> For the complete documentation index, see [llms.txt](https://docs.struct.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.struct.com/pim3/tutorials/helpful-guides/how-to-use-business-rules/how-to-automatically-calculate-the-size-of-a-variant.md).
# How to automatically calculate the size of a variant
To make a rule that will calculate the volume of a [variant](/pim3/fundamental-concepts/products-and-variants.md) based on the dimensions of that [variant](/pim3/fundamental-concepts/products-and-variants.md), make a [trigger](/pim3/reference/product-model-setup/business-rules/business-rule-details/triggers/trigger-details.md) that [triggers](/pim3/reference/product-model-setup/business-rules/business-rule-details/triggers/trigger-details.md) when any of the dimensions change.
Business rule setup page
Then add an [action](/pim3/reference/product-model-setup/business-rules/business-rule-details/actions/action-details.md) and choose "Calculate value". When configuring the [action](/pim3/reference/product-model-setup/business-rules/business-rule-details/actions/action-details.md) click "+Add math calculation" and choose "Add math operation".\
\
In this example we start by multiplying the height and width. That result is then multiplied with the length. Finally that result is multiplied with 0,001 to make sure the units are correct.
Configure action
Finally choose what [attribute](/pim3/fundamental-concepts/attributes.md) the result should be written to.
{% hint style="info" %}
**Note** By clicking the operator between the values you can choose the operator (x,/,+,-).
{% endhint %}
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