# Using Drivers in Formulas

This article explains how to **use drivers in formulas** in Model Reef.

You will learn:

* How to reference drivers in the Formula modal.
* How drivers combine with variables and other series.
* Patterns for building driver based formulas safely.

Drivers are read-only time series that you plug into variable formulas and custom chart formulas.

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### Referencing drivers in the Formula modal

When you open the **Formula modal** for a variable or custom series:

* A sidebar shows available drivers grouped by type.
* You can click a driver to insert a reference into the formula.
* You can also start typing the driver name and use autocomplete.

The inserted reference represents the entire time series, not a single cell.
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### Arithmetic-only formula engine

The formula engine supports arithmetic only:

* Addition `+`
* Subtraction `-`
* Multiplication `*`
* Division `/`
* Parentheses `( )`

No IF statements, logical operators or cell-based references are allowed. Formulas operate on entire time series at once.
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### Common driver formula patterns

Examples of patterns using drivers:

* Price and volume
  * `Revenue = Units driver × Price driver`
* Inflation or growth
  * `Cost = Base cost × (1 + Inflation driver)`
* FX adjustment
  * `Local amount = Foreign amount × FX driver`
* Scenario multiplier
  * `Scenario result = Base result × Scenario modifier driver`

Because everything is vectorised, the formula is applied across all periods of the model horizon.
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{% step %}

### Combining drivers with variables

You can combine drivers with variables, for example:

* `New variable = Existing variable × Modifier driver`
* `Margin % = (Revenue variable - COGS variable) ÷ Revenue variable`

The engine takes care of aligning time indexes so that operations occur period by period.
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{% step %}

### Validation and previews

The Formula modal provides:

* Syntax validation to check the formula is structurally correct.
* A preview of the resulting series, so you can see if the pattern looks right.

If a driver reference is removed or renamed, you will see an error until the formula is updated.
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## Related articles

* [Linking Drivers to Variables](/syntax/drivers-syntax/linking-drivers-to-variables.md)
* [Overriding Drivers](/syntax/scenario-syntax/overriding-drivers.md)
* [Economic Drivers](/help/drivers-variables-and-timing/economic-drivers.md)
* [Build a Staffing Cost Model](/how-tos/operations-and-unit-economics/build-a-staffing-cost-model.md)


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