Help

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.

As of July 2025, there are two known issues caused by Microsoft code changes. These are a RefEdit issue that breaks the Model dialog (see #6 below; a user can easily fix this) and a DLL access change that impacts NOMAD (see #10 below; fixing this requires editting VBA code).

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 https://www.or-exchange.org/.

Common Software Problems

Excel can have problems dealing with add-ons. Visit http://www.add-ins.com/how_to_repair_office.htm 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:

Windows

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 http://support.microsoft.com/kb/166273, which talks about this.

Mac

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’ and ‘Compile error: variable not defined’: Missing Reference & Blocked ActiveX RefEdit Errors (updated July 2025)

July 2026 update: OpenSolver uses the ActiveX “RefEdit” control.  If this is not accessible, then you may get a “compile error: variable not defined” error (at a line containing “refObj”) when you open the Model dialog. I have also had a “Run-time  error -2147352573 (800200003)  Could not find the specified object” occur when clicking the “Model” button.

According to StackOverflow, the default setting in Excel has recently changed to a stricter level and this disables controls from external libraries like RefEdit. So, in recent Excel versions, permission to use ActiveX controls must be turned on in the Excel Trust center. To do this, use File…. Options… Trust Center… Trust Center Settings to select “Prompt me before enabling all controls with minimal restrictions” (with “Safe mode” turned on); see this screenshot. (Thanks to jkpieterse for replying on StackOverflow with this information, and for Ziad for sharing the settings that worked for them.)  It also works for me selecting the slightly safer “Prompt me before enabling Unsafe for Initialization (UFI) controsl ith additional restritions and Safe for Initialzatrion (SFI) controls with minimal restrictions”. Note that this change also causes Frontline’s Solver to crash with the error “Solver: An unexpected internal error occurred or available memory was exhausted.”

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.

9. OpenSolver does not work in Excel installed from the Windows Store

Windows Store versions of Office applications do not support Add-ins such as OpenSolver; see this link, or this support page for another add-in, or this technical article from Add-in Express. This Dell article explains a little more. If you have a Windows Store version, it will display this in the Excel Version information, such as in the following example:

Version 1807 (Build 10325.20082 Microsoft Store)

The solution is to un-install Excel, and install the click-to-run versions, in which case the Excel version will change from ‘Microsoft Store’ to ‘Click-to-Run’. Standard installs (meaning not Office 365) don’t have this issue.

10. “About OpenSolver” menu crashes; NOMAD crashes (updated July 2025)

July 2025: A recent update to Excel has blocked access to the “OpenSolverNomad.dll” file that we use to execute the NOMAD solver. Symptoms of this include the “About OpenSolver” window crashing with an error: Run-time error: ’53’: File not found: OpenSolverNomad.dll
(This error occurs even though OpenSolverNomad.dll is located in the Opensolver folder under Solvers/win32 and Solvers/win64.) OpenSolver Errors also occur when trying to use the NOMAD solver.

These errors occur because Microsoft have restricted Excel’s access to DLL’s (which is what we use for Nomad). To fix this in OpenSolver you need to edit the code. (We will release a fix at some stage.) For more information, see this Microsoft Community post (also available locally). One of our users, Ed, says “I can confirm that adding the full path in the [NOMAD DLL] declare statement works”. See also this StackOverflow post.

Note: This has been confirmed for “Microsoft® Excel® for Microsoft 365 MSO (Version 2507 Build 16.0.19029.20136) 64-bit” installed as part of “Microsoft 365 Apps for Enterprise” on 12 Aug 2025.

2,231 thoughts on “Help”

  1. Hye,

    I am new in using this open solver. So, can this be used for ‘grg nonliner’ engine as in standard solver. How? Or does it can only be used for linear problems?

    Hope to receive your prompt reply.

    Regards,
    Smurt

    Reply

    1. NOMAD and the NEOS-based solvers (apart from CBC) can be used for non-linear models. NOMAD will work on any spreadsheet, but with no guarantees on results. For the NEOS solvers (except CBC), we parse the spreadsheet formulae, and so OpenSolver maty not work if you have formulae in your model we do not (yet) support. Please try it out and tell us how you get on. Thanks, Andrew

      Reply

      1. Thanks for the response Andrew.

        I’ve try using NOMAD, but there is run time error ’13’ window pop up. Do you have any suggestion?

