Equation Tab

In the Model window, use the Equation tab to specify the equation or initial value for a selected building block. The Equation tab is for assigning units to a building block and selecting options and parameters that affect the building block's equation.

To view the Equation tab, click the at the bottom of the Properties panel. For more information about using the Properties panel, see Using the Properties panel.


The options available in this section depend on the type of building block you select.

Required Inputs

This displays a list of all inputs that must be used in the equation for the selected variable. Required inputs are anything connected to the selected variable using connectors. This list only appears if the selected variable is a flow or converter.

Note:This list doesn't appear for Summing Converters, which can take any inputs.

Tips: You can also see a list of all allowable inputs for the selected variable (including required inputs) by using the Find window. You can then drag a listed input from the Find window into the Equation box. You can also drag a required input variable from the model diagram into the Equation box. For more information, see Finding Variables in Models.


This displays a list of builtins that you can include in the equation for the selected building block.

Note: Regarding converters, this list doesn't appear for summing converters, which create their equations automatically.

Tip: By default, the list displays all builtins in alphabetical order. To make it easier to find a specific builtin, select a category in the box at the top of the Builtins list.


This is the list of variables whose values will be added for the selected summing converter.

Note: This box only appears when a summing converter is selected on the model diagram.


Use the buttons in the Operators and Calculator sections to insert numbers and operators into the equation for the building block.

Note: The full calculator is only available when you're viewing the Equation tab for a stock, or when the Properties panel is sufficiently wide. For other variables, and when the Properties panel is at its default width, only the Operators section is available.

Tip: You can also insert operators and numbers into the equation by typing them.

Tips: To transpose arrays in an equation, you can use the apostrophe (') as a transposition operator. For example, to transpose the array A[X, Y] as A[Y, X] in an equation, you can type A'. The apostrophe operator doesn't appear as a button in the Operator section. You have to type it.

If you're defining an equation for an arrayed variable that has more than one dimension with the same name, use the @ operator instead of the dimension name to refer to a particular instance of the dimension. Use @1 for the first use of the dimension, @2 for the second use of the dimension, etc.


The options available in this section depend on the type of building block you select.

Tip: For parameters where you can specify a model variable as a value, you can type the variable's name in the parameter box, or drag a variable from the Find window.


Use this box to specify the initial value or equation for the selected building block. The initial value specified here is used only at the start of a simulation run.

Note: The name and icon for the selected building block appear just above the Equation box.

The method for defining the initial value and equation depends on the type of building block you select. The following general procedures apply to defining equations for all types of building blocks. For information on defining equations for a specific type of building block, click the appropriate link below.

Note: You can also generate initial and subsequent values for variables, via a link from an Excel or .CSV file, by using the data import features. For more information, see Importing and Exporting Data.

Apply To All

This check box appears if the selected variable has been defined as an arrayed variable.

For more information about specifying equations for arrays, see Creating Arrayed Variables. You can also open the Arrayed Equation editing dialog by clicking on the button.


Use the options in this section to assign and manage units of measure.

Note: You can't assign or change units for a variable whose units are defined by other variables. For example, the unit of measure for a flow attached to a stock is defined by the stock's unit of measure. In this case, the flow's units are displayed in the Units box, but you can't edit them directly; to change the flow's units, change the stock's units.

If you're enforcing unit consistency, when you click at the bottom of the tab to apply your changes, the software will perform a unit consistency check on the equation.

For more information, see Assigning Units to Variables, Creating and Deleting Units, and Checking Unit Consistency.

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