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## Resource: BillingTaxCalculation



## Article: Introduction

## Article Link: https://dev.wix.com/docs/velo/events-service-plugins/billing/service-plugins/billing-tax-calculation/introduction.md

## Article Content:

# Introduction
The Tax Calculation service plugin (formerly SPI) allows you to integrate a custom tax calculation service with Wix. 

Use the Tax Calculation service plugin together with the [Tax Groups API](https://dev.wix.com/docs/velo/api-reference/wix-billing-v2/tax-groups.md) and the [Tax Regions API](https://dev.wix.com/docs/velo/api-reference/wix-billing-v2/tax-regions.md) to customize your tax calculations. 

Learn more:
+ [Velo: Custom App Extensions Using service plugins](https://support.wix.com/en/article/velo-custom-business-app-extensions-using-spis-beta)

### To add a service plugin
1. [Add the plugin to your site](https://dev.wix.com/docs/develop-websites/articles/coding-with-velo/integrations/custom-extensions-spis/custom-app-extensions-using-sp-is.md#step-1-create-a-new-extension-on-your-wix-site).
2. Update the [`getConfig()`](https://dev.wix.com/docs/velo/api-reference/wix-billing-v2/service-plugins-spis/billing-tax-calculation/get-config.md) function in the **-config.js** file that is added to your site during step 1.
3. Update the [`calculateTax()`](https://dev.wix.com/docs/velo/api-reference/wix-billing-v2/service-plugins-spis/billing-tax-calculation/calculate-tax.md) function in the **.js** file that is added to your site during step 1.

### Terminology

+ **Tax Group:** A category of products that share the same tax treatment. Tax is calculated based on the tax group and the tax region associated with a line item. See the [Tax Groups API](https://dev.wix.com/docs/velo/api-reference/wix-billing-v2/tax-groups.md).
+ **Tax Region:** A location, defined by `country` and `subdivision` with a specific tax treatment. Tax is calculated based on the tax region and the tax group associated with a line item. See the [Tax Regions API](https://dev.wix.com/docs/velo/api-reference/wix-billing-v2/tax-regions.md).
+ **Tax Calculator:** An app that is used under the hood to calculate tax for a Wix site. The app calculates tax based on the tax group of the product and the tax region of the sale. Use the Tax Calculation service plugin to become a tax calculator.