> For the complete documentation index, see [llms.txt](https://help.modelreef.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://help.modelreef.io/help/drivers-variables-and-timing/units-and-bounds.md).

# Units & Bounds

This article explains how units and bounds work in Model Reef and how they help you keep variables well behaved and easy to interpret.

You will learn:

* What units are and why they matter.
* How bounds prevent unrealistic or broken assumptions.
* Where to see and edit units and bounds.
* Good practice for setting units and bounds across a model.

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### What units are

Each variable and driver in Model Reef has **units**, for example:

* Currency: GBP, USD, EUR, AUD.
* Counts: units, customers, orders, FTEs, hours.
* Rates: percent, basis points, index values.

Units are used for:

* Display formatting in tables, charts and dashboards.
* Tool tips and labels in the UI.
* Helping humans interpret what a series represents.

The model calculations treat values as numeric but units make that numeric content meaningful.
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### What bounds are

**Bounds** define sensible minimum and maximum values for a variable or driver, for example:

* Revenue cannot be negative below a certain threshold in a given scenario.
* Volumes cannot go below zero.
* Discount rates should stay within a realistic range.
* Growth rates should not exceed a hard cap without explicit decision.

Bounds do not replace judgement, but they help catch typos and extreme inputs.

{% hint style="warning" %}
Bounds are safeguards to catch unrealistic or extreme inputs — they are not a substitute for modelling judgement.
{% endhint %}
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### How units and bounds are used in the UI

In the variable or driver editor you can:

* Set or change units from a defined list or by entering custom units.
* Set lower and upper bounds for values.
* See warnings or validation messages if values fall outside the bounds.

In dashboards and charts:

* Units influence axis labels, legends and KPI card formatting.
* Bounds can help you notice unusual behaviour when a series suddenly hits a cap or floor.

These features improve readability and reduce modelling errors.
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### Good practice for units

Guidelines:

* Use the **model currency** for all monetary variables by default.
* Use counts for volumes and headcount, not implicit currency.
* Use percent or index units for growth, margins and modifiers.
* Keep units consistent across similar drivers and variables, especially when combining them in formulas.

Clear units make cross checking much easier for reviewers and collaborators.
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### Good practice for bounds

Guidelines:

* Set bounds narrowly for sensitive or critical assumptions to catch out of range inputs.
* Use wider bounds for series where volatility is expected, but still prevent impossible states such as negative headcount.
* Document in notes where bounds represent a hard constraint rather than just a warning.
* Review bounds when shifting to new scenarios or business environments.

Bounds are especially helpful when junior modellers or non finance users are editing assumptions.
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## Related articles

* [New Store Expansion Model](/use-cases/retail-single-and-multi-site/new-store-expansion-model.md)
* [Build a Capital Raise Model](/how-tos/investment-and-transactions/build-a-capital-raise-model.md)
* [Mapping Units & Frequency](/help/importing-and-data-inputs/mapping-units-and-frequency.md)
* [Units & Bounds Panel](/syntax/understanding-the-interface/units-and-bounds-panel.md)