> ## Documentation Index > Fetch the complete documentation index at: https://docs.nebuly.com/llms.txt > Use this file to discover all available pages before exploring further. # Custom metrics Built-in variables cover most needs, but when creating or editing an Analytics or Table chart you can also define **custom variables**. Custom variables support **mathematical formulas**, so you can express calculations that aren't possible with built-in variables alone. Formulas can reference both built-in and custom variables, and filters can be applied as usual, letting your team track the metrics that matter most to how you measure the AI product. ## Creating a new variable To create a custom variable, open the chart builder for an Analytics or Table chart. In the **Metrics** panel on the left, click **+ Create New Metric**. Creating a new variable In the dialog that opens, give the variable a **Name** and, optionally, a **Description**. Then build the **Formula**: type to search for variables and press Enter or Tab to insert each one as a chip, adding operators and numbers as needed (for example, `( Interactions / Conversations ) * 100`). Turn on **Display as percentage** if the result should be shown as a percentage, optionally add a **Filter** to scope the variable, then click **Create**.
Creating a new variable, formula
The new variable is added to the chart and can be referenced inside other custom variables. ## Forecasting metrics For any time-series line chart in Reports, you can extend the chart with a **forecast** of where the metric is heading through the end of the current period. This is especially useful for monitoring goals ("are we on track to hit our monthly target?") and for executive reports where the audience cares about the trajectory, not just the historical line.