        Regards,
        Smurt

        Reply

        1. Thanks for trying out our experimental release. Please confirm you are running the latest version (2.5.3), and what versions of Office and Windows you are using. Also, would you be able to email us your spreadsheet so we can test it on our system, to a dot mason at auckland ac nz. Thanks, Andrew

          Reply

        2. Thanks for letting us know about this. Please try the newest release 2.5.4 and see if that fixes the problem.

          Thanks,
          Jack

          Reply

        3. We think the latest release should have fixed this. Please let us know if we succeeded in getting the bug! Thanks, Andrew

          Reply

  2. Hi,

    I would like to start the OpenSolver with VBA.

    I have defined the whole model, so I just need to start the Solver with a button.

    But I have no Idea how to do it.

    1. The User opens the excel file (no VBA)
    2. The User pushes the butten “optimize the total costs” (VBA: Starting the Open Solver (I mean, starting the calculation process “solve”))

    I tried to record the macro but it still doesn’t work:(.

    Maybe someone has the solution for my problem:). Thanks for your help in advance!

    Reply

  3. Hi,
    Opensolver has reported that it cannot find a solution and pointed to my objective function cell. I suppose it means the objective function cell is not built right for opensolver or cannot be handled by opensolver as it is. Please help me to resolve why this cell is presumably causing the error. I look forward to any assistance and will appreciate it.

    Reply

      1. Hi Andrew,
        Thank you so much for coming back to me. I do not know what exactly is the error (I am fairly new to use of opensolver). The program pointed me to one cell containing objective function after reportng that opensolver cannot find a solution. I can send my file for any help in checking what could be the error in the built model; will really appreciate that.

        Reply

        1. Thanks. However, we really need to know exactly what OpenSolver is saying. Please solve the problem again and explain in more detail what error OpenSolver reported (i.e. give us the OpenSolver error message text). Thanks. Andrew

          Reply

          1. Thanks Andrew. Below are the details of the error message:
            “OpenSolver could not find an optimal solution, and reported:Infeasible
            The model contains an Excel constraint “1.2INPUT DATA’!$B$414:$G$414=1′ for which instance 1 does not depend on the decision variables and is not satisfied.
            Constraint specifies:LHS=B414=0=RHS=1=1
            No solution was available to load into the spreadsheet.
            CBC Solver reported: Unsolved”

            On a separate but related note, I had to delete all constraints in relation to binary variable cells that I wanted to set as additional variable cells (decision variables) after getting the error “A cell specified as bin or int could not be found in the decision variable cells”. I wish to also find out if Opensolver supports this possibility of setting additional variables cells as binary?

            I look forward to your valuable insights and assistance.

            Reply

            1. You have made a mistake in your model. Specifically, you have a constraint that says B414=1, but cell B414 (which is currently 0) does not depend on the decision cells, and so there is no way to satisfy your constraint. And, no, OpenSolver does not support specifying a non-decision cell as being integer or binary. You can, of course, add a new integer variable, and force it to equal the non-decision cell via a constraint. Cheers, Andrew

              Reply

  4. Hi,I am a new user of opensolver and running excel 2007 (on a windows 8 laptop) and excel 2010 (on a windows 7 work PC) and wish to ask for help about the model I have built. The model has a really large number of variables (some with 2 subscripts, others 3).I wish to also find out if open solver can handle this kind of model from the experts (after my reading informed me that it can handle more complex problems than solver). I am confident about most equations but one (involving variable with 3 subscripts). Please help me to check if the model I have built is right for and can be handled by open solver. Also interested to check how much time it will take open solver to produce an optimal solution. I can send an e-mail with my spreadsheet model and would really appreciate any assistance to get the model right for open solver.

    Reply

  5. Hi,

    I have win8 and MS Office 2013 installed.

    I downloaded the latest version and tried to solve a model I built- got this error:

    “Open Solver encountered an error:
    The CBC solver did not create a solution file. No new solution is available. (at line 12300) (at line 19540)

    Source= opensolver Model Solving, ErrNumber= -2147220503”

    What shall I do??

