256GB card tests as smaller? What to do

Your new memory card says 256GB on the package, but your computer shows less space, or a capacity check finds far less than expected. That can be normal, or it can mean the card is counterfeit or failing.

First, compare the displayed size with the normal difference between advertised gigabytes and the units some devices use. Then use the Fake USB & SD Check to write test data to the card and read it back. The test works in Chrome and Edge on a computer.

Verify the card's capacity safely

1. Copy anything you want to keep off the card. A full-capacity test writes across the card, so do not assume existing files will survive.
2. Put the card in a reliable card reader connected to your computer. If possible, connect the reader directly instead of through a hub.
3. Open the Fake USB & SD Check in Chrome or Edge. Follow the on-screen prompt to select the memory card, then run the capacity check.
4. Let the write and read-back process finish. Avoid removing the card, closing the browser tab, or letting the computer sleep during the test.
5. Compare the verified capacity with the advertised 256GB, keeping the normal decimal-versus-binary difference in mind.

Do not use a card with important files as the test target. A counterfeit card may report the advertised size while storing much less, and writing past its real limit can corrupt earlier files. For more on that risk, see why fake drives overwrite data without warning.

What the result means

The card shows about 238GB or 238GiB

This is usually normal for a genuine 256GB card. Manufacturers use decimal units: 256GB means 256 billion bytes. A computer that reports binary units divides those bytes into gibibytes, giving about 238.4GiB before space used by formatting. Some systems label that binary figure as GB, which makes a normal card look smaller than its package claim.

The test verifies close to the expected capacity

If the test writes and reads back nearly the full capacity, the card passed this check. The computer may still show slightly less available space because the file system uses some room to organize files. This result does not prove that the card will last forever, but it is a good sign that it can store data across its advertised space.

The verified capacity is much lower than expected

A result such as a few dozen gigabytes on a card sold as 256GB is not explained by unit conversion. The card may be counterfeit, defective, or incorrectly identified by the reader. Do not trust it with photos, backups, or other files until you have checked it again with a different reader.

The test cannot finish or reports read-back errors

The card, reader, connection, or browser may be interrupting the test. A card that repeatedly fails the read-back check is not safe for storing files, even if the computer still displays its full advertised capacity.

What to do if the result looks wrong

1. Check the normal size difference. A 256GB card contains 256,000,000,000 advertised bytes, or about 238.4GiB. A number near 238 is not evidence of a fake card by itself.
2. Confirm which card you selected. If more than one drive is connected, check the card's name and size before running another test. Do not guess based on a drive letter or icon.
3. Try another reader and port. Reconnect the card, then repeat the test with a known-working reader if available. This helps separate a card problem from a loose adapter or unstable connection.
4. Check what the operating system reports. In Windows 11, open File Explorer › This PC, right-click the card, and choose Properties. On macOS, open Applications › Utilities › Disk Utility, select the card, and use View › Show All Devices if you need to see the device as well as its volume. These screens show reported size, but they do not prove that every part of the card can store data correctly.
5. Check the card in a phone only for mounting issues. On Android, open Settings › Storage › SD card; the exact labels vary by phone. A phone showing the card's size is not a full capacity verification. Use a computer and the read-back test to check whether the whole card works.
6. Stop using it if the low result repeats. Do not reformat the card in the hope of restoring missing capacity. Formatting changes its file system, not the amount of physical storage inside it.

When to return or replace the card

If the check repeatedly verifies far less than 256GB on different readers, treat the card as counterfeit or defective. Stop saving new files to it and request a refund or replacement from the seller. Include the card label, purchase details, and test result if you need to explain the problem. If the card has already held files, do not run another full test on it before copying off anything that is still readable.

A card that verifies close to its expected capacity but later loses files may have a different fault, such as unreliable flash memory or a damaged connector. Replace it rather than trusting it for important storage. A format or partition change cannot repair failed memory cells.

Why the package and computer use different numbers

Storage makers count one gigabyte as 1,000,000,000 bytes. Some operating systems display the same bytes in gibibytes, where one GiB is 1,073,741,824 bytes. That is why a 256GB card works out to about 238.4GiB. Formatting uses a small amount more, so the space available for files can be lower still.

The Fake USB & SD Check tests the card by writing data and reading it back in your browser. Keep the card connected until the check is finished, and only use a card whose contents you have already backed up.