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.

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.

1,887 thoughts on “Help”

  1. Hi Andrew, I have noticed a bug in the Open Solver. It incorrectly identifies a linear constraint as non-linear (part of the linearity check is reproduced below). I am pretty certain that my formulation is correct, and I have managed to solve it on another platform. Is this a known error?

    This error only arises if the formula in cell G7 is changed from SUM(G12:G75) to SUMPRODUCT(G12:G75,E12:E75), where G12:G75 contains binary variables and E12:E75 contains constants
    In the constraint: G7 <= I7,
    the model appears to be non-linear in the decision variables: G12, G13, G14, G15, G16, G17, G18, G19, G20, G21, G22, G23, G24, G25, G26, G27, G28, G29, G30, G31, G32, G35, G36, G37, G38, G39, G40, G41, G42, G43, G44, G45, G46, G47, G48, G51, G52, G53, G54, G55, G56, G57, G58, G59, G60, G61, G62, G63, G64, G67, G68, G69, G70, G71, G72, G73, G74, G75

    1. Thanks for the report. Please confirm that E12:E75 are all constants (i.e. simple numeric values). Andrew

          1. E12:E75 are numeric constants and not formulae in the cells.
            G12:G75 are binary variables (not formulae). The error only arises when the formula in cell G7 is changed from SUM(G12:G75) to SUMPRODUCT(G12:G75,E12:E75)

            1. Are you happy sending the spreadsheet to me? False non-linearity reports can often arise because of numerical issues where the coefficients are very large or very small; such models are poorly behaved both for solving and for linearity checking. It would be helpful to have an example of such a case that we could include in our testing. If you are happy, are you able to send the spreadsheet to email hidden; JavaScript is required ? Thanks, Andrew

  2. Greetings, Dr. Mason

    I have a number of large Minimum Cost Network Flow problems to do. These should have, among their solutions, entirely integer allocation sets, which I prefer to non-integer solutions.

    In lieu of integer constraints, OpenSolver returns non-integer allocations with the correct objective function value. This suggests to me that it is using a barrier algorithm. (?) Is there a way to demand the Simplex algorithm for these – or to otherwise guide the solver? With large problems (75k variables), I fear that the introduction of integer variables adds significant time, even in the MCNF realm. (Perhaps I am deceived in this estimation.)

    Note that I can reproduce the difficulty even with a very small problem, for which the Excel solver yields integer allocations and OpenSolver does not.

    Also, OpenSolver’s .lp file indicates that it recognizes the network problem. (As one would expect.)
    / Network!$I$2:$L$25 < 1
    / Network!$I$30:$L$33 = Network!$I$37:$L$40
    / Network!$O$2:$O$25 < 1

    Any hints?

    -Larry Curcio

    1. Larry: OpenSolver uses CBC, which uses the simplex method. How fractional are your solutions; values like 0.9999 and 1 are equivalent for all practical purposes, and you should treat them as such. However, if you get 1/2 or 1/3 in a solution, then I’d be surprised, but as you say such solutions may still be optimal. It is possible that CBC is finding the solutions using a heuristic, but I’d be surprised. By the way, the “Network” reference in the .lp file is your sheet name, not a network identification. Andrew

  3. Hello, I am attempting to run open solver on my Macbook air. I was able to successfully download the program, but when I engage any of the buttons on the solver, it does not respond. I have attempted to uninstall, then re-install excel and the solver, but had no luck. Any suggestions?

  4. Greetings,

    I am trying to run OpenSolver via VBA but I am getting a “Runtime Error -2147220503, Automation Error” and I can’t seem to find out why. My code is appended below.
    Sub C_Solve()

    Dim Schedule As Worksheet
    Set Schedule = Sheets(“Data”)

    With Schedule

    ‘C Crew Last Row
    Dim crewlRowC As Long
    Dim crewblRowC As Long
    crewlRowC = Cells(Rows.Count, 2).End(xlUp).Row
    crewblRowC = .Range(.Cells(118, 2), .Cells(crewlRowC, 2)).End(xlDown).Row

    ‘C MaxDuty Row
    Dim lRowC As Long
    Dim blRowC As Long
    lRowC = Cells(Rows.Count, 20).End(xlUp).Row
    blRowC = .Range(.Cells(1, 20), .Cells(lRowC, 20)).End(xlDown).Row

