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I'm reformating and installing windows vista on friends computer. I'm making one partition for OS, one for data. As a bit of OCD i want to make one of the partitions so it appears in windows explorer as a nice round number e.g 100Gb.

however i've found in the past if i make it 102,400MB (=100x1024) it shows as slightly less than 100Gb

What size would in MB would i need to make the partition so it shows up as 100Gb? bonus: why?

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2 Answers

up vote 10 down vote accepted

There's a difference between the partition size that is specified in MB in the installer, and the disk capacity that is shown 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 that 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 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 still 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).

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This is spot on and it also happens on Windows 7; you just have to add 1 MB. On a side note, to get the same effect on Win XP (and maybe 2000) you must add 4 MB instead. The reason is that the end of an XP partition seems to be aligned on multiples of 8 MB, so the total size can be up to ±4 MB different from the expected size. –  efotinis Dec 2 '10 at 0:11
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make it purely as a power of 2


2^10 MB =1 GB

so, make it 2^20 kB for 1 GB

100GB=(2^20)*100 kB

so, specify the size as 1 048 57600 kB

Its like that because acc. to HDD manufacturers, 1kB=1000 Bytes, while the computer identifies 1kB=1024 Bytes

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Sorry, the value in MB would still be 102400 MB (2^10*100=102400) this actually shows up as less than 100GB in windows explorer. –  Jaips Dec 1 '10 at 5:05
you would have to set it to 107374 MB I guess then... wolframalpha.com/input/?i=100+GiB+to+MB –  Akash Dec 1 '10 at 5:09
The answer I had given originally was assuming you could set the partition size in kB –  Akash Dec 1 '10 at 5:11
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