What Is This Drive Actually Costing Me per Terabyte?
By Devin Chua · Data checked by Owen Nakamura · Updated Drives are sold in decimal terabytes and reported by Windows and most Linux tools in binary tebibytes, which is why a 4 TB drive shows as roughly 3.64 TB. Nothing is missing and no space has been taken by formatting: the two numbers count in different units. RAID, filesystem overhead and reserved blocks come out of the usable figure on top of this.
Cost per terabyte is price divided by capacity, and it is the only honest way to compare drives of different sizes. The tool also shows cost per usable tebibyte, because a 4 TB drive reports as about 3.64 TB in your operating system and that gap confuses more buyers than any other storage number.
The formula
cost per TB = price / advertised capacity TB
usable TiB = advertised TB x 1,000,000,000,000 / 1024^4
cost per TiB = price / usable TiB
1 TB (decimal, how drives are sold) = 0.909 TiB (binary, how Windows reports)Worked examples
| Case | Inputs | Working | Result |
|---|---|---|---|
| 4 TB NVMe | 249 for 4 TB | 249 / 4 | 62.25 per TB |
| Same drive, usable | 249 for 3.64 TiB | 249 / 3.638 | 68.44 per TiB |
| 20 TB hard drive | 380 for 20 TB | 380 / 20 | 19.00 per TB |
Why the number your drive reports is smaller
The most common storage question people ask is why a 4 TB drive shows up as 3.64 TB. The answer is a unit difference rather than missing space. Drives are sold in decimal terabytes, where one terabyte is a trillion bytes. Windows and most Linux tools report in binary tebibytes, where one tebibyte is 1,099,511,627,776 bytes, but label the result TB. The two count in different units and the gap is about 9%.
Nothing has been taken by formatting, and no manufacturer is cheating. Filesystem metadata does consume some space on top of this, but it is a small fraction next to the unit difference. Knowing which number you are looking at is the difference between sizing an array correctly and coming up short.
Comparing drives honestly
Cost per terabyte is the only sensible way to compare drives of different capacities, and within a product line it almost always favours the larger drive, because the controller, board, packaging and warranty cost roughly the same regardless of how much the drive holds. Across product lines it settles much less than people assume: a DRAM-less SSD and a flagship at the same figure are different products, and a SATA drive will undercut any NVMe drive while being several times slower.
What comes out of the usable figure next
RAID takes its share after the unit conversion, not instead of it. A four-drive RAID 5 array of 4 TB drives gives you three drives' worth of usable space, then the filesystem takes its own overhead on top, then ZFS or Btrfs may reserve more. If you are sizing an array rather than a single drive, work out usable capacity first and apply cost per terabyte to that figure, because the headline capacity you are paying for is not the capacity you will be able to use.
Questions
Why does my 4 TB drive show as 3.64 TB?
Because the two numbers count in different units. Drives are sold in decimal terabytes of 1,000,000,000,000 bytes; Windows and most Linux tools report binary tebibytes of 1,099,511,627,776 bytes but label them TB. No space is missing and formatting did not take it.
Should I compare on cost per TB or cost per TiB?
Per TB for comparing purchases, since that is how every drive is sold and the ratio is identical across them. Per TiB when you are sizing an array against a capacity requirement, because that is the number your operating system will show you.
Does RAID overhead come out of this?
Yes, and on top of the unit difference. A four-drive RAID 5 array of 4 TB drives gives you three drives' worth of usable space, then the filesystem takes its own share. Our RAID calculator does that part.
Is the cheapest cost per TB always the best buy?
No. The lowest figure is usually the largest drive, which concentrates more data behind one failure and lengthens rebuild times. It also ignores recording technology, warranty and workload rating, all of which matter more in an array than a few dollars per terabyte.
Why do SSD and hard drive costs per TB differ so much?
Flash and spinning media have different cost structures, though the gap narrowed through the 2026 shortage as both rose. Our SSD versus hard drive page tracks the current spread.