Pourquoi la capacité réelle de la clé USB est-elle inférieure à la capacité indiquée sur l'étiquette ?

The primary reason the actual capacity of a USB flash drive is smaller than the labeled capacity is that manufacturers and computer systems use different methods to calculate capacity. Manufacturers use the decimal system (1GB = 1000MB, 1MB = 1000KB) for capacity calculations, while computer systems recognize capacity based on the binary system (1GB = 1024MB, 1MB = 1024KB). When converting between these two systems, the actual displayed capacity will naturally be smaller than the labeled value. This is a normal technical discrepancy and not a product defect.

USB Drive Capacity Calculation Method

(1) Manufacturer’s Decimal Calculation

USB manufacturers use the decimal system to label storage capacities. The core conversion rules are: 1GB = 1000MB, 1MB = 1000KB, 1KB = 1000B. This standard aligns with the storage industry’s common measurement practices, enabling users to intuitively understand the product’s nominal capacity range. It also serves as the unified specification for manufacturers when labeling capacities on USB drives, hard disks, and other storage devices.

Manufacturers use the decimal standard for capacity labeling as follows:

• 1GB = 1000MB

• 1MB = 1000KB

• 1KB = 1000B

For example, a 64GB USB drive has a total byte count calculated as: 64 × 1000 × 1000 × 1000 = 64,000,000,000 bytes.

(2) Binary Calculation in Operating Systems

The binary calculation method for USB drives is the standard used by computer systems to recognize storage capacity. Applicable scenarios: When operating systems like Windows or Mac read storage devices such as USB drives or hard disks, this is the default method for calculating actual available capacity. Computer systems recognize capacity according to binary standards, specifically:

• 1GB = 1024MB

• 1MB = 1024KB

•1KB = 1024B

Therefore, the actual capacity of a 64GB USB drive in the system is: 64,000,000,000 ÷ 1024 ÷ 1024 ÷ 1024 ≈ 59.6GB. This result closely matches the commonly observed scenario where “64GB shows 57GB” (remaining capacity may further decrease due to other factors).

USB Drive Decimal to Binary Conversion Parameter Comparison Table

Comparaison	Decimal Calculation	Binary Calculation
Conversion factor	In multiples of 1000	1024 is a multiple (2 to the power of 10)
Conversion relationship	1GB=1000MB	1GB=1024MB
	1MB=1000KB	1MB=1024KB
	1KB=1000B	1KB=1024B
Unit	Bety	Bety
scene	Storage Manufacturer Labeled USB Drive	Windows, Mac, and other systems recognize the actual usable capacity of the device.
	Nominal capacity of hard drives and other devices

Reference for Actual Usable Capacity of Commonly Rated Capacities

The following table provides reference values for capacity differences calculated using various methods. If the results fall within this range, it is considered normal and does not indicate a USB drive quality issue.

Capacité nominale	Theoretical vs. Actual Capacity	System displays available capacity
4GB	about 3.73GB	about 3.5-3.7GB
8GB	about 7.45GB	about 7.0-7.3GB
16GB	about 14.9GB	about 14.0-14.5GB
32GB	about 29.8GB	about 28.0-29.0GB
64GB	about 59.6GB	about 57.0-59.0GB
128GB	about 119.2GB	about 115.0-118.0GB

In addition to the differing calculation methods mentioned above, there are other factors that may contribute to minor discrepancies, though these factors account for only a small portion of the total:

(1) Space Occupied by System Files

When formatting a USB drive, partition tables (e.g., MBR/GPT) and file systems (e.g., FAT32/NTFS/exFAT) must be created. These structures occupy a portion of the storage space. For example:

• The FAT32 file system requires reserved space for the File Allocation Table (FAT) and root directory area, typically consuming several hundred KB to several MB of space;
• The NTFS system allocates space for log files, metadata areas, etc., occupying relatively more space.

(2) Pre-installed Software / Encryption Features

Some USB drives come with pre-installed programs like encryption tools or backup software, or include built-in secure partitions, all of which occupy a portion of the available capacity.

(3) Firmware and Hidden Partitions

The firmware embedded in the USB drive’s controller chip (responsible for data read/write and device recognition) occupies a small amount of physical capacity, typically tens to hundreds of MB. Some USB drives contain hidden partitions (e.g., areas for boot functions or firmware upgrades), which are inaccessible to users and also reduce usable capacity.

(4) Bad Blocks and Wear Leveling

It is normal for a USB drive to display a capacity lower than its labeled value. This is typically caused by a combination of technical specifications (calculation methods, file systems) and hardware design (manufacturer-reserved space, bad block handling). If the capacity discrepancy falls within a reasonable range (e.g., a 64GB drive showing 57-59GB, or a 128GB drive showing 119-121GB), this is a reasonable phenomenon due to calculation methods, file systems, and manufacturer-reserved space, and there is no need for concern.

If the displayed capacity falls below 80% of the theoretical value (e.g., 64GB showing < 50GB), or if the drive cannot store files matching its displayed capacity (e.g., showing 57GB but only storing 10GB), this may indicate a counterfeit drive or hardware failure.