    ‘CP MaxDuty Row
    Dim lRowCP As Long
    Dim blRowCP As Long
    lRowCP = Cells(Rows.Count, 22).End(xlUp).Row
    blRowCP = .Range(.Cells(1, 22), .Cells(lRowCP, 22)).End(xlDown).Row

    End With

    ‘Reset all previously used models
    OpenSolver.ResetModel Sheet:=Schedule

    ‘Set Solver
    OpenSolver.SetChosenSolver “Bonmin”, Sheet:=Schedule

    ‘Objective definition
    OpenSolver.SetObjectiveFunctionCell Schedule.Cells(3, 26), Sheet:=Schedule
    OpenSolver.SetObjectiveSense MaximiseObjective, Sheet:=Schedule

    ‘Variables definition
    OpenSolver.SetDecisionVariables Union(Schedule.Range(Cells(2, 118), Cells(2, crewblRowC)), Schedule.Range(Cells(32, 118), Cells(32, crewblRowC))), Sheet:=Schedule

    ‘Constraints definition
    ‘1. Variables are binary
    OpenSolver.AddConstraint Union(Schedule.Range(Cells(2, 118), Cells(2, crewblRowC)), Schedule.Range(Cells(32, 118), Cells(32, crewblRowC))), RelationBIN, Sheet:=Schedule
    ‘2. Max of 1 crew scheduled per day
    OpenSolver.AddConstraint Schedule.Range(Cells(2, 143), Cells(32, 143)), RelationEQ, Cells(25, 6), Sheet:=Schedule
    ‘3. Max duty count
    OpenSolver.AddConstraint Schedule.Range(Cells(33, 118), Cells(33, crewblRowC)), RelationLE, Schedule.Range(Cells(20, 2), Cells(20, blRowC)), Sheet:=Schedule
    ‘4. Consec duty count
    OpenSolver.AddConstraint Schedule.Range(Cells(34, 118), Cells(34, crewblRowC)), RelationEQ, Cells(25, 9), Sheet:=Schedule

    OpenSolver.RunOpenSolver Sheet:=Schedule

    End Sub

    Appreciate any help. Thanks alot.

  5. I’ve now transitioned to Windows products due to the fact I couldn’t use opensolver on google. I am now getting an “Error 4551: Your organization used device guard to block this app”. what can I do? Do i need to pay money to get this unblocked. I have no problem doing so. I just need this solver to work.

    1. We have not had this reported to us before. No, it is nothing to do with paying money. From my Googling, it looks like a Windows feature used on work machines. Maybe your IT department can help? Please let us know how you get on. Andrew

      1. No IT. Use solver for Daily Fantasy. I got it working by changing the Engine to NEOS using COIN-OR CBC (Linear Solver). I believe I was trying to solve with the COIN-OR.

  6. Hi,

    I keep getting an error message “Method ‘Range’ of object ‘_Worksheet’ failed” when using the non-linear engines.

    Any thoughts on what I’m doing wrong ?


    1. David: Are you using the latest beta release? From memory, this error is a VBA problem that we added a small delay to fix. Andrew

      1. Andrew,

        Yes I am. Seems to be an issue with how Excel looks at the Add-In, I need to disable then enable OpenSolver between runs, not a big issue.



  7. Hello,
    I am helping a user with his new install of OpenSolver v2.8.5.
    When he tried to choose a different Solver of, COIN-OR Bonmin (Non-linear solver) he gets the message;
    Unable to find Bonmin (“Bonmin.exe”) Folders searched:

    We are very locked down here and I am questioning if he needs a newer version of OpenSolver ot to load/install bonmin.exe?


    1. You need the Advanced version of OpenSolver. And no, it is nothing to do with COIN-OR’s move to GitHub. Andrew

  8. Hello, I am helping a user with OpenSolver v2.8.5 add-in, it was just recently installed.
    When he tried to choose a different Solver of, COIN-OR Bonmin (Non-linear slover) he gets the message;
    Unable to find Bonmin (“Bonmin.exe”) Folders searched:

    We are very locked down and I am questioning if he needs a newer version of OpenSolver or to load/install bonmin.exe?
    Could it be related to has moved to an issue?

    1. You need the Advanced version of OpenSolver which includes Bonmin. And no, it is nothing to do with COIN-OR’s move to GitHub. Make sure that you can run the bonmin.exe execuable from a command line; if you don’t have permissions to do so, then OpenSolver won’t be able to either. Andrew

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