# Introduction to Units

When we build models in Excel, we often use units only as an afterthought. Maybe we add a column in which we write a `$` symbol to indicate that a line is in US Dollars. But this is only done as a hint to the spreadsheet user and has no direct impact on any calculations.

In [**Models**](/models/home-tab.md)**,** Units are built into the fundamental workings of the software. Each [**Variable**](/variables/variable-basics.md) has a Unit, and if a [**Calculation**](/calculations/introduction-to-calculations.md) produces an inconsistent Unit, you will see a warning, letting you know that there may be a logical error in your Model.

Units in Models also allow you to automatically move between items of different size, e.g. $ to $000 to $m, (see [**Factors**](/units/key-unit-concepts.md#factors)).

Units also enable much of the advanced functionality in Models, such as:

* Automatically [**matching Assumptions**](/assumptions/creating-referencing-assumptions.md#time-segment-editing) to Variables.
* [**Aggregation**](/aggregation/introduction-to-aggregation.md) of multiple Variables
* Performing automatic and custom Dimension [**references**](/calculations/calculation-parts/variable-references/referencing-dimensions.md) in Calculations

{% hint style="info" %}
For the most part, you don't *have* to use Units to produce a working Model. Inconsistent Units will only produce warnings, not errors, and calculations will still be performed. And if you want to, you can create a Model with only `Unitless` Variables.

The Models UI is designed to make the use of Units as easy as possible, and to add only a small overhead to your build process.

We hope that once you understand how Units work, it will help you build better, more-readable Models, with fewer errors.
{% endhint %}


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