Assigning Units to Variables

You manually assign units to stocks and converters in the model.

All stocks in a main chain are automatically assigned the same units that you assign to any stock in the chain.

Units for flows are automatically assigned by the software based on the units you've assigned to their associated stocks. For example, if you assign a unit of measure called "widgets" to a stock, and your model is running in months, any flows into or out of the stock would automatically be assigned the units "widgets per month", a built in unit definition with equation widgets/month.

Notes: If you add stocks or flows to the chain after you assign units, the new variables won't be automatically assigned units. To assign units to the new variables, assign units to any stock in the chain; everything else in the chain will automatically be assigned units.

If there's a unit-converted flow (one for which the Unit conversion check box is selected) in the chain, any flows or stocks beyond that flow will not be automatically assigned the same unit of measure; you have to manually assign units to those stocks yourself.

When you assign units to a variable, you can select from existing units, select from units already used in the model, create new units, or have the software suggest the appropriate units based on the variable's equation.

To Assign Units to a Variable

Use the following procedure to assign units to a variable by using the Units options on the Equation panel.

Select the variable in the model diagram.
Click the Equation tab of the Properties panel.
Use the Units box at the bottom of the Equation panel to assign units to the variable:

To assign existing units, type the name of the units you want to assign to the variable. As you type, existing units that match the letters you've typed so far will appear in a dropdown list. Use the arrow keys to highlight the unit you want to assign, and then press Enter.
To select existing units from a list of categories, click to the right of the Units box, choose the category you want, and then choose the units you want to assign.
To select units that have already been assigned to another variable in the model, click , choose Used in this model, and then choose the units you want.
To select units that you've created previously, click , choose My Units, and then choose the units you want.
To assign units that don't already exist, type the name that you want to give the units, or type in an equation. If you want to provide aliases (synonyms) or define an equation for the units you type in, see Creating and Deleting Units.
To have the software specify the appropriate units, click , and then choose Suggest units. If possible, the software will determine the correct units and display them in the Units box.

Note: If the units can't be determined from the equation, an error message will be provided.

When the units you want to assign are displayed in the Units box, move on to your next task, or click to apply the units to the variable.