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?

  • There's a big difference between Gb and GB. 100Gb is neither 100000MB or 102400MB
    – phuclv
    Jun 11, 2018 at 6:57

5 Answers 5


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).

  • 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, 2010 at 0:11

For every 10GB add 8MB.

For every 100GB add 80MB.

For Perfect 100GB Drive.

100GB Drive = 100 X 1024 = 102400MB (Displayed as 99.99GB)

Perfect 100GB Drive = 10 x 10240 + 10 x 8 = 102480MB (Displayed as 100.00GB)

  • Awww. I missed. When I purely install windows on new drive, it will create additional partition for windows file. I try using your formula for 300GB partition for C drive and calculation result: 307,440 but it end up 299.6GB something.
    – Luiey
    Jul 10, 2019 at 1:50

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

  • 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.
    – Hemi
    Dec 1, 2010 at 5:05
  • 1
    you would have to set it to 107374 MB I guess then... wolframalpha.com/input/?i=100+GiB+to+MB
    – Akash
    Dec 1, 2010 at 5:09
  • The answer I had given originally was assuming you could set the partition size in kB
    – Akash
    Dec 1, 2010 at 5:11

I'm OCD and this has plagued me for years. The 1024 thing doesn't work, apparently because there's some administrative overhead (partition table or superblock or some such) that the OS subtracts from the physical space before it displays the free space, even with a spanking-new (=empty) partition.

Today I installed Linux and tried 1032 (1024 plus a smidge) and the swap partition I wanted to come out as exactly 8gb game out as 7.69. I booted the same box using a Live Windows thumb drive and it agreed that that partition was 7.69gb. So I interpolated and came up with 1073.6, which struck me as too high but I figured I'd try it and see.

So I partitioned my swap slice as 8,589mb during the install and the OS reports it at 8.0gb on the nose. And the other partitions I wanted to come out on the even GB also came out point-zero exact.


102511mb in Disk management gives me exactly 100gb of 100gb in Explorer enter image description here

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