> ## Documentation Index
> Fetch the complete documentation index at: https://docs.getnovaplan.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Budget Rules

> Allocate and derive budget across departments, accounts, and periods with rules.

Budget Rules let you express how a number should spread or derive instead of typing it cell by cell. Allocate a pool across departments, drive one account off another, or apply a percentage across periods, and the rule re-applies when its source changes so allocations stay consistent.

## What Rules Can Do

* **Allocate** a total across departments, accounts, or periods by weight or driver.
* **Derive** one line from another (for example, a percentage of revenue or of headcount).
* **Spread** an annual figure across months with a chosen shape.

## How to Use It

<Steps>
  <Step title="Describe it">
    Define the target, the source, and the allocation logic. You can ask Nova to draft a rule from a plain-language description and refine it.
  </Step>

  <Step title="Preview">
    See exactly which cells the rule will write before you apply it, so nothing changes by surprise.
  </Step>

  <Step title="Apply">
    The rule populates the plan and re-applies when the source changes, so a derived line stays in step with what it depends on.
  </Step>
</Steps>

## A Few Examples

| Goal                                             | Rule shape                                                  |
| ------------------------------------------------ | ----------------------------------------------------------- |
| Spread a software pool across teams by headcount | Allocate a total across departments, weighted by head count |
| Budget T\&E as 4 percent of revenue              | Derive an account as a percentage of another                |
| Turn an annual marketing number into months      | Spread an annual figure across periods by a seasonal shape  |

## Common Questions

<AccordionGroup>
  <Accordion title="What happens when the source of a derived rule changes?">
    The rule re-applies. If you budget an account as a percent of revenue and the revenue plan moves, the derived line moves with it the next time the rule runs.
  </Accordion>

  <Accordion title="Will a rule overwrite numbers I typed by hand?">
    The preview shows you exactly which cells the rule writes before you apply, so you can see whether it would touch a hand-entered cell and adjust the rule's scope.
  </Accordion>

  <Accordion title="How is this different from a seasonality curve in Expense Planning?">
    A seasonality curve shapes one expense line's annual total across months. A budget rule is broader: it can allocate across departments and accounts, or derive one line from another, not just shape timing.
  </Accordion>

  <Accordion title="Can I use rules and mid-year updates together?">
    Yes. Budget Rules shape the plan; [reforecast rules](/planning/reforecast) adjust the forward outlook mid-cycle. They serve different stages of the cycle.
  </Accordion>
</AccordionGroup>

## If the Numbers Look Off

* A derived line is stale: re-apply the rule so it re-reads its source, or re-run the forecast if the source is a derived universe.
* An allocation does not sum to the pool: check the weights or driver. If a department has a zero driver value it receives nothing, which can leave a remainder.
* A rule wrote cells you did not expect: revisit the target scope in the preview before applying; narrowing the dimension or period range fixes most surprises.