Welcome to the OpenSolver on-line help.

We hope that most of your questions will be answered on the Using OpenSolver page. However, if you still have a question, please post it below.

Building  Linear Programming models

Writing optimisation models that only use linear mathematical equations and inequalities is not easy. However, most of the time you want to build these “linear programming” models (and avoid non-linear models) because these are easier and more reliable to solve using packages such as OpenSolver. (OpenSolver uses CBC by default to solve these linear programmes.) Check out this Wiki page on linear programming.

The AIMMS folks have a great online chapter describing how to take non-linear requirements (local copy of pdf) and model them using linear programming. (Thanks to Stu Mitchell of PuLP fame for pointing out this resource.) The NEOS guide is also a great resource. If you still have a modelling question (e.g., how do I express this requirement using the rules for a linear program), please ask it on

Common Software Problems

Excel can have problems dealing with add-ons. Visit for a guide on fixing these.

1. OpenSolver not loading – July 2016 Office update (no error message)

In July 2016, MicroSoft released an update to Office 365 which prevents OpenSolver from loading unless the .zip file is “unblocked” before the files are extracted. (We expect this change to appear in other versions of Excel.) The symptoms are simply that OpenSolver does not appear in the Data tab; there is no error message shown. To fix this, delete your old OpenSolver files (but not the downloaded .zip file), right click the downloaded OpenSolver .zip file, choose Properties, and click the Unblock button which will show if the file is blocked. Once the file is unblock, close the properties dialog, and then un-zip the files and open up OpenSolver.xlam as usual.

2. Unable to find the external solver …

If OpenSolver reports this problem, then it could not find a solver file that comes with OpenSolver. Please check that you have extracted (i.e. un-zipped or uncompressed) all the files from the OpenSolver download.

3. The solver did not create a solution file.

If this error is reported, then it is likely that OpenSolver was unable to run the solver executable. This may happen if the OpenSolver files  are in an ‘untrusted’ location, and so Windows will not let them be run. First check whether the solver is shown as found in the “About OpenSolver” form. If not, go to the folder identified in the “About OpenSolver” form and double click on this solver file. You should then see a command window open that gives the solver name and version information etc. If you don’t see this, then Windows should give you an error message to help you diagnose the problem. It may help to move the OpenSolver folder (and all the files it contains) into a location such as your Documents, or even into Program Files, and try again.

4. Solver crashes; OpenSolver reports no solution file

The files in the “Solver” folder provided with OpenSolver contain the actual solver code. These solvers are being continually upgraded as bugs are reported and fixed. If OpenSolver reports that no solution file was generated, then the solver may have crashed. This can happen on big problems when the solver runs out of memory, but also on smaller problems that happen to hit a solver bug. In such cases, you should check if a newer version of OpenSolver is available since we regularly update the solvers included with OpenSolver as they are released.

With CBC, a typical OpenSolver error for thois problem message might be: ‘The CBC solver did not complete, but aborted with the error code -1073741510’

If needed, you can download the latest version of CBC, Bonmin or Couenne from AMPL. Download the either the Windows or Mac version as appropriate. For Windows, get the 32-bit version if you have 32-bit Windows, or the 64-bit version otherwise. (The 64 bit version will solve much larger problems than the standard 32 bit version included in OpenSolver, so always choose this if possible. Right-clicking on Computer, and choosing Properties, will tell you if have a 32 or 64 bit system type.) Right click on the downloaded .zip file, choose Properties, and click the Unblock button if there is one. Then, open the .zip file, and drag the files into the right location in your OpenSolver Solvers folder (the win32, win64 or osx folder as appropriate), replacing the existing files. This may fix your problem.

Report a CBC bug

If after installing the latest version of CBC, CBC still crashes on you, then you may wish to report a bug. Please be sure to include the .lp file produced by OpenSolver; you can open this file using “View Last Model .lp File”. CBC bugs can be reported by either:

  1. The online CBC TRAC bug reporting system (which you will have to register for), or
  2. The CBC mail list

Switch from CBC to Gurobi
Since August 2014, OpenSolver has also supported solving models using the Gurobi solver. You can obtain a copy of this solver at no charge if you are at a university. Gurobi is faster and more stable than CBC, and comes highly recommended.

5. Repairing Excel

Sometimes it is useful (if all else fails) to repair your Office installation:


See these Microsoft Office 2010 and later instructions (or in Excel 2007, use Excel’s application menu, then Excel Options, then Resources, then “run Microsoft Office Diagnostics”). You might also want to look at, which talks about this.


Unfortunately there is no easy way to repair the Office 2011 installation. The best option is to follow these instructions to completely uninstall Office. You can then install Office again from scratch to get a clean version installed.

