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Why is the capacity of the SD card smaller than the catalog specs?

When you look at the SD card used in the mirror cam on your computer, you will notice that the capacity is smaller than the catalog specs.

For example, if you look at the contents of a 32GB SD card in Windows, you will see something like this.

This is not a defect in the SD card or mirror cam.
In a word, SD cards are like that.


The first major difference is caused by the difference in the way computer operating systems (Windows and Mac) and storage manufacturers calculate capacity.

Storage manufacturer: 1KB=1000Calculated in Bytes
Computer OS: 1KB=1024Calculated in bytes

In other words, for 32 GB of storage, the formula is as follows
32,000,000,000 ÷ 1,024 ÷ 1,024 ÷ 1,024 = 29.8023223877 (approximately)29.8GB)


Furthermore, in reality, about 1-3% of the storage (depending on the manufacturer) is used for administrative purposes, so the user-used space is even smaller.

Assuming 3%, the calculation would be
29.8 * 0.97 = 28.906 (approximately)28.9GB)

This is exactly the same capacity as displayed on the computer.

ReferenceTranscend Product Support "Why is the actual capacity of the product less than the capacity indicated in the product specifications?


In MirrorCam, 20% of the total capacity is originally reserved for overwrite protection files in case of shock detection. Another 6-7% is used for system administration.
The area indicated as "free space" above is that area.

Excluding that portion, the area that can be used for normal recording files is calculated to be as follows.

28.9 * 0.735 = 21.2415 (approx.21.2GB)

This matches the size of the space used above.


If you are using an SD card for a drive recorder such as a mirror cam, 32 GB should be sufficient for general use.

First of all, even if you use a 32GB SD card, it will be full after about 3 to 4 hours of driving.

However, recordings are saved separately every minute as standard.
When the capacity fills up the card.automatically overwrites the oldest recordings first.Files at the time of the accident are automatically locked for overwrite protection by shock sensor detection.
Therefore, in normal use, there is no need to be aware of the remaining capacity of the card, and it is safe to leave a single SD card in the system.

Some people often misunderstand that formatting an SD card is to "free up the remaining capacity," but as mentioned above, there is no need to format for the sake of capacity.

See alsoWhy Regular SD Card Formatting is Necessary for DVRs

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