What size of partition you should set at installation if you want perfect round numbers?
First you need to realize, that there is a difference between the partition size that is specified in the MB in the installer and the disk capacity that is displayed in Windows Explorer.
That difference is the size of one block or “allocation unit”, which is 4KB for any NTFS volume over 2GB; and that block is the first block on the partition, to hold the boot sector of the disk (512 bytes). The rest of the block is unused, so all blocks are “aligned” on the disk.
So if you declare a partition to be 20480 MB, that’s exactly 20.00 GB (or GiB, for sticklers). But the resulting disk capacity will be (20480 MB – 4 KB, shown as) 19.99 GB. It appears that Windows Explorer does not round up or to the nearest, but truncates after two decimal places at most.
So you can create a partition that is one MB larger than the desired size, and after 4 KB is subtracted for the boot block, the disk capacity should be shown as you want. But it is not exactly that size.
While the installer and DISKPART work in MB, the partition table is expressed in sectors, so you can manually edit it to create a partition that is exactly the size you want plus 8 sectors (4 KB).
20580 MB = Exact 20 GB Partition
154100 MB = Exact 150 GB Partition
1049000 MB = Exact 1.00 TB Partition
Step by step guide – partition HDD after installation
Step 1: Decide on your partition size
For this example, I want a 2TB partition and a 500GB partition.
Step 2: Convert your target to MB
Each of the next few steps varies depending on the size of your target partition.
TB x 1,000,000 = MB
Example: 2 TB x 1,000,000 = 2,000,000 MB
GB x 1,000 = MB
Example: 500 GB x 1,000 = 500,000 MB
Step 3: Convert to mebibytes
Due to the difference between how Windows and hard drive manufacturers report the size of a kilobyte/kibibyte (beyond the scope of this how-to), you must convert to mebibytes.
TB in MB x 1.048576 = MiB
Example: 2,000,000 MB x 1.048576 = 2,097,152 MiB
GB in MB x 1.024 = MiB
Example: 500,000 MB x 1.024 = 512,000 MiB
Step 4: Compensate for partition overhead
As is you will end up just short of your target. Add extra bytes to your result.
TB in MiB + 4.096 KB = Total MiB
Example: 2,097,152 MiB + 4,096 KB = 2,101,248 MiB
GB in MiB + 1,024 KB = Total MiB
Example: 512,000 MiB + 1,024 KB = 513,024 MiB
Step 5: Open the disk management MMC
Open Server Manager (server) or Computer Management (desktop) and expand Storage > Disk Management. Alternatively, run diskmgmt.msc.
Step 6: Launch the partition wizard
Right click in the unallocated space on your target drive/array and select “New Simple Volume.”
Step 7: Specify volume size
Click “Next” and then enter your result from Step #4 into the “Simple volume size in MB” field. Click “Next” again.
2 TB = 2,101,248
500 GB = 513,024