Why a 128GB Card Shows 119GB — Nominal vs Usable Capacity
A 128GB card shows around 119GB in your operating system, and that is normal rather than a defect. Nominal capacity is quoted in decimal units — a gigabyte of 1,000,000,000 bytes — while operating systems report in binary units of 1,073,741,824 bytes, and the file system also needs space for its own structures. The gap works out at roughly seven per cent before formatting and a little more after. Knowing where that line sits is a practical skill: a shortfall of a few per cent is arithmetic, a shortfall of tens of per cent is a device reporting capacity it does not have, and only a full write-and-read-back test tells you which.
Where the shortfall comes from
| Nominal capacity | Binary bytes | Roughly usable in the OS |
|---|---|---|
| 8GB | 7.45 GB | Roughly 7.4 GB |
| 32GB | 29.8 GB | Roughly 29.8 GB |
| 64GB | 59.6 GB | Roughly 59.6 GB |
| 128GB | 119.2 GB | Roughly 119 GB |
| 256GB | 238.4 GB | Roughly 238 GB |
Why the gap exists at all
Two effects combine. The first is unit definition: capacity is sold in decimal units, the way drive makers have long quoted them, while your computer counts in binary. The second is that the file system needs space to hold its own directory and allocation structures.
Neither is a defect and neither is recoverable. You cannot claim the missing gigabytes back, so the useful response is to know the figure your devices will report before you size an order against them.
Where the shortfall is a defect
- A 128GB card that reports 8GB — the classic misrepresentation
- A device that stops accepting writes part-way through a full write
- Reported capacity materially below the binary equivalent, for example a 32GB card reporting under 28GB
When the shortfall is normal
- A 128GB card reporting around 119GB
- A small spread between individual units in the same batch
How to check it yourself
The standard method is a full write-and-read-back: fill the device with test data, read it back, and confirm every byte survives the round trip. H2testw is the usual tool on Windows; F3 does the same on macOS and Linux. It takes from a few minutes on a small card to several hours on a large one, and it is slow because it is thorough.
- Empty the card or drive first — the test fills it completely
- Test a random sample from the batch you receive, not a unit the supplier selected
- Read the reported result against the binary figures above, not against the number printed on the label
What to specify on an order
- State the nominal capacity you require and accept the usable shortfall as normal — it always exists
- Do not specify in usable gigabytes unless you have a reason to; the convention across the market is nominal
- If capacity is critical, work backwards from your content size to the nominal figure that comfortably covers it, with headroom
- For a recording workload, capacity that looks implausibly generous is usually a sign of a misrepresented device rather than a bargain
Not sure which applies to your product?
Send your workload — what writes, how often, at what temperature — and we will propose a configuration with the reasoning. We reply to business enquiries within 1 business day.
Request a quoteFrequently asked questions
Is a 128GB card showing 119GB fake?
No. That is the normal decimal-versus-binary gap plus file system overhead. A card showing 8GB when the label says 128GB is the real problem.
Can I get the missing capacity back by reformatting?
No. The difference is arithmetic and file system overhead, both unavoidable. A reformat can only recover the space taken by a poorly chosen file system, not the decimal-versus-binary gap.
How much should a 256GB card show?
Roughly 238GB. If it shows materially less than that, test it properly before you rely on it.
Will your products pass a capacity test?
Yes. Every unit is capacity-verified by a full write-and-read before packing, and the method is published so you can repeat it on anything you buy from us or from anyone else.