6. ‘Can’t find project or library’ : Missing Reference Errors

OpenSolver uses the “RefEdit” control, which we need but which can cause problems because of missing references caused by version changes. To check for missing references, using the VBA editor (which you access using Alt-F11), select OpenSolver (openSolver.xlam) in the Project window, choose Tools… References, and look for a “Missing:” line. If you see this, un-check that line, close the dialog, and choose File … Save OpenSolver.xlam to update the file. If this doesn’t help, try the above steps for repairing Excel.

7. Error 70: Unable to delete the file: C:\Users\XXXXXX\AppData\Local\Temp\log1.tmp

This errors happens if Excel crashes while OpenSolver is running, leaving log1.tmp open. Please restart your computer, which will close this file, and try again.

8. OpenSolver was working on my Mac. Then I updated to Excel 16.16.7 (released late 2018) and OpenSolver stopped working. What do I do?

We have had numerous users report this issue. We are looking for a Mac user with VBA experience to help us diagnose this problem. Please contact us at email hidden; JavaScript is required if you can help.

1,710 thoughts on “Help”

    1. The front page notes that OpenSolver is released under the GPL. If you really need a different license (because you want to modify and distribute the code, rather than just use OpenSolver), then contact me. Andrew

      1. Hi Andrew,

        I work for a corporation where legal will need to review the EULA as we’re distributing the software to numerous users. Please advise what type of license this will fall under? What is the best way to contact you?


        1. Great that you are finding OpenSolver useful. OpenSolver is under the GPL. Distribution is fine as I doubt you will be changing the OpenSolver code. (If you do change the code, then we would like to get those improvements added to our code base.) Cheers, Andrew

  1. Dear All,

    I have just started to use OpenSolver in my courses and I have recently installed an academic version of Gurobi. I would like to know if it is sufficient to select that solver from the drop-down menu in OpenSolver in order to use it (I did this and found the .py file in the OpenSolver directory). I am just a practitioner so maybe this question is trivial: sorry for that!

  2. Hi,

    First thanks for opensolver. I am trying to use opensolver add-on to google sheets but I’m having a problem.

    I installed the add-on ok. I am able to open the sidebar. I select the Objective Cell and press Update on the sidebar and the Objective Cell input is updated. Next I select the range of the Variable Cells and press Update in the Variable Cells area. The variable cells range is updated in the Variable Cells textarea, but THE OBJECTIVE CELL INPUT IS CLEARED.

    I cannot input and Objective Cell and Variable Range at the same time. Same thing when adding constraints. Objective or Variable cells are cleared when adding a constraint.

    I have tried firefox, chrome, and safari. Thanks for looking at my report.

  3. I normally use a higher end laptop on my very well-running project. Today, I attempted to use this same identical project on my rather generic Dell laptop. It appeared to churn through the data (albeit quite a bit slower) but rather than a solution, it showed a ’10’ (Limited SubOptimal). Does this make sense or is something else at play? I am using VBA to initiate the program. It always runs nicely on my other computer and produces a solution. I searched by did not see any question/comment addressing this other than the size of one’s problem is limited to the size of their computer’s memory. Is it possibly a memory issue? Regardless, I ABSOLUTELY love OpenSolver!!!

    1. I wanted to give a follow up because other tests have been conducted and the culprit has been IDENTIFIED. A little background on the problem: it is a linear scheduling problem with 23,199 variables and 1,085 constraints.

      I pulled two more computers out and tested my project… unfortunately these, too, showed a ‘Limited Suboptimal’ result when I know that the project I was running has a solution! This issue was causing me a bit of grief as I simply could not understand what was happening – why would one computer provide a solution while (three) others fail?

      I looked further and saw that my preferred computer (the one that ALWAYS yielded a solution!) was running an older version of OpenSolver (2.6) whereas all three other computers were using the 2.9.0 version – and these are the computers that were NOT generating a solution for the same, identical problem. Bewildered, I begin looking within the OpenSolver options and saw that the 2.6 version had the ‘max number of iterations’ grayed out whereas the 2.9.0 version was changeable and preset at 100. After changing this from 100 to 10,000 a PERFECT solution was presented!

      This is an amazing tool!

      1. Thanks for your sleuthing around this. Yes, the newer OpenSolver versions allow the user to set the integer programme iteration limits (the number of branch-and-bound nodes). I’m not sure why it was defaulting to 100; we will double check our code, as this number seems too low. Users should definitely increase this (to, eg, 999999) if they want optimal integer solutions. Andrew

  4. Hi –
    Is it possible to access SolveStatusString through the OpenSolverResult object?
    So if I have:
    Dim rResult as OpenSolverResult
    Dim sResult as String
    I could write:
    sResult = rResult.SolveStatusString
    Thank you,

  5. I’m attempting to solve an integer problem but it doesn’t seem to be getting the optimal solution. I’ve set the branch and bound tolerance to 0% and it still is having problems. As I add constraints the given solution becomes more optimal, which shouldn’t happen when adding constraints. What am I doing wrong?

