At Storage Manager > Volume, the hard drive size appears smaller than that claimed by the HDD manufacturer, because file systems (of Synology NAS or regular computers) adopt the binary definition of storage units while HDD manufacturers adopt the decimal definition.
- Decimal definition (adopted by HDD manufacturers):
1 TB = 1000 GB; 1 GB = 1000 MB; 1 MB = 1000 KB; 1 KB=1000 Bytes
Therefore, 1 TB = 10004 Bytes, and 1 GB = 10003 Bytes - Binary definition (adopted by file systems):
1 TB = 1024 GB; 1 GB = 1024 MB; 1 MB = 1024 KB; 1 KB=1024 Bytes
Therefore, 1 TB = 10244 Bytes, and 1GB = 10243 Bytes
As a result, if you buy 2 hard drives labeled as 500 GB and 1 TB in storage capacity respectively, and install them on your Synology NAS, the actual sizes for each drive displayed on the Volume page will be as follows:
500 GB x 10003 / 10243 = 465 GB
1000 GB x 10003 / 10243 = 931 GB