's what keeps the drive performing well for longer.
Adding up all four reasons
For a 1 TB SSD it's reasonable to see roughly 900-920 GB available after adding up decimal/binary, recovery partition, formatting and over-provisioning. For a 1 TB mechanical HDD (with no over-provisioning), expect something closer to 920-931 GB. Neither scenario indicates a problem — both are the expected, documented behavior of each technology.
When you should actually be concerned
If the gap between advertised and shown capacity exceeds 15-20% with no clear reason (for example, large partitions created by accident, or a drive reporting less capacity than it technically should), it's worth checking the operating system's disk management before assuming it's "normal" — at that point it could be a misconfigured partition, not the expected math.
Verifiable sources
The technical explanation of the decimal/binary difference is documented by NIST; the combined explanation of all four causes (including reserved partitions and over-provisioning) is officially documented by Intel for its own storage products.