    1. Sorry it is mis-behaving. This should not happen with a linear problem. Please check the .lp model file to check your maths is as you expect. Please also turn on the option to display progress and look for anything unusual. You can also try another solver such as Gurobi. Let us know how you get on. Andrew

  6. Hi,

    First off, I love your product. Great work! However, I’m having a few problems right now. I don’t seem to be getting an optimal solution with an integer problem even with branch and bound set to 0%. In fact, I get a more optimal solution sometimes with it at 100%. What am I doing wrong?

  7. I have a model created in FrontLineSYstems – trying to run in OpenSOlver I get this error:

    OpenSolver 2.8.6 encountered an error:
    Method ‘Range’ of object ‘_Worksheet’ failed

    All my Named Ranges are valid. What does this error mean?

    1. Sorry it is not working. I assume you have a non-linear problem, and so are not using CBC? The BonMin and Couenne solvers attempt to read all the formulae off the sheet, which sometimes fails (particularly if the formulae are complex). I suggest you try the NOMAD solver, which uses a more standard approach which should work for you. Andrw

  8. I have set up a sudoku problem and been trying to solve. OpenSolver gives an error message Error -2147220504: Unrecognised relationship for constraint
    I am not able to understand which is the unrecognised constraint. The excel sheet indicates Alldifferent ( unknown). Is this the error? Does it mean that alldiff is not recognised though its is provided in
    the dropdown list?

  9. The independent variables are cycling between + E113 to -E113, using the NOMAD to fit the parameters in the for the cumulative Weilbul function. Works fine using Excel Solver. Process Control engineer wants a non-MS application to run on the PLC server.

    Can you take a look at the set up and see if I can get the non-linear to work?

    1. Have you tried another solver choice such as Couenne or Bonmin? If you want paid consulting support, then email me at email hidden; JavaScript is required. Andrew

  10. Hi guys, I am having the same issue as above. I have been using open solver for a while now, but since yesterday (when I updated Excel) I can’t continue working.

    I am getting “Path/File access error”, every time I try to solve my model. Please help!


    1. Hello,
      I tried it on both Mac Excel 16.18 and 16.19, there is an issue with the newer Mac Excel update from 16.18, just delete only excel 16.18 or 16.19 and download the office package revision 16.17 from Microsoft website and reinstall, it worked again with me on 16.17

      until someone in dig deeper and find the glitch, this will solve the problem.

      Hope this helps !
      Mostafa. A

      1. Hi There,

        What is the easiest way to downgrade to previous versions? I am currently running 16.19 and would like to downgrade to 16.17 to be able to run an opensolver model. Can you provide a link and/or some instructions?



  11. When I attempt to solve my model in opensolver I am getting an error that says
    Path/File access error

    It says an error log has been saved but when I click more details it says there is no error log.

    Any Ideas?

      1. I’m getting this same error this morning, and I’ve used OpenSolver many times in the last few months without any problems. So for me at least, it’s not about extracting files.

        1. Are you Windows or Mac? Has your Excel updated recently (perhaps with a new ribbon look)? What is your Excel version? This may be an Excel change. Andrew

      2. The solver worked fine on my computer before but since yesterday I get the same error and also no error log. (I’m a Mac user if that helps)
        Any suggestions what to try? I already deinstalled and installed Excel and the Solver.

      3. Yes I have extracted the files. I have used opensolver before and am, just like the others, just two days ago getting the error. I think excel may have updated since then though.

        1. Can you please tell us the Excel version you have? This may be a widespread issue; thanks for reporting it. Thanks. Andrew

            1. Something in Excel 16.18 for Mac has broken OpenSolver. We don’t have a Mac user on our team any more so we are looking for someone in the OpenSolver community to help debug this. Any VBA Mac programmers out there who can help? Andrew

  12. OpenSolver won’t solve the problem because it says that “Error -2147220504: Could not find the Solvers folder in the folder containing OpenSolver.xlam, indicating OpenSolver has not been properly installed. Make sure you have unzipped all files from the downloaded zip file to the same place.” I’m not sure what this means because it is all in the same folder on my computer

  13. Hi,
    Due to the large size of my non-linear model (on year time series with hourly resolution) I though using opensolver will be the solution, however it couldn´t even solve the same problem with limited size which worked in solver, I try both solver and opensolver with exactly the same problem but opensolver either gice an error or not able to find an optimal solution.

    Why is this happening?

  14. I am using OpenSolver to test models with different levels of data granularity (i.e no of rows = number of constraints where result must be <= 1 + a few other constraints) . Although I can quite easily find the cpu run time, most of the time is taken in the setting up of the model.
    Does OpenSolver record model set up time anywhere ?

    1. I don’t think this is recorded, sorry. Please double check the model.lp file to be sure. Andrew

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