You can specify a calculation that will set the value of a field on a form. For example, you could easily calculate a line item amount on an invoice from the price and number of units fields. Calculations may be performed across several forms. A calculated field could also update subtotal and total fields when, for example, a line item on an invoice is added, deleted or updated.

With **formdef**, you define each field on a data entry form with** formdef**'s Field Description screen. See Chapter 5, *Creating Formfiles* for more information on creating forms. On the second Field Description Screen, there is a field labeled *Copy/Calculate.* This chapter describes the possible formulas (calculations) that may be used for calculating field values.

When **form **evaluates the *Copy/Calculate* formula of a field, **form** stores the result of the formula in that field. Normally, a *Copy/Calculate* field has *Entry Allowed* set to **no**. A *Copy/Calculate* field can be an *Invisible *field, allowing you to calculate values that are not displayed on the form. **Form **evaluates the *Copy/Calculate *formula in a field in the following situations

A new form is displayed (for example, by pressing the NEXTFORM key)

An existing form is cleared (for example, by pressing the ADD key)

A new record is displayed in an existing form (for example, by pressing the FIND key)

Another field changes and the *Copy/Calculate *formula depends on the other field value (for example, the **amount** field has the *Copy/Calculate* formula **quantity * cost** and you type a new value in the **cost **field)