    Reply

  6. Hi andrew,

    my question is, has the Excel SOLVER than 32 changing cells, few Open Solver changing cells have?, Or Open Solver only increases the number of variables and constraints but not varables decision?

    Reply

  7. Andrew,

    thanks a lot for the great piece of software. I find opensolver extremely extremely powerful

    I have a very quick question about integration with CPLEX as a solver (instead of CBC). Does anybody try this? I assume the .lp generated as input for CBC is fully compatible and can be loaded directly to CPLEX. But, is there a smart way to parse .sol (CPLEX output)?

    I would love to hear any experience you guys have had with similar gimmicks. Due to some constraints (with other people understanding the model) I can’t directly upgrade to more advanced solutions (like Solver Studio) while CPLEX Excel plug-in seems to be useless.

    thanks for any hints

    Reply

  8. Hello. Why is SolverStudio 6.0 faster than OpenSolver? My understanding is that OpenSolver uses CBC written in C++. Where does the speed difference come from?

    Reply

    1. An OpenSolver spreadsheet with 1000 variables takes 1000 spreadsheet recalculations to extract the model equations ready for CBC to then start solving. These 1000 recalculations can be slow. SolverStudio stores the model equations directly (using user input via a modelling language), and so is often faster. Hope this helps, Andrew

      Reply

  9. How do i use open solver in a macro? is there something similar to “solver.solve” for the open solver ?

    Regards, and thanks a lot.

    BTW this tool works great!

    Reply

      1. Thanks a lot Andrew. The installation challenge on Windows 8 was related to a file extension issue. I read your other post about how to resolve this issue.I moved the extracted files to My Docoments, clicked on “view” and checked the box “file extension name”. I was then able to see cbc.exe (instead of the previous cbc) and all was good.

        Thanks

        Reply

  10. Hi, i’m very new in use of open solver, because i do not do this calculation usually at work. but now i have to solve this and after different trials with no success,i really need your help: i have 694 rows with different values. i have to find exactly 239 rows which sum is a exact number. the only thing is that i am sure that there are those rows. can you please help me in building the right model for this, or should i send an e-mail with my spreadsheet? do you think is it possible and does it take too long to calculate? I will really appreciate any help. Thank you (i have excel 2010 and 2003)

    Reply

    1. Costa,

      This is known as the knapsack problem: http://en.wikipedia.org/wiki/Knapsack_problem

      It is possible to use Solver (and OpenSolver to find this).

      You need three columns: Each cell of Column A should contain a data point that you need to sum, Column B will be filled with zeros (as long as column A), and column C will contain the formula ColA * ColB.

      For example A1 = 4 B1 = 0 C1 = A1*B1
      A2 = 5 B2 = 0 C2 = A2*B2
      … and so on for all 694 rows.

      In the cell immediately below the last cell in columnC, sum up the values in column C (=Sum(C1:C694))

      Then, set your solver as follows:

      Objective = The cell with the sum formula above
      Set to value = Put the sum that you are looking for here

      Constraints: Column B = binary

      That should do it!

      Reply

  11. Hi,
    How do I install opensolver on Windows 8? I get a command (black) screen with Coin: when I click on the application, after downloading and extracting all the files to my Program files location. Please help?

    Reply

  12. I have problem with usage DEA (in Add-Ins menu)in following message:
    There are more than 20 DMUs,
    Please get the full version of the software.
    please advise me. thanks.

    Reply

  13. I have problem with usage DEA (in Add-Ins menu)in following message:
    There are more than 20 DMUs,
    Please get the full version of the software.
    please advise me. thanks.

    Reply

  14. Respected sir,
    I have four inputs and two outputs ,out of four inputs one of them is text term that has been varied ie chemical name 1 mpms,aps,mtmos these are varied along with other parameters and out put is been effected by all input parameters.This is done by taguchi l9 array.can i use this sexcel solver to solve my problem

    Reply

  15. Have a large LP where I have to try out different values for just one coefficient in the objective function; everything else the same. Can I use Quick Solve? Thanks for a help.

    Reply

    1. Unfortunately, no. This is not for any good reason, but simply because we have not needed this functionality. Note that the next release of OpenSolver will give cost sensitivity analysis, which may help you. Andrew

      Reply

Leave a Reply to Jack Cancel reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.