Hard disk space increase

john masterton
john masterton used Ask the Experts™
Hi tricky one (for me).

I have a few systems needing to have 20% free hard disk space.
The systems are already in use and I cannot delete files.
So the Final total size of each disk must have 20% free.
Not as simple as getting a 39GB Drive and clearing 20% on it, since there is (for example) already 6GB in use..
I tried to write an excel formula, but I'm not sure this is straightforward. Can some one help.
I have been given figures for: 1. total disk space 2. Free space (GB) 3. Free Space (%) and I need to increment the hard disk size (new total) to have 20% free.
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Lasse BodilsenSystem Administrator

Something along the lines of.

Total Space - Free Space(GB) = Used Space
Used Space + 25% = New Total
New Total - Total Space = Needed size increment.

When you add 25% to the Used space, you will get the figure for how big the drive space is needed to have Total size - 20% = Used Space.

in excel:
B1 = Total Space
B2 = Free Space (GB)

then the formula for Needed increment would be: =(B1-B2)*1,25-B1
and for New Total, just remove the -B1 at the end.

EDIT: Added excel info.
Aaron TomoskyDirector of Solutions Consulting

To get the percent free to be what you want after expanding you can't just multiply by the percent increase as the total size increase changes the percentage.
If you have a 100gb drive 100% full and add 20% (100*1.2) that's 120. But 100/120 = 16.7% free

The trick is to do this:
Current size / (1-percent increase)
To find the percent increase just do:
Desired percent - existing percent
Lasse BodilsenSystem Administrator

That's why i multiplied by 1.25 to add 25%  

eg. 100*1.25 = 125  
and 125 - 20% = 100.
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Director of Solutions Consulting
Correct, in this specific case, 125*.8 = 100. However if the 20% is arbitrary, and needs to change, I thought the actual formula would be handy.


Thanks to everyone!
Lasse BodilsenSystem Administrator

no problem.

even though i think you choose the wrong comment for the "Accepted Solution"
Aaron TomoskyDirector of Solutions Consulting

Agree, our first two comments are the good ones. The selected answer is just some additional discussion

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