ONTAP Discussions

NetApp Usable Space Calculator


A very basic version of NetApp Usable Space Calculator. Please provide feedback.


March 18, 2014 - Latest version 2.1 - Minor updates to include some new disks and a few disk calculation bugs


Sep 5, 2013 Update: Software Version 2.1. Minor updates - Added 2 new disk sizes ( 1.2 TB SAS & 4 TB SATA )


Aug 8, 2013 Update: I have updated the software version to 2.0 and fixed some bugs.


Also this includes a new feature called Raid Group Size Estimator where you key in disks values, raid type and disk type - the software will attempt to provide the best RG size values either based on NetApp recommendations or Optimal Capacity.(Please note: The Raid group size input in the calculator is only for disk space calculations and this is ignored for the raid group estimator)

I have developed this Raid Group Size Estimator based on many users request. As usual please provide feedback if you do some testing.


Screenshot of new version 2.1 (Software zip attached to this post)




(The below image is a old version. New version available which is 2.1)




A few best practices on root volume.
Root volume contains all configurations files (ONTAP Files). It is always preferred to separate the root volume from the data volumes on the filer. For optimal management, it is better to restrict the root volume to just 2 disks (minimum required for a RAID-4 parity group). This enables faster reconstruction times in case of a disk failure on the root volume
Refer netapp link here - https://library.netapp.com/ecmdocs/ECMM1278325/html/sysadmin/planning/concept/c_oc_plan_root-vol-recommend.html#c_oc_plan_root-vol-recommend

with your hand calculation

24HDD-2Parity-2Spare-1root volume diks=19HDD
Real capacity of 900gb about 850gb
14535gb-3% aggr reserve = 14098.95

This will be how I will calculate / size it.

Total of 24 HDD
2 for root volume
2 for spares

we lose 4 disks and 20 remain. So RG size can be 20
With a RG size of 20 we lose 2 disks for parity so net usable disks becomes 18

900 GB SAS disk's right sized approx value is 837 GB
18 x 837 = 15066
15066 - 10% WAFL = 13559.4
13559.4 - 3% AGGR = 13152.61
then if you need volume reserve you need to subtract 20% from 13152.61

Hope this gives an idea ?



Great work!

But why you allow change WAFL reserve? It's 10% always, isn't?


Yes it is 10%. Just if some one wants to play with numbers I made that field as a changeable one.


Are the numbers valid for Clustered Data ONTAP as well?



Great tool, thanks for your hard work in putting it together!


Really useful tool.  Now I don't have to bug our engineers when I need a usable capacity.


a really usefull tool ....thanks

bye nico



great tool!

It's sometime a long task to centralize all informations you need to know how much space you will have after aggr configuration.

In a future release could you had 800GB SSD?

Thanks in advance,




I can not run this, virus scanner (F-Secure) complains that

it contains Trojan?

Unfortunately the message is not more specific




I have had many users used and this is the first complaint about a virus message on the executable. Not sure. Sorry it is not working for you.



I sent the calculator to FSecure and got following answer:

“Thank you for your submission.

The file you submitted is indeed clean. A database update will be released to resolve this issue.”

So it was a false alarm, sorry about this.




I sent the calculator to FSecure to check  and got following answer:

“Thank you for your submission. The file you submitted is indeed clean. A database update will be released to resolve this issue.”


So it was a false alarm, sorry about this.



This is a great idea.  Thank you for taking the time to create this tool and to share it with us. 

I have a question though about how the tool uses the various values I input to calculate Usable Capacity.  For example, with the following values:

  • Total Disks = 12
  • Disk Size/Type = 900GB SAS
  • Spare Disks = 0
  • Disks Allocated for Root Volume = 0
  • Volume reserve = 20%
  • Aggregate Reserve = 5%
  • Raid Group Type = Raid-DP
  • WAFL Reserve = 10%

Why do Raid Group sizes of 5, 6, 7, and 8 all give a usable capacity of 4494.70 GB and all Raid Group sizes greater than 8 yield a usable capacity of 5618.38 GB? 

In other words, what factor is raid group size playing in the usable capacity calculation? 



Hello kensumrall

Thanks for the feedback. Yes I do see an issue. Thanks for testing and finding a bug. I will correct it in a day or two and post the new version.



Thank you so much!  I also was a bit confused about the following as well:  

Say I have 12 total disks, 0 disks allocated for a root volume, 0 allocated as Spare Disks, and have a raid-dp group size of 6.

I noticed that Usable Capacity was decreased if I added disks to the Disks Allocated For Root Volume field even though I have plenty of disks left from the total of 12 to not take away from the Raid Group size of 6.

The same is true if I increase the number of spare disks.  Each additional spare disk takes away from Usable Capacity even if there are plenty of available disks with a Raid Group size of 6.

I understand on the one hand that the tool just does plain disk calculations, but on the other hand it leaves me wondering what the Raid Group Size is used for then.  (Or maybe a better way to put it is, how should I properly use the Raid Group Size value in the way you intended.)

Thanks again for such a great tool. 


Hello kensurmrall

Download the latest version which is 2.0 and let me know the feedback. I fixed some bugs and included a new feature called Raid Group Size Estimator for a given set of disk inputs, Raid Type and Disk Type.



very good , i just downloaded version 2.0 , but i dont see never drives 1.2TB and 4TB .




Hello - I just updated this version to 2.1 which includes the 1.2 TB and 4 TB drives. Check it out.


Aug 8, 2013 Update - New version 2.0 is available. Please check original post.

I have uploaded the zip of the new version 1.2 to my original post.  Please provide feedback.

Updates in this version are

- Choice of display the output in TB/GB

- Included more different capacity disks and disk types

- Informational message for recommended values of Raid Group Sizes


Nice tool for those that don't know the math. Some feedback:

* Choice to display capacity in either GB or TB.

* Choosing ONTAP version, because there can be differences in what is allowed or default values. E.g. max raid group sizes are different between ONTAP versions and volume reserve space default space.

* Choice for syncmirror or not.

* Show the actual RAID groups since the RAID group size you choose can have some weird results.

What would make this tool really great is suggesting optimal RAID group sizes for a given number of disks and according to NetApp recommendations. My storage admins seem to struggle the most with deciding on how to do it.


Thanks for the feedback. Working on a new version